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666 lines
17 KiB
666 lines
17 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* |
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* Copyright(c) 2017 Intel Corporation. All rights reserved. |
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* |
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* This code is based in part on work published here: |
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* |
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* https://github.com/IAIK/KAISER |
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* |
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* The original work was written by and and signed off by for the Linux |
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* kernel by: |
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* |
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* Signed-off-by: Richard Fellner <[email protected]> |
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* Signed-off-by: Moritz Lipp <[email protected]> |
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* Signed-off-by: Daniel Gruss <[email protected]> |
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* Signed-off-by: Michael Schwarz <[email protected]> |
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* |
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* Major changes to the original code by: Dave Hansen <[email protected]> |
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* Mostly rewritten by Thomas Gleixner <[email protected]> and |
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* Andy Lutomirsky <[email protected]> |
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*/ |
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#include <linux/kernel.h> |
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#include <linux/errno.h> |
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#include <linux/string.h> |
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#include <linux/types.h> |
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#include <linux/bug.h> |
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#include <linux/init.h> |
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#include <linux/spinlock.h> |
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#include <linux/mm.h> |
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#include <linux/uaccess.h> |
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#include <linux/cpu.h> |
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|
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#include <asm/cpufeature.h> |
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#include <asm/hypervisor.h> |
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#include <asm/vsyscall.h> |
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#include <asm/cmdline.h> |
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#include <asm/pti.h> |
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#include <asm/tlbflush.h> |
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#include <asm/desc.h> |
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#include <asm/sections.h> |
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#include <asm/set_memory.h> |
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|
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#undef pr_fmt |
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#define pr_fmt(fmt) "Kernel/User page tables isolation: " fmt |
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|
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/* Backporting helper */ |
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#ifndef __GFP_NOTRACK |
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#define __GFP_NOTRACK 0 |
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#endif |
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/* |
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* Define the page-table levels we clone for user-space on 32 |
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* and 64 bit. |
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*/ |
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#ifdef CONFIG_X86_64 |
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#define PTI_LEVEL_KERNEL_IMAGE PTI_CLONE_PMD |
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#else |
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#define PTI_LEVEL_KERNEL_IMAGE PTI_CLONE_PTE |
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#endif |
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static void __init pti_print_if_insecure(const char *reason) |
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{ |
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if (boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN)) |
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pr_info("%s\n", reason); |
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} |
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static void __init pti_print_if_secure(const char *reason) |
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{ |
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if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN)) |
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pr_info("%s\n", reason); |
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} |
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static enum pti_mode { |
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PTI_AUTO = 0, |
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PTI_FORCE_OFF, |
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PTI_FORCE_ON |
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} pti_mode; |
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|
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void __init pti_check_boottime_disable(void) |
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{ |
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char arg[5]; |
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int ret; |
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/* Assume mode is auto unless overridden. */ |
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pti_mode = PTI_AUTO; |
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if (hypervisor_is_type(X86_HYPER_XEN_PV)) { |
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pti_mode = PTI_FORCE_OFF; |
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pti_print_if_insecure("disabled on XEN PV."); |
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return; |
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} |
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ret = cmdline_find_option(boot_command_line, "pti", arg, sizeof(arg)); |
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if (ret > 0) { |
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if (ret == 3 && !strncmp(arg, "off", 3)) { |
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pti_mode = PTI_FORCE_OFF; |
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pti_print_if_insecure("disabled on command line."); |
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return; |
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} |
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if (ret == 2 && !strncmp(arg, "on", 2)) { |
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pti_mode = PTI_FORCE_ON; |
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pti_print_if_secure("force enabled on command line."); |
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goto enable; |
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} |
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if (ret == 4 && !strncmp(arg, "auto", 4)) { |
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pti_mode = PTI_AUTO; |
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goto autosel; |
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} |
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} |
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if (cmdline_find_option_bool(boot_command_line, "nopti") || |
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cpu_mitigations_off()) { |
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pti_mode = PTI_FORCE_OFF; |
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pti_print_if_insecure("disabled on command line."); |
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return; |
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} |
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autosel: |
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if (!boot_cpu_has_bug(X86_BUG_CPU_MELTDOWN)) |
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return; |
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enable: |
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setup_force_cpu_cap(X86_FEATURE_PTI); |
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} |
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pgd_t __pti_set_user_pgtbl(pgd_t *pgdp, pgd_t pgd) |
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{ |
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/* |
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* Changes to the high (kernel) portion of the kernelmode page |
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* tables are not automatically propagated to the usermode tables. |
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* |
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* Users should keep in mind that, unlike the kernelmode tables, |
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* there is no vmalloc_fault equivalent for the usermode tables. |
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* Top-level entries added to init_mm's usermode pgd after boot |
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* will not be automatically propagated to other mms. |
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*/ |
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if (!pgdp_maps_userspace(pgdp)) |
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return pgd; |
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|
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/* |
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* The user page tables get the full PGD, accessible from |
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* userspace: |
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*/ |
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kernel_to_user_pgdp(pgdp)->pgd = pgd.pgd; |
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/* |
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* If this is normal user memory, make it NX in the kernel |
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* pagetables so that, if we somehow screw up and return to |
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* usermode with the kernel CR3 loaded, we'll get a page fault |
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* instead of allowing user code to execute with the wrong CR3. |
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* |
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* As exceptions, we don't set NX if: |
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* - _PAGE_USER is not set. This could be an executable |
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* EFI runtime mapping or something similar, and the kernel |
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* may execute from it |
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* - we don't have NX support |
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* - we're clearing the PGD (i.e. the new pgd is not present). |
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*/ |
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if ((pgd.pgd & (_PAGE_USER|_PAGE_PRESENT)) == (_PAGE_USER|_PAGE_PRESENT) && |
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(__supported_pte_mask & _PAGE_NX)) |
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pgd.pgd |= _PAGE_NX; |
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/* return the copy of the PGD we want the kernel to use: */ |
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return pgd; |
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} |
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/* |
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* Walk the user copy of the page tables (optionally) trying to allocate |
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* page table pages on the way down. |
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* |
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* Returns a pointer to a P4D on success, or NULL on failure. |
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*/ |
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static p4d_t *pti_user_pagetable_walk_p4d(unsigned long address) |
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{ |
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pgd_t *pgd = kernel_to_user_pgdp(pgd_offset_k(address)); |
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gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO); |
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if (address < PAGE_OFFSET) { |
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WARN_ONCE(1, "attempt to walk user address\n"); |
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return NULL; |
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} |
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if (pgd_none(*pgd)) { |
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unsigned long new_p4d_page = __get_free_page(gfp); |
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if (WARN_ON_ONCE(!new_p4d_page)) |
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return NULL; |
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set_pgd(pgd, __pgd(_KERNPG_TABLE | __pa(new_p4d_page))); |
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} |
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BUILD_BUG_ON(pgd_large(*pgd) != 0); |
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return p4d_offset(pgd, address); |
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} |
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/* |
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* Walk the user copy of the page tables (optionally) trying to allocate |
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* page table pages on the way down. |
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* |
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* Returns a pointer to a PMD on success, or NULL on failure. |
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*/ |
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static pmd_t *pti_user_pagetable_walk_pmd(unsigned long address) |
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{ |
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gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO); |
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p4d_t *p4d; |
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pud_t *pud; |
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p4d = pti_user_pagetable_walk_p4d(address); |
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if (!p4d) |
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return NULL; |
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BUILD_BUG_ON(p4d_large(*p4d) != 0); |
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if (p4d_none(*p4d)) { |
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unsigned long new_pud_page = __get_free_page(gfp); |
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if (WARN_ON_ONCE(!new_pud_page)) |
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return NULL; |
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set_p4d(p4d, __p4d(_KERNPG_TABLE | __pa(new_pud_page))); |
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} |
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pud = pud_offset(p4d, address); |
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/* The user page tables do not use large mappings: */ |
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if (pud_large(*pud)) { |
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WARN_ON(1); |
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return NULL; |
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} |
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if (pud_none(*pud)) { |
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unsigned long new_pmd_page = __get_free_page(gfp); |
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if (WARN_ON_ONCE(!new_pmd_page)) |
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return NULL; |
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set_pud(pud, __pud(_KERNPG_TABLE | __pa(new_pmd_page))); |
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} |
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return pmd_offset(pud, address); |
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} |
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/* |
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* Walk the shadow copy of the page tables (optionally) trying to allocate |
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* page table pages on the way down. Does not support large pages. |
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* |
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* Note: this is only used when mapping *new* kernel data into the |
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* user/shadow page tables. It is never used for userspace data. |
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* |
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* Returns a pointer to a PTE on success, or NULL on failure. |
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*/ |
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static pte_t *pti_user_pagetable_walk_pte(unsigned long address) |
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{ |
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gfp_t gfp = (GFP_KERNEL | __GFP_NOTRACK | __GFP_ZERO); |
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pmd_t *pmd; |
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pte_t *pte; |
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pmd = pti_user_pagetable_walk_pmd(address); |
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if (!pmd) |
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return NULL; |
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/* We can't do anything sensible if we hit a large mapping. */ |
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if (pmd_large(*pmd)) { |
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WARN_ON(1); |
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return NULL; |
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} |
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if (pmd_none(*pmd)) { |
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unsigned long new_pte_page = __get_free_page(gfp); |
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if (!new_pte_page) |
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return NULL; |
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set_pmd(pmd, __pmd(_KERNPG_TABLE | __pa(new_pte_page))); |
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} |
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pte = pte_offset_kernel(pmd, address); |
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if (pte_flags(*pte) & _PAGE_USER) { |
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WARN_ONCE(1, "attempt to walk to user pte\n"); |
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return NULL; |
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} |
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return pte; |
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} |
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#ifdef CONFIG_X86_VSYSCALL_EMULATION |
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static void __init pti_setup_vsyscall(void) |
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{ |
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pte_t *pte, *target_pte; |
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unsigned int level; |
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pte = lookup_address(VSYSCALL_ADDR, &level); |
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if (!pte || WARN_ON(level != PG_LEVEL_4K) || pte_none(*pte)) |
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return; |
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target_pte = pti_user_pagetable_walk_pte(VSYSCALL_ADDR); |
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if (WARN_ON(!target_pte)) |
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return; |
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*target_pte = *pte; |
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set_vsyscall_pgtable_user_bits(kernel_to_user_pgdp(swapper_pg_dir)); |
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} |
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#else |
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static void __init pti_setup_vsyscall(void) { } |
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#endif |
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enum pti_clone_level { |
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PTI_CLONE_PMD, |
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PTI_CLONE_PTE, |
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}; |
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static void |
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pti_clone_pgtable(unsigned long start, unsigned long end, |
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enum pti_clone_level level) |
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{ |
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unsigned long addr; |
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/* |
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* Clone the populated PMDs which cover start to end. These PMD areas |
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* can have holes. |
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*/ |
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for (addr = start; addr < end;) { |
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pte_t *pte, *target_pte; |
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pmd_t *pmd, *target_pmd; |
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pgd_t *pgd; |
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p4d_t *p4d; |
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pud_t *pud; |
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/* Overflow check */ |
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if (addr < start) |
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break; |
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pgd = pgd_offset_k(addr); |
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if (WARN_ON(pgd_none(*pgd))) |
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return; |
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p4d = p4d_offset(pgd, addr); |
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if (WARN_ON(p4d_none(*p4d))) |
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return; |
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pud = pud_offset(p4d, addr); |
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if (pud_none(*pud)) { |
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WARN_ON_ONCE(addr & ~PUD_MASK); |
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addr = round_up(addr + 1, PUD_SIZE); |
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continue; |
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} |
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pmd = pmd_offset(pud, addr); |
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if (pmd_none(*pmd)) { |
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WARN_ON_ONCE(addr & ~PMD_MASK); |
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addr = round_up(addr + 1, PMD_SIZE); |
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continue; |
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} |
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if (pmd_large(*pmd) || level == PTI_CLONE_PMD) { |
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target_pmd = pti_user_pagetable_walk_pmd(addr); |
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if (WARN_ON(!target_pmd)) |
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return; |
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/* |
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* Only clone present PMDs. This ensures only setting |
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* _PAGE_GLOBAL on present PMDs. This should only be |
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* called on well-known addresses anyway, so a non- |
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* present PMD would be a surprise. |
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*/ |
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if (WARN_ON(!(pmd_flags(*pmd) & _PAGE_PRESENT))) |
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return; |
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/* |
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* Setting 'target_pmd' below creates a mapping in both |
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* the user and kernel page tables. It is effectively |
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* global, so set it as global in both copies. Note: |
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* the X86_FEATURE_PGE check is not _required_ because |
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* the CPU ignores _PAGE_GLOBAL when PGE is not |
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* supported. The check keeps consistency with |
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* code that only set this bit when supported. |
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*/ |
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if (boot_cpu_has(X86_FEATURE_PGE)) |
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*pmd = pmd_set_flags(*pmd, _PAGE_GLOBAL); |
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/* |
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* Copy the PMD. That is, the kernelmode and usermode |
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* tables will share the last-level page tables of this |
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* address range |
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*/ |
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*target_pmd = *pmd; |
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addr += PMD_SIZE; |
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} else if (level == PTI_CLONE_PTE) { |
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/* Walk the page-table down to the pte level */ |
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pte = pte_offset_kernel(pmd, addr); |
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if (pte_none(*pte)) { |
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addr += PAGE_SIZE; |
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continue; |
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} |
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/* Only clone present PTEs */ |
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if (WARN_ON(!(pte_flags(*pte) & _PAGE_PRESENT))) |
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return; |
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/* Allocate PTE in the user page-table */ |
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target_pte = pti_user_pagetable_walk_pte(addr); |
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if (WARN_ON(!target_pte)) |
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return; |
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/* Set GLOBAL bit in both PTEs */ |
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if (boot_cpu_has(X86_FEATURE_PGE)) |
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*pte = pte_set_flags(*pte, _PAGE_GLOBAL); |
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/* Clone the PTE */ |
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*target_pte = *pte; |
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addr += PAGE_SIZE; |
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} else { |
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BUG(); |
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} |
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} |
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} |
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#ifdef CONFIG_X86_64 |
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/* |
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* Clone a single p4d (i.e. a top-level entry on 4-level systems and a |
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* next-level entry on 5-level systems. |
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*/ |
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static void __init pti_clone_p4d(unsigned long addr) |
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{ |
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p4d_t *kernel_p4d, *user_p4d; |
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pgd_t *kernel_pgd; |
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user_p4d = pti_user_pagetable_walk_p4d(addr); |
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if (!user_p4d) |
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return; |
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kernel_pgd = pgd_offset_k(addr); |
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kernel_p4d = p4d_offset(kernel_pgd, addr); |
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*user_p4d = *kernel_p4d; |
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} |
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/* |
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* Clone the CPU_ENTRY_AREA and associated data into the user space visible |
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* page table. |
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*/ |
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static void __init pti_clone_user_shared(void) |
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{ |
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unsigned int cpu; |
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pti_clone_p4d(CPU_ENTRY_AREA_BASE); |
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for_each_possible_cpu(cpu) { |
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/* |
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* The SYSCALL64 entry code needs one word of scratch space |
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* in which to spill a register. It lives in the sp2 slot |
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* of the CPU's TSS. |
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* |
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* This is done for all possible CPUs during boot to ensure |
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* that it's propagated to all mms. |
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*/ |
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unsigned long va = (unsigned long)&per_cpu(cpu_tss_rw, cpu); |
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phys_addr_t pa = per_cpu_ptr_to_phys((void *)va); |
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pte_t *target_pte; |
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target_pte = pti_user_pagetable_walk_pte(va); |
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if (WARN_ON(!target_pte)) |
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return; |
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*target_pte = pfn_pte(pa >> PAGE_SHIFT, PAGE_KERNEL); |
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} |
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} |
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#else /* CONFIG_X86_64 */ |
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/* |
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* On 32 bit PAE systems with 1GB of Kernel address space there is only |
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* one pgd/p4d for the whole kernel. Cloning that would map the whole |
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* address space into the user page-tables, making PTI useless. So clone |
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* the page-table on the PMD level to prevent that. |
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*/ |
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static void __init pti_clone_user_shared(void) |
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{ |
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unsigned long start, end; |
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start = CPU_ENTRY_AREA_BASE; |
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end = start + (PAGE_SIZE * CPU_ENTRY_AREA_PAGES); |
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pti_clone_pgtable(start, end, PTI_CLONE_PMD); |
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} |
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#endif /* CONFIG_X86_64 */ |
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/* |
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* Clone the ESPFIX P4D into the user space visible page table |
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*/ |
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static void __init pti_setup_espfix64(void) |
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{ |
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#ifdef CONFIG_X86_ESPFIX64 |
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pti_clone_p4d(ESPFIX_BASE_ADDR); |
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#endif |
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} |
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/* |
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* Clone the populated PMDs of the entry text and force it RO. |
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*/ |
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static void pti_clone_entry_text(void) |
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{ |
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pti_clone_pgtable((unsigned long) __entry_text_start, |
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(unsigned long) __entry_text_end, |
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PTI_CLONE_PMD); |
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} |
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|
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/* |
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* Global pages and PCIDs are both ways to make kernel TLB entries |
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* live longer, reduce TLB misses and improve kernel performance. |
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* But, leaving all kernel text Global makes it potentially accessible |
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* to Meltdown-style attacks which make it trivial to find gadgets or |
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* defeat KASLR. |
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* |
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* Only use global pages when it is really worth it. |
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*/ |
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static inline bool pti_kernel_image_global_ok(void) |
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{ |
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/* |
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* Systems with PCIDs get little benefit from global |
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* kernel text and are not worth the downsides. |
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*/ |
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if (cpu_feature_enabled(X86_FEATURE_PCID)) |
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return false; |
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|
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/* |
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* Only do global kernel image for pti=auto. Do the most |
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* secure thing (not global) if pti=on specified. |
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*/ |
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if (pti_mode != PTI_AUTO) |
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return false; |
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|
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/* |
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* K8 may not tolerate the cleared _PAGE_RW on the userspace |
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* global kernel image pages. Do the safe thing (disable |
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* global kernel image). This is unlikely to ever be |
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* noticed because PTI is disabled by default on AMD CPUs. |
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*/ |
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if (boot_cpu_has(X86_FEATURE_K8)) |
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return false; |
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|
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/* |
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* RANDSTRUCT derives its hardening benefits from the |
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* attacker's lack of knowledge about the layout of kernel |
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* data structures. Keep the kernel image non-global in |
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* cases where RANDSTRUCT is in use to help keep the layout a |
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* secret. |
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*/ |
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if (IS_ENABLED(CONFIG_GCC_PLUGIN_RANDSTRUCT)) |
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return false; |
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|
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return true; |
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} |
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|
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/* |
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* For some configurations, map all of kernel text into the user page |
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* tables. This reduces TLB misses, especially on non-PCID systems. |
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*/ |
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static void pti_clone_kernel_text(void) |
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{ |
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/* |
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* rodata is part of the kernel image and is normally |
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* readable on the filesystem or on the web. But, do not |
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* clone the areas past rodata, they might contain secrets. |
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*/ |
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unsigned long start = PFN_ALIGN(_text); |
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unsigned long end_clone = (unsigned long)__end_rodata_aligned; |
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unsigned long end_global = PFN_ALIGN((unsigned long)_etext); |
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|
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if (!pti_kernel_image_global_ok()) |
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return; |
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|
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pr_debug("mapping partial kernel image into user address space\n"); |
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|
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/* |
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* Note that this will undo _some_ of the work that |
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* pti_set_kernel_image_nonglobal() did to clear the |
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* global bit. |
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*/ |
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pti_clone_pgtable(start, end_clone, PTI_LEVEL_KERNEL_IMAGE); |
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|
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/* |
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* pti_clone_pgtable() will set the global bit in any PMDs |
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* that it clones, but we also need to get any PTEs in |
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* the last level for areas that are not huge-page-aligned. |
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*/ |
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|
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/* Set the global bit for normal non-__init kernel text: */ |
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set_memory_global(start, (end_global - start) >> PAGE_SHIFT); |
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} |
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|
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static void pti_set_kernel_image_nonglobal(void) |
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{ |
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/* |
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* The identity map is created with PMDs, regardless of the |
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* actual length of the kernel. We need to clear |
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* _PAGE_GLOBAL up to a PMD boundary, not just to the end |
|
* of the image. |
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*/ |
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unsigned long start = PFN_ALIGN(_text); |
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unsigned long end = ALIGN((unsigned long)_end, PMD_PAGE_SIZE); |
|
|
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/* |
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* This clears _PAGE_GLOBAL from the entire kernel image. |
|
* pti_clone_kernel_text() map put _PAGE_GLOBAL back for |
|
* areas that are mapped to userspace. |
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*/ |
|
set_memory_nonglobal(start, (end - start) >> PAGE_SHIFT); |
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} |
|
|
|
/* |
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* Initialize kernel page table isolation |
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*/ |
|
void __init pti_init(void) |
|
{ |
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if (!boot_cpu_has(X86_FEATURE_PTI)) |
|
return; |
|
|
|
pr_info("enabled\n"); |
|
|
|
#ifdef CONFIG_X86_32 |
|
/* |
|
* We check for X86_FEATURE_PCID here. But the init-code will |
|
* clear the feature flag on 32 bit because the feature is not |
|
* supported on 32 bit anyway. To print the warning we need to |
|
* check with cpuid directly again. |
|
*/ |
|
if (cpuid_ecx(0x1) & BIT(17)) { |
|
/* Use printk to work around pr_fmt() */ |
|
printk(KERN_WARNING "\n"); |
|
printk(KERN_WARNING "************************************************************\n"); |
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printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! **\n"); |
|
printk(KERN_WARNING "** **\n"); |
|
printk(KERN_WARNING "** You are using 32-bit PTI on a 64-bit PCID-capable CPU. **\n"); |
|
printk(KERN_WARNING "** Your performance will increase dramatically if you **\n"); |
|
printk(KERN_WARNING "** switch to a 64-bit kernel! **\n"); |
|
printk(KERN_WARNING "** **\n"); |
|
printk(KERN_WARNING "** WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! **\n"); |
|
printk(KERN_WARNING "************************************************************\n"); |
|
} |
|
#endif |
|
|
|
pti_clone_user_shared(); |
|
|
|
/* Undo all global bits from the init pagetables in head_64.S: */ |
|
pti_set_kernel_image_nonglobal(); |
|
/* Replace some of the global bits just for shared entry text: */ |
|
pti_clone_entry_text(); |
|
pti_setup_espfix64(); |
|
pti_setup_vsyscall(); |
|
} |
|
|
|
/* |
|
* Finalize the kernel mappings in the userspace page-table. Some of the |
|
* mappings for the kernel image might have changed since pti_init() |
|
* cloned them. This is because parts of the kernel image have been |
|
* mapped RO and/or NX. These changes need to be cloned again to the |
|
* userspace page-table. |
|
*/ |
|
void pti_finalize(void) |
|
{ |
|
if (!boot_cpu_has(X86_FEATURE_PTI)) |
|
return; |
|
/* |
|
* We need to clone everything (again) that maps parts of the |
|
* kernel image. |
|
*/ |
|
pti_clone_entry_text(); |
|
pti_clone_kernel_text(); |
|
|
|
debug_checkwx_user(); |
|
}
|
|
|