From: Ben Hutchings <benh@xxxxxxxxxx> The mitigation for PBRSB includes adding LFENCE instructions to the RSB filling sequence. However, RSB filling is done on some older CPUs that don't support the LFENCE instruction. Define and use a BARRIER_NOSPEC macro which makes the LFENCE conditional on X86_FEATURE_LFENCE_RDTSC, like the barrier_nospec() macro defined for C code in <asm/barrier.h>. Reported-by: Martin-Éric Racine <martin-eric.racine@xxxxxx> References: https://bugs.debian.org/1017425 Cc: stable@xxxxxxxxxxxxxxx Cc: regressions@xxxxxxxxxxxxxxx Cc: Daniel Sneddon <daniel.sneddon@xxxxxxxxxxxxxxx> Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx> Fixes: 2b1299322016 ("x86/speculation: Add RSB VM Exit protections") Fixes: ba6e31af2be9 ("x86/speculation: Add LFENCE to RSB fill sequence") Signed-off-by: Ben Hutchings <benh@xxxxxxxxxx> --- Re-sending this with properly matched From address and server. Apologies if you got 2 copies. Ben. arch/x86/include/asm/nospec-branch.h | 11 +++++++---- 1 file changed, 7 insertions(+), 4 deletions(-) diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h index e64fd20778b6..b1029fd88474 100644 --- a/arch/x86/include/asm/nospec-branch.h +++ b/arch/x86/include/asm/nospec-branch.h @@ -34,6 +34,11 @@ #define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */ +#ifdef __ASSEMBLY__ + +/* Prevent speculative execution past this barrier. */ +#define BARRIER_NOSPEC ALTERNATIVE "", "lfence", X86_FEATURE_LFENCE_RDTSC + /* * Google experimented with loop-unrolling and this turned out to be * the optimal version - two calls, each with their own speculation @@ -62,9 +67,7 @@ dec reg; \ jnz 771b; \ /* barrier for jnz misprediction */ \ - lfence; - -#ifdef __ASSEMBLY__ + BARRIER_NOSPEC; /* * This should be used immediately before an indirect jump/call. It tells @@ -138,7 +141,7 @@ int3 .Lunbalanced_ret_guard_\@: add $(BITS_PER_LONG/8), %_ASM_SP - lfence + BARRIER_NOSPEC .endm /*
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