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

Jim Mattson
2024-08-16T18:26:06Z

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);
+ }
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