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

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Message-ID: <Z8HCXP0hFYs0dUxM@google.com>
Date: Fri, 28 Feb 2025 06:04:12 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Ravi Bangoria <ravi.bangoria@....com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Xiaoyao Li <xiaoyao.li@...el.com>, rangemachine@...il.com, whanos@...gal.fun
Subject: Re: [PATCH v3 0/6] KVM: SVM: Fix DEBUGCTL bugs

On Fri, Feb 28, 2025, Ravi Bangoria wrote:
> 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>

Thank you for all your help, much appreciated!