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x86/alternative: Try inline spectre_v2=retpoline,amd

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Try and replace retpoline thunk calls with:

  LFENCE
  CALL    *%\reg

for spectre_v2=retpoline,amd.

Specifically, the sequence above is 5 bytes for the low 8 registers,
but 6 bytes for the high 8 registers. This means that unless the
compilers prefix stuff the call with higher registers this replacement
will fail.

Luckily GCC strongly favours RAX for the indirect calls and most (95%+
for defconfig-x86_64) will be converted. OTOH clang strongly favours
R11 and almost nothing gets converted.

Note: it will also generate a correct replacement for the Jcc.d32
case, except unless the compilers start to prefix stuff that, it'll
never fit. Specifically:

  Jncc.d8 1f
  LFENCE
  JMP     *%\reg
1:

is 7-8 bytes long, where the original instruction in unpadded form is
only 6 bytes.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Borislav Petkov <bp@suse.de>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Tested-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/r/20211026120310.359986601@infradead.org
This commit is contained in:
Peter Zijlstra 2021-10-26 14:01:44 +02:00
parent 2f0cbb2a8e
commit bbe2df3f6b
1 changed files with 14 additions and 2 deletions
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16
arch/x86/kernel/alternative.c
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@ -389,6 +389,7 @@ static int emit_indirect(int op, int reg, u8 *bytes)
*
* CALL *%\reg
*
* It also tries to inline spectre_v2=retpoline,amd when size permits.
*/
static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
{
@ -405,7 +406,8 @@ static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
/* If anyone ever does: CALL/JMP *%rsp, we're in deep trouble. */
BUG_ON(reg == 4);
if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
if (cpu_feature_enabled(X86_FEATURE_RETPOLINE) &&
!cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD))
return -1;
op = insn->opcode.bytes[0];
@ -418,8 +420,9 @@ static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
* into:
*
* Jncc.d8 1f
* [ LFENCE ]
* JMP *%\reg
* NOP
* [ NOP ]
* 1:
*/
/* Jcc.d32 second opcode byte is in the range: 0x80-0x8f */
@ -434,6 +437,15 @@ static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
op = JMP32_INSN_OPCODE;
}
/*
* For RETPOLINE_AMD: prepend the indirect CALL/JMP with an LFENCE.
*/
if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) {
bytes[i++] = 0x0f;
bytes[i++] = 0xae;
bytes[i++] = 0xe8; /* LFENCE */
}
ret = emit_indirect(op, reg, bytes + i);
if (ret < 0)
return ret;