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

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 01, 2025, Maxim Levitsky wrote:
> On Thu, 2025-02-27 at 14:24 -0800, 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
> > 
> 
> 
> Hi,
> 
> Amusingly there is another DEBUGCTL issue, which I just got to the bottom of.
> (if I am not mistaken of course).
> 
> We currently don't let the guest set DEBUGCTL.FREEZE_WHILE_SMM and neither
> set it ourselves in GUEST_IA32_DEBUGCTL vmcs field, even when supported by the host
> (If I read the code correctly, I didn't verify this in runtime)

Ugh, SMM.  Yeah, KVM doesn't propagate DEBUGCTLMSR_FREEZE_IN_SMM to the guest
value.  KVM intercepts reads and writes to DEBUGCTL, so it should be easy enough
to shove the bit in on writes, and drop it on reads.

> This means that the host #SMIs will interfere with the guest PMU.  In
> particular this causes the 'pmu' kvm-unit-test to fail, which is something
> that our CI caught.
> 
> I think that kvm should just set this bit, or even better, use the host value
> of this bit, and hide it from the guest, because the guest shouldn't know
> about host's smm, and we AFAIK don't really support freezing perfmon when the
> guest enters its own emulated SMM.

Agreed.  Easy thing is to use the host's value, so that KVM doesn't need to check
for its existence.  I can't think of anything that would go sideways by freezing
perfmon if the host happens to take an SMI.

> What do you think? I'll post patches if you think that this is a good idea.
> (A temp hack to set this bit always in GUEST_IA32_DEBUGCTL fixed the problem for me)
> 
> I also need to check if AMD also has this feature, or if this is Intel specific.

Intel only.  I assume/think/hope AMD's Host/Guest Only field in the event selector
effectively hides SMM from the guest.