146 lines
4.6 KiB
C
146 lines
4.6 KiB
C
// 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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