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145 lines
4.6 KiB
145 lines
4.6 KiB
// SPDX-License-Identifier: GPL-2.0-or-later |
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#include <string.h> |
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#include <objtool/special.h> |
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#include <objtool/builtin.h> |
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#define X86_FEATURE_POPCNT (4 * 32 + 23) |
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#define X86_FEATURE_SMAP (9 * 32 + 20) |
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void arch_handle_alternative(unsigned short feature, struct special_alt *alt) |
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{ |
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switch (feature) { |
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case X86_FEATURE_SMAP: |
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/* |
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* If UACCESS validation is enabled; force that alternative; |
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* otherwise force it the other way. |
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* |
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* What we want to avoid is having both the original and the |
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* alternative code flow at the same time, in that case we can |
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* find paths that see the STAC but take the NOP instead of |
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* CLAC and the other way around. |
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*/ |
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if (uaccess) |
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alt->skip_orig = true; |
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else |
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alt->skip_alt = true; |
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break; |
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case X86_FEATURE_POPCNT: |
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/* |
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* It has been requested that we don't validate the !POPCNT |
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* feature path which is a "very very small percentage of |
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* machines". |
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*/ |
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alt->skip_orig = true; |
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break; |
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default: |
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break; |
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} |
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} |
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bool arch_support_alt_relocation(struct special_alt *special_alt, |
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struct instruction *insn, |
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struct reloc *reloc) |
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{ |
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/* |
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* The x86 alternatives code adjusts the offsets only when it |
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* encounters a branch instruction at the very beginning of the |
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* replacement group. |
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*/ |
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return insn->offset == special_alt->new_off && |
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(insn->type == INSN_CALL || is_jump(insn)); |
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} |
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/* |
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* There are 3 basic jump table patterns: |
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* |
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* 1. jmpq *[rodata addr](,%reg,8) |
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* |
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* This is the most common case by far. It jumps to an address in a simple |
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* jump table which is stored in .rodata. |
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* |
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* 2. jmpq *[rodata addr](%rip) |
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* |
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* This is caused by a rare GCC quirk, currently only seen in three driver |
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* functions in the kernel, only with certain obscure non-distro configs. |
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* |
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* As part of an optimization, GCC makes a copy of an existing switch jump |
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* table, modifies it, and then hard-codes the jump (albeit with an indirect |
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* jump) to use a single entry in the table. The rest of the jump table and |
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* some of its jump targets remain as dead code. |
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* |
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* In such a case we can just crudely ignore all unreachable instruction |
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* warnings for the entire object file. Ideally we would just ignore them |
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* for the function, but that would require redesigning the code quite a |
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* bit. And honestly that's just not worth doing: unreachable instruction |
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* warnings are of questionable value anyway, and this is such a rare issue. |
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* |
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* 3. mov [rodata addr],%reg1 |
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* ... some instructions ... |
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* jmpq *(%reg1,%reg2,8) |
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* |
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* This is a fairly uncommon pattern which is new for GCC 6. As of this |
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* writing, there are 11 occurrences of it in the allmodconfig kernel. |
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* |
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* As of GCC 7 there are quite a few more of these and the 'in between' code |
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* is significant. Esp. with KASAN enabled some of the code between the mov |
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* and jmpq uses .rodata itself, which can confuse things. |
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* |
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* TODO: Once we have DWARF CFI and smarter instruction decoding logic, |
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* ensure the same register is used in the mov and jump instructions. |
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* |
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* NOTE: RETPOLINE made it harder still to decode dynamic jumps. |
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*/ |
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struct reloc *arch_find_switch_table(struct objtool_file *file, |
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struct instruction *insn) |
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{ |
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struct reloc *text_reloc, *rodata_reloc; |
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struct section *table_sec; |
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unsigned long table_offset; |
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/* look for a relocation which references .rodata */ |
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text_reloc = find_reloc_by_dest_range(file->elf, insn->sec, |
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insn->offset, insn->len); |
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if (!text_reloc || text_reloc->sym->type != STT_SECTION || |
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!text_reloc->sym->sec->rodata) |
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return NULL; |
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table_offset = text_reloc->addend; |
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table_sec = text_reloc->sym->sec; |
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if (text_reloc->type == R_X86_64_PC32) |
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table_offset += 4; |
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/* |
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* Make sure the .rodata address isn't associated with a |
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* symbol. GCC jump tables are anonymous data. |
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* |
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* Also support C jump tables which are in the same format as |
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* switch jump tables. For objtool to recognize them, they |
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* need to be placed in the C_JUMP_TABLE_SECTION section. They |
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* have symbols associated with them. |
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*/ |
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if (find_symbol_containing(table_sec, table_offset) && |
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strcmp(table_sec->name, C_JUMP_TABLE_SECTION)) |
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return NULL; |
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/* |
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* Each table entry has a rela associated with it. The rela |
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* should reference text in the same function as the original |
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* instruction. |
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*/ |
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rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset); |
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if (!rodata_reloc) |
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return NULL; |
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/* |
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* Use of RIP-relative switch jumps is quite rare, and |
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* indicates a rare GCC quirk/bug which can leave dead |
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* code behind. |
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*/ |
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if (text_reloc->type == R_X86_64_PC32) |
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file->ignore_unreachables = true; |
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return rodata_reloc; |
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}
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