diff options
| author | Peter Zijlstra <peterz@infradead.org> | 2021-10-26 14:01:44 +0200 |
|---|---|---|
| committer | Peter Zijlstra <peterz@infradead.org> | 2021-10-28 23:25:28 +0200 |
| commit | bbe2df3f6b6da7848398d55b1311d58a16ec21e4 (patch) | |
| tree | 2abeeafd8f025733e94cf8e6a0e96bfd2ea5d004 | |
| parent | 2f0cbb2a8e5bbf101e9de118fc0eb168111a5e1e (diff) | |
x86/alternative: Try inline spectre_v2=retpoline,amd
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
| -rw-r--r-- | arch/x86/kernel/alternative.c | 16 |
1 files changed, 14 insertions, 2 deletions
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index 1dea2f6c5546..c89824c69ff7 100644 --- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -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; |