Messages in this thread Patch in this message | | | Date | Wed, 20 Oct 2021 12:44:53 +0200 | From | Peter Zijlstra <> | Subject | [PATCH v2 11/14] 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.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- arch/x86/kernel/alternative.c | 20 +++++++++++++++++--- 1 file changed, 17 insertions(+), 3 deletions(-)
--- a/arch/x86/kernel/alternative.c +++ b/arch/x86/kernel/alternative.c @@ -389,12 +389,13 @@ static int emit_indirect(int op, int reg * * 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) { + u8 cc, op = insn->opcode.bytes[0]; void (*target)(void); int reg, ret, i = 0; - u8 op, cc; target = addr + insn->length + insn->immediate.value; reg = (target - &__x86_indirect_thunk_rax) / @@ -406,11 +407,23 @@ static int patch_retpoline(void *addr, s /* If anyone ever does: CALL/JMP *%rsp, we're in deep trouble. */ BUG_ON(reg == 4); + if (cpu_feature_enabled(X86_FEATURE_RETPOLINE_AMD)) { + /* + * Can't do nothing about the Jcc case here. + */ + if (op != JMP32_INSN_OPCODE && op != CALL_INSN_OPCODE) + return -1; + + bytes[i++] = 0x0f; + bytes[i++] = 0xae; + bytes[i++] = 0xe8; /* lfence */ + + goto indirect; + } + if (cpu_feature_enabled(X86_FEATURE_RETPOLINE)) return -1; - op = insn->opcode.bytes[0]; - /* * Convert: * @@ -433,6 +446,7 @@ static int patch_retpoline(void *addr, s op = JMP32_INSN_OPCODE; } +indirect: ret = emit_indirect(op, reg, bytes + i); if (ret < 0) return ret;
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