cachepc-linux

Fork of AMDESE/linux with modifications for CachePC side-channel attack
git clone https://git.sinitax.com/sinitax/cachepc-linux
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special.c (4737B)

      1// SPDX-License-Identifier: GPL-2.0-or-later
      2#include <string.h>
      3
      4#include <objtool/special.h>
      5#include <objtool/builtin.h>
      6
      7#define X86_FEATURE_POPCNT (4 * 32 + 23)
      8#define X86_FEATURE_SMAP   (9 * 32 + 20)
      9
     10void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
     11{
     12	switch (feature) {
     13	case X86_FEATURE_SMAP:
     14		/*
     15		 * If UACCESS validation is enabled; force that alternative;
     16		 * otherwise force it the other way.
     17		 *
     18		 * What we want to avoid is having both the original and the
     19		 * alternative code flow at the same time, in that case we can
     20		 * find paths that see the STAC but take the NOP instead of
     21		 * CLAC and the other way around.
     22		 */
     23		if (opts.uaccess)
     24			alt->skip_orig = true;
     25		else
     26			alt->skip_alt = true;
     27		break;
     28	case X86_FEATURE_POPCNT:
     29		/*
     30		 * It has been requested that we don't validate the !POPCNT
     31		 * feature path which is a "very very small percentage of
     32		 * machines".
     33		 */
     34		alt->skip_orig = true;
     35		break;
     36	default:
     37		break;
     38	}
     39}
     40
     41bool arch_support_alt_relocation(struct special_alt *special_alt,
     42				 struct instruction *insn,
     43				 struct reloc *reloc)
     44{
     45	/*
     46	 * The x86 alternatives code adjusts the offsets only when it
     47	 * encounters a branch instruction at the very beginning of the
     48	 * replacement group.
     49	 */
     50	return insn->offset == special_alt->new_off &&
     51	       (insn->type == INSN_CALL || is_jump(insn));
     52}
     53
     54/*
     55 * There are 3 basic jump table patterns:
     56 *
     57 * 1. jmpq *[rodata addr](,%reg,8)
     58 *
     59 *    This is the most common case by far.  It jumps to an address in a simple
     60 *    jump table which is stored in .rodata.
     61 *
     62 * 2. jmpq *[rodata addr](%rip)
     63 *
     64 *    This is caused by a rare GCC quirk, currently only seen in three driver
     65 *    functions in the kernel, only with certain obscure non-distro configs.
     66 *
     67 *    As part of an optimization, GCC makes a copy of an existing switch jump
     68 *    table, modifies it, and then hard-codes the jump (albeit with an indirect
     69 *    jump) to use a single entry in the table.  The rest of the jump table and
     70 *    some of its jump targets remain as dead code.
     71 *
     72 *    In such a case we can just crudely ignore all unreachable instruction
     73 *    warnings for the entire object file.  Ideally we would just ignore them
     74 *    for the function, but that would require redesigning the code quite a
     75 *    bit.  And honestly that's just not worth doing: unreachable instruction
     76 *    warnings are of questionable value anyway, and this is such a rare issue.
     77 *
     78 * 3. mov [rodata addr],%reg1
     79 *    ... some instructions ...
     80 *    jmpq *(%reg1,%reg2,8)
     81 *
     82 *    This is a fairly uncommon pattern which is new for GCC 6.  As of this
     83 *    writing, there are 11 occurrences of it in the allmodconfig kernel.
     84 *
     85 *    As of GCC 7 there are quite a few more of these and the 'in between' code
     86 *    is significant. Esp. with KASAN enabled some of the code between the mov
     87 *    and jmpq uses .rodata itself, which can confuse things.
     88 *
     89 *    TODO: Once we have DWARF CFI and smarter instruction decoding logic,
     90 *    ensure the same register is used in the mov and jump instructions.
     91 *
     92 *    NOTE: RETPOLINE made it harder still to decode dynamic jumps.
     93 */
     94struct reloc *arch_find_switch_table(struct objtool_file *file,
     95				    struct instruction *insn)
     96{
     97	struct reloc  *text_reloc, *rodata_reloc;
     98	struct section *table_sec;
     99	unsigned long table_offset;
    100
    101	/* look for a relocation which references .rodata */
    102	text_reloc = find_reloc_by_dest_range(file->elf, insn->sec,
    103					      insn->offset, insn->len);
    104	if (!text_reloc || text_reloc->sym->type != STT_SECTION ||
    105	    !text_reloc->sym->sec->rodata)
    106		return NULL;
    107
    108	table_offset = text_reloc->addend;
    109	table_sec = text_reloc->sym->sec;
    110
    111	if (text_reloc->type == R_X86_64_PC32)
    112		table_offset += 4;
    113
    114	/*
    115	 * Make sure the .rodata address isn't associated with a
    116	 * symbol.  GCC jump tables are anonymous data.
    117	 *
    118	 * Also support C jump tables which are in the same format as
    119	 * switch jump tables.  For objtool to recognize them, they
    120	 * need to be placed in the C_JUMP_TABLE_SECTION section.  They
    121	 * have symbols associated with them.
    122	 */
    123	if (find_symbol_containing(table_sec, table_offset) &&
    124	    strcmp(table_sec->name, C_JUMP_TABLE_SECTION))
    125		return NULL;
    126
    127	/*
    128	 * Each table entry has a rela associated with it.  The rela
    129	 * should reference text in the same function as the original
    130	 * instruction.
    131	 */
    132	rodata_reloc = find_reloc_by_dest(file->elf, table_sec, table_offset);
    133	if (!rodata_reloc)
    134		return NULL;
    135
    136	/*
    137	 * Use of RIP-relative switch jumps is quite rare, and
    138	 * indicates a rare GCC quirk/bug which can leave dead
    139	 * code behind.
    140	 */
    141	if (text_reloc->type == R_X86_64_PC32)
    142		file->ignore_unreachables = true;
    143
    144	return rodata_reloc;
    145}