Re: [PATCH v2 2/2] KVM: x86: AMD's IBPB is not equivalent to Intel's IBPB

Sean Christopherson
2024-08-22T19:17:20Z

On Fri, Aug 16, 2024, Jim Mattson wrote:
> >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].
>
> However, some AMD processors have an "enhanced IBPB" [terminology
> mine] which does clear the return address predictor. This feature is
> enumerated by CPUID.80000008:EDX.IBPB_RET[bit 30] [4].
>
> Adjust the cross-vendor features enumerated by KVM_GET_SUPPORTED_CPUID
> accordingly.
>
> [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
> [4] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24594.pdf
>
> Fixes: 0c54914d0c52 ("KVM: x86: use Intel speculation bugs and features as derived in generic x86 code")
> Suggested-by: Venkatesh Srinivas <venkateshs@chromium.org>
> Signed-off-by: Jim Mattson <jmattson@google.com>

Venkatesh, can I grab a review from you on this? You know this way better than
I do, and I honestly don't feel like reading mitigation disclosures right now :-)

> ---
> v2: Use IBPB_RET to identify semantic equality (Venkatesh)
>
> arch/x86/kvm/cpuid.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 2617be544480..044bdc9e938b 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -690,7 +690,9 @@ 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))
> + if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) &&
> + boot_cpu_has(X86_FEATURE_AMD_IBPB) &&
> + boot_cpu_has(X86_FEATURE_AMD_IBRS))
> kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL);
> if (boot_cpu_has(X86_FEATURE_STIBP))
> kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
> @@ -759,8 +761,10 @@ void kvm_set_cpu_caps(void)
> * arch/x86/kernel/cpu/bugs.c is kind enough to
> * record that in cpufeatures so use them.
> */
> - if (boot_cpu_has(X86_FEATURE_IBPB))
> + if (boot_cpu_has(X86_FEATURE_IBPB)) {
> kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB);
> + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET);
> + }
> if (boot_cpu_has(X86_FEATURE_IBRS))
> kvm_cpu_cap_set(X86_FEATURE_AMD_IBRS);
> if (boot_cpu_has(X86_FEATURE_STIBP))
> --
> 2.46.0.184.g6999bdac58-goog
>