| Message ID | 20240823185323.2563194-5-jmattson@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | Distinguish between variants of IBPB | expand |
On 8/23/24 13:53, 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> > --- > arch/x86/kvm/cpuid.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > index ec7b2ca3b4d3..c8d7d928ffc7 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); > @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) > + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET without AMD_IBPB, but it just looks odd seeing them set with separate checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. That's just me, though, not worth a v4 unless others feel the same. Thanks, Tom > if (boot_cpu_has(X86_FEATURE_IBPB)) > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > if (boot_cpu_has(X86_FEATURE_IBRS))
On Fri, Aug 23, 2024, Tom Lendacky wrote: > On 8/23/24 13:53, 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> > > --- > > arch/x86/kvm/cpuid.c | 6 +++++- > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > index ec7b2ca3b4d3..c8d7d928ffc7 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); > > @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) > > + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > > If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET > without AMD_IBPB, but it just looks odd seeing them set with separate > checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. > That's just me, though, not worth a v4 unless others feel the same. You thinking something like this (at the end, after the dust settles)? if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); > > Thanks, > Tom > > > if (boot_cpu_has(X86_FEATURE_IBPB)) > > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > > if (boot_cpu_has(X86_FEATURE_IBRS))
On Fri, Aug 23, 2024 at 1:51 PM Sean Christopherson <seanjc@google.com> wrote: > > On Fri, Aug 23, 2024, Tom Lendacky wrote: > > On 8/23/24 13:53, 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> > > > --- > > > arch/x86/kvm/cpuid.c | 6 +++++- > > > 1 file changed, 5 insertions(+), 1 deletion(-) > > > > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > > index ec7b2ca3b4d3..c8d7d928ffc7 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); > > > @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) > > > + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > > > > If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET > > without AMD_IBPB, but it just looks odd seeing them set with separate > > checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. > > That's just me, though, not worth a v4 unless others feel the same. > > You thinking something like this (at the end, after the dust settles)? > > if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && > !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) > kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); > > Ugh. No. I think it would be better to replace the subsequent vendor-neutral tests with something like: if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) { kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); kvm_cpu_cap_set(X86_FEATURE_AMD_IBRS); } Again, my real preference is to leave the cross-vendor enumeration to userspace, but I guess that ship has sailed. > > Thanks, > > Tom > > > > > if (boot_cpu_has(X86_FEATURE_IBPB)) > > > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > > > if (boot_cpu_has(X86_FEATURE_IBRS))
On 8/23/24 15:51, Sean Christopherson wrote: > On Fri, Aug 23, 2024, Tom Lendacky wrote: >> On 8/23/24 13:53, 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> >>> --- >>> arch/x86/kvm/cpuid.c | 6 +++++- >>> 1 file changed, 5 insertions(+), 1 deletion(-) >>> >>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c >>> index ec7b2ca3b4d3..c8d7d928ffc7 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); >>> @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) >>> + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); >> >> If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET >> without AMD_IBPB, but it just looks odd seeing them set with separate >> checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. >> That's just me, though, not worth a v4 unless others feel the same. > > You thinking something like this (at the end, after the dust settles)? > > if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && > !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) > kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); I was just thinking more along the lines of: if (boot_cpu_has(X86_FEATURE_IBPB)) { kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); } Thanks, Tom >> > >> Thanks, >> Tom >> >>> if (boot_cpu_has(X86_FEATURE_IBPB)) >>> kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); >>> if (boot_cpu_has(X86_FEATURE_IBRS))
On Fri, Aug 23, 2024 at 3:12 PM Tom Lendacky <thomas.lendacky@amd.com> wrote: > > On 8/23/24 15:51, Sean Christopherson wrote: > > On Fri, Aug 23, 2024, Tom Lendacky wrote: > >> On 8/23/24 13:53, 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> > >>> --- > >>> arch/x86/kvm/cpuid.c | 6 +++++- > >>> 1 file changed, 5 insertions(+), 1 deletion(-) > >>> > >>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > >>> index ec7b2ca3b4d3..c8d7d928ffc7 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); > >>> @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) > >>> + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > >> > >> If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET > >> without AMD_IBPB, but it just looks odd seeing them set with separate > >> checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. > >> That's just me, though, not worth a v4 unless others feel the same. > > > > You thinking something like this (at the end, after the dust settles)? > > > > if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && > > !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) > > kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); > > I was just thinking more along the lines of: > > if (boot_cpu_has(X86_FEATURE_IBPB)) { > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > } AFAICT, there are just two reasons that X86_FEATURE_IBPB gets set: 1. The CPU reports CPUID.(EAX=7,ECX=0):EDX[bit 26] (aka X86_FEATURE_SPEC_CTRL) 2. The CPU reports CPUID Fn8000_0008_EBX[IBPB] (aka X86_FEATURE_AMD_IBPB) Clearly, in the second case, the KVM cpu capability for AMD_IBPB will already be set, since it's specified in the mask for CPUID_8000_0008_EBX. If this block of code is just trying to populate CPUID Fn8000_0008_EBX on Intel processors, I'd rather change all of the predicates to test for Intel features, rather than vendor-neutral features, so that the derivation is clear. But maybe this block of code is also trying to populate CPUID Fn8000_0008_EBX on AMD processors that may have some of these features, but don't enumerate them via CPUID? > Thanks, > Tom > > >> > > > >> Thanks, > >> Tom > >> > >>> if (boot_cpu_has(X86_FEATURE_IBPB)) > >>> kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > >>> if (boot_cpu_has(X86_FEATURE_IBRS))
On Fri, Aug 23, 2024 at 3:48 PM Jim Mattson <jmattson@google.com> wrote: > > On Fri, Aug 23, 2024 at 3:12 PM Tom Lendacky <thomas.lendacky@amd.com> wrote: > > > > On 8/23/24 15:51, Sean Christopherson wrote: > > > On Fri, Aug 23, 2024, Tom Lendacky wrote: > > >> On 8/23/24 13:53, 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> > > >>> --- > > >>> arch/x86/kvm/cpuid.c | 6 +++++- > > >>> 1 file changed, 5 insertions(+), 1 deletion(-) > > >>> > > >>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > >>> index ec7b2ca3b4d3..c8d7d928ffc7 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); > > >>> @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) > > >>> + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > > >> > > >> If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET > > >> without AMD_IBPB, but it just looks odd seeing them set with separate > > >> checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. > > >> That's just me, though, not worth a v4 unless others feel the same. > > > > > > You thinking something like this (at the end, after the dust settles)? > > > > > > if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && > > > !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) > > > kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); > > > > I was just thinking more along the lines of: > > > > if (boot_cpu_has(X86_FEATURE_IBPB)) { > > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > > if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) > > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > > } > > AFAICT, there are just two reasons that X86_FEATURE_IBPB gets set: > 1. The CPU reports CPUID.(EAX=7,ECX=0):EDX[bit 26] (aka X86_FEATURE_SPEC_CTRL) > 2. The CPU reports CPUID Fn8000_0008_EBX[IBPB] (aka X86_FEATURE_AMD_IBPB) > > Clearly, in the second case, the KVM cpu capability for AMD_IBPB will > already be set, since it's specified in the mask for > CPUID_8000_0008_EBX. > > If this block of code is just trying to populate CPUID Fn8000_0008_EBX > on Intel processors, I'd rather change all of the predicates to test > for Intel features, rather than vendor-neutral features, so that the > derivation is clear. But maybe this block of code is also trying to > populate CPUID Fn8000_0008_EBX on AMD processors that may have some of > these features, but don't enumerate them via CPUID? There's another argument for just nuking these cross-vendor derivations. How do we factor in CVE-2022-26373 (Post-barrier Return Stack Buffer Predictions)? Intel CPUs without IA32_ARCH_CAPABILITIES.PBRSB_NO[bit 24] have a weaker IBPB than AMD CPUs with CPUID Fn8000_0008_EBX[IBPB_RET], and probably should not be enumerating that CPUID bit. Trying to derive cross-vendor mitigation equivalence is just going to end in tears. > > Thanks, > > Tom > > > > >> > > > > > >> Thanks, > > >> Tom > > >> > > >>> if (boot_cpu_has(X86_FEATURE_IBPB)) > > >>> kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > > >>> if (boot_cpu_has(X86_FEATURE_IBRS))
On Fri, Aug 23, 2024, Jim Mattson wrote: > On Fri, Aug 23, 2024 at 3:48 PM Jim Mattson <jmattson@google.com> wrote: > > > > On Fri, Aug 23, 2024 at 3:12 PM Tom Lendacky <thomas.lendacky@amd.com> wrote: > > > > > > On 8/23/24 15:51, Sean Christopherson wrote: > > > > On Fri, Aug 23, 2024, Tom Lendacky wrote: > > > >> On 8/23/24 13:53, 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> > > > >>> --- > > > >>> arch/x86/kvm/cpuid.c | 6 +++++- > > > >>> 1 file changed, 5 insertions(+), 1 deletion(-) > > > >>> > > > >>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c > > > >>> index ec7b2ca3b4d3..c8d7d928ffc7 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); > > > >>> @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) > > > >>> + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > > > >> > > > >> If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET > > > >> without AMD_IBPB, but it just looks odd seeing them set with separate > > > >> checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB. > > > >> That's just me, though, not worth a v4 unless others feel the same. > > > > > > > > You thinking something like this (at the end, after the dust settles)? > > > > > > > > if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) && > > > > !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB))) > > > > kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET); > > > > > > I was just thinking more along the lines of: > > > > > > if (boot_cpu_has(X86_FEATURE_IBPB)) { > > > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); > > > if (boot_cpu_has(X86_FEATURE_SPEC_CTRL)) > > > kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); > > > } > > > > AFAICT, there are just two reasons that X86_FEATURE_IBPB gets set: > > 1. The CPU reports CPUID.(EAX=7,ECX=0):EDX[bit 26] (aka X86_FEATURE_SPEC_CTRL) > > 2. The CPU reports CPUID Fn8000_0008_EBX[IBPB] (aka X86_FEATURE_AMD_IBPB) > > > > Clearly, in the second case, the KVM cpu capability for AMD_IBPB will > > already be set, since it's specified in the mask for > > CPUID_8000_0008_EBX. > > > > If this block of code is just trying to populate CPUID Fn8000_0008_EBX > > on Intel processors, I'd rather change all of the predicates to test > > for Intel features, rather than vendor-neutral features, so that the > > derivation is clear. But maybe this block of code is also trying to > > populate CPUID Fn8000_0008_EBX on AMD processors that may have some of > > these features, but don't enumerate them via CPUID? > > There's another argument for just nuking these cross-vendor > derivations. How do we factor in CVE-2022-26373 (Post-barrier Return > Stack Buffer Predictions)? > Intel CPUs without IA32_ARCH_CAPABILITIES.PBRSB_NO[bit 24] have a > weaker IBPB than AMD CPUs with CPUID Fn8000_0008_EBX[IBPB_RET], and > probably should not be enumerating that CPUID bit. > > Trying to derive cross-vendor mitigation equivalence is just going to > end in tears. Agreed, but I also don't want to break existing setups. Is there a bare minimum of sorts that we can advertise to userspace? E.g. something that might be imperfect, but has acceptable tradeoffs/risks for the existing code? And then put a stake in the ground saying no more of these shenanigans.
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c index ec7b2ca3b4d3..c8d7d928ffc7 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); @@ -759,6 +761,8 @@ 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_SPEC_CTRL)) + kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET); if (boot_cpu_has(X86_FEATURE_IBPB)) kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB); if (boot_cpu_has(X86_FEATURE_IBRS))
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> --- arch/x86/kvm/cpuid.c | 6 +++++- 1 file changed, 5 insertions(+), 1 deletion(-)