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104 lines
2.5 KiB
104 lines
2.5 KiB
// SPDX-License-Identifier: GPL-2.0-only |
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/* |
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* sysret_ss_attrs.c - test that syscalls return valid hidden SS attributes |
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* Copyright (c) 2015 Andrew Lutomirski |
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* |
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* On AMD CPUs, SYSRET can return with a valid SS descriptor with with |
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* the hidden attributes set to an unusable state. Make sure the kernel |
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* doesn't let this happen. |
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*/ |
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#define _GNU_SOURCE |
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#include <stdlib.h> |
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#include <unistd.h> |
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#include <stdio.h> |
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#include <string.h> |
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#include <sys/mman.h> |
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#include <err.h> |
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#include <stddef.h> |
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#include <stdbool.h> |
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#include <pthread.h> |
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static void *threadproc(void *ctx) |
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{ |
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/* |
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* Do our best to cause sleeps on this CPU to exit the kernel and |
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* re-enter with SS = 0. |
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*/ |
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while (true) |
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; |
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return NULL; |
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} |
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#ifdef __x86_64__ |
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extern unsigned long call32_from_64(void *stack, void (*function)(void)); |
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asm (".pushsection .text\n\t" |
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".code32\n\t" |
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"test_ss:\n\t" |
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"pushl $0\n\t" |
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"popl %eax\n\t" |
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"ret\n\t" |
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".code64"); |
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extern void test_ss(void); |
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#endif |
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int main() |
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{ |
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/* |
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* Start a busy-looping thread on the same CPU we're on. |
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* For simplicity, just stick everything to CPU 0. This will |
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* fail in some containers, but that's probably okay. |
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*/ |
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cpu_set_t cpuset; |
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CPU_ZERO(&cpuset); |
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CPU_SET(0, &cpuset); |
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if (sched_setaffinity(0, sizeof(cpuset), &cpuset) != 0) |
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printf("[WARN]\tsched_setaffinity failed\n"); |
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pthread_t thread; |
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if (pthread_create(&thread, 0, threadproc, 0) != 0) |
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err(1, "pthread_create"); |
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#ifdef __x86_64__ |
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unsigned char *stack32 = mmap(NULL, 4096, PROT_READ | PROT_WRITE, |
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MAP_32BIT | MAP_ANONYMOUS | MAP_PRIVATE, |
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-1, 0); |
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if (stack32 == MAP_FAILED) |
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err(1, "mmap"); |
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#endif |
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printf("[RUN]\tSyscalls followed by SS validation\n"); |
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for (int i = 0; i < 1000; i++) { |
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/* |
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* Go to sleep and return using sysret (if we're 64-bit |
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* or we're 32-bit on AMD on a 64-bit kernel). On AMD CPUs, |
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* SYSRET doesn't fix up the cached SS descriptor, so the |
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* kernel needs some kind of workaround to make sure that we |
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* end the system call with a valid stack segment. This |
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* can be a confusing failure because the SS *selector* |
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* is the same regardless. |
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*/ |
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usleep(2); |
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#ifdef __x86_64__ |
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/* |
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* On 32-bit, just doing a syscall through glibc is enough |
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* to cause a crash if our cached SS descriptor is invalid. |
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* On 64-bit, it's not, so try extra hard. |
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*/ |
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call32_from_64(stack32 + 4088, test_ss); |
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#endif |
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} |
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printf("[OK]\tWe survived\n"); |
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#ifdef __x86_64__ |
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munmap(stack32, 4096); |
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#endif |
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return 0; |
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}
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