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

Jim Mattson
2024-08-23T17:33:41Z

On Fri, Aug 23, 2024 at 9:06 AM Tom Lendacky <thomas.lendacky@amd.com> wrote:
>
> On 8/16/24 13:25, 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>
> > ---
> > 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);
>
> Should IBPB_RET be conditionally set? I would think that you would only
> want to set IBPB_RET if either IBPB_RET or SPEC_CTRL is set on the hypervisor.
>
> if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) ||
> boot_cpu_has(X86_FEATURE_SPEC_CTRL)
> kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET);
>
> Right?

Right. This clause is intended to set cross-vendor capabilities, so it
should be:

if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET);

Passing through AMD_IBPB_RET from the hardware should be done by
adding the bit to the mask for CPUID_8000_0008_EBX.

I'll send out a v3.

Thanks!

> Thanks,
> Tom
>
> > + }
> > if (boot_cpu_has(X86_FEATURE_IBRS))
> > kvm_cpu_cap_set(X86_FEATURE_AMD_IBRS);
> > if (boot_cpu_has(X86_FEATURE_STIBP))