linux-kernel - Re: [PATCH v3 0/6] KVM: SVM: Fix DEBUGCTL bugs

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Message-ID: <653c3c6e-bdfc-4604-bda0-3b67970a0c62@amd.com>
Date: Fri, 28 Feb 2025 15:01:49 +0530
From: Ravi Bangoria <ravi.bangoria@....com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Xiaoyao Li <xiaoyao.li@...el.com>, rangemachine@...il.com,
 whanos@...gal.fun, Ravi Bangoria <ravi.bangoria@....com>
Subject: Re: [PATCH v3 0/6] KVM: SVM: Fix DEBUGCTL bugs

On 28-Feb-25 3:54 AM, Sean Christopherson wrote:
> Fix a long-lurking bug in SVM where KVM runs the guest with the host's
> DEBUGCTL if LBR virtualization is disabled.  AMD CPUs rather stupidly
> context switch DEBUGCTL if and only if LBR virtualization is enabled (not
> just supported, but fully enabled).
> 
> The bug has gone unnoticed because until recently, the only bits that
> KVM would leave set were things like BTF, which are guest visible but
> won't cause functional problems unless guest software is being especially
> particular about #DBs.
> 
> The bug was exposed by the addition of BusLockTrap ("Detect" in the kernel),
> as the resulting #DBs due to split-lock accesses in guest userspace (lol
> Steam) get reflected into the guest by KVM.
> 
> Note, I don't love suppressing DEBUGCTL.BTF, but practically speaking that's
> likely the behavior that SVM guests have gotten the vast, vast majority of
> the time, and given that it's the behavior on Intel, it's (hopefully) a safe
> option for a fix, e.g. versus trying to add proper BTF virtualization on the
> fly.
> 
> v3:
>  - Suppress BTF, as KVM doesn't actually support it. [Ravi]
>  - Actually load the guest's DEBUGCTL (though amusingly, with BTF squashed,
>    it's guaranteed to be '0' in this scenario). [Ravi]
> 
> v2:
>  - Load the guest's DEBUGCTL instead of simply zeroing it on VMRUN.
>  - Drop bits 5:3 from guest DEBUGCTL so that KVM doesn't let the guest
>    unintentionally enable BusLockTrap (AMD repurposed bits). [Ravi]
>  - Collect a review. [Xiaoyao]
>  - Make bits 5:3 fully reserved, in a separate not-for-stable patch.
> 
> v1: https://lore.kernel.org/all/20250224181315.2376869-1-seanjc@google.com

For the series,

Reviewed-and-tested-by: Ravi Bangoria <ravi.bangoria@....com>

Thanks,
Ravi