From: Jim Mattson <jmattson@google.com>
To: kvm@vger.kernel.org, Sean Christopherson <seanjc@google.com>,
Paolo Bonzini <pbonzini@redhat.com>
Cc: Jim Mattson <jmattson@google.com>
Subject: [PATCH] KVM: x86: AMD's IBPB is not equivalent to Intel's IBPB
Date: Thu, 11 Apr 2024 13:58:51 -0700 [thread overview]
Message-ID: <20240411205911.1684763-1-jmattson@google.com> (raw)
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@google.com>
---
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
next reply other threads:[~2024-04-11 20:59 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-11 20:58 Jim Mattson [this message]
2024-04-12 1:32 ` [PATCH] KVM: x86: AMD's IBPB is not equivalent to Intel's IBPB Venkatesh Srinivas
2024-04-12 2:57 ` Jim Mattson
2024-05-07 20:32 ` Jim Mattson
2024-05-08 15:42 ` Sean Christopherson
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