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

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Message-ID: <e5fe6cad-9304-479c-944a-8d8456fcdefa@chromium.org>
Date: Fri, 23 Aug 2024 16:47:17 +0000
From: Venkatesh Srinivas <venkateshs@...omium.org>
To: Sean Christopherson <seanjc@...gle.com>, Jim Mattson <jmattson@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 Paolo Bonzini <pbonzini@...hat.com>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
 Kai Huang <kai.huang@...el.com>, Sandipan Das <sandipan.das@....com>,
 linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 2/2] KVM: x86: AMD's IBPB is not equivalent to Intel's
 IBPB



On 8/22/24 19:17, Sean Christopherson wrote:
> 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@...omium.org>
>> Signed-off-by: Jim Mattson <jmattson@...gle.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 :-)

Got lost in my mailbox!

Reviewed-by: Venkatesh Srinivas <venkateshs@...omium.org>

> 
>> ---
>>   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
>>