>From Intel's documention [1], "CPUID.(EAX=07H,ECX=0):EDX[26] enumerates support for indirect branch restricted speculation (IBRS) and the indirect branch predictor barrier (IBPB)." Further, from [2], "Software that executed before the IBPB command cannot control the predicted targets of indirect branches (4) executed after the command on the same logical processor," where footnote 4 reads, "Note that indirect branches include near call indirect, near jump indirect and near return instructions. Because it includes near returns, it follows that **RSB entries created before an IBPB command cannot control the predicted targets of returns executed after the command on the same logical processor.**" [emphasis mine] On the other hand, AMD's "IBPB may not prevent return branch predictions from being specified by pre-IBPB branch targets" [3]. Since Linux sets the synthetic feature bit, X86_FEATURE_IBPB, on AMD CPUs that implement the weaker version of IBPB, it is incorrect to infer from this and X86_FEATURE_IBRS that the CPU supports the stronger version of IBPB indicated by CPUID.(EAX=07H,ECX=0):EDX[26]. Stop making this inference. [1] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/cpuid-enumeration-and-architectural-msrs.html [2] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/speculative-execution-side-channel-mitigations.html#Footnotes [3] https://www.amd.com/en/resources/product-security/bulletin/amd-sb-1040.html Fixes: 0c54914d0c52 ("KVM: x86: use Intel speculation bugs and features as derived in generic x86 code") Signed-off-by: Jim Mattson <jmattson@xxxxxxxxxx> --- arch/x86/kvm/cpuid.c | 2 -- 1 file changed, 2 deletions(-) diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index bfc0bfcb2bc6..66f2761b2836 100644 --- a/arch/x86/kvm/cpuid.c +++ b/arch/x86/kvm/cpuid.c @@ -677,8 +677,6 @@ void kvm_set_cpu_caps(void) kvm_cpu_cap_set(X86_FEATURE_TSC_ADJUST); kvm_cpu_cap_set(X86_FEATURE_ARCH_CAPABILITIES); - if (boot_cpu_has(X86_FEATURE_IBPB) && boot_cpu_has(X86_FEATURE_IBRS)) - kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL); if (boot_cpu_has(X86_FEATURE_STIBP)) kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP); if (boot_cpu_has(X86_FEATURE_AMD_SSBD)) -- 2.44.0.683.g7961c838ac-goog
