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Date: Fri, 16 Feb 2024 09:10:47 -0800
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Subject: Re: [PATCH 1/2] KVM: x86: Mark target gfn of emulated atomic
 instruction as dirty
From: Sean Christopherson <seanjc@google.com>
To: David Matlack <dmatlack@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Pasha Tatashin <tatashin@google.com>, Michael Krebs <mkrebs@google.com>
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On Thu, Feb 15, 2024, Sean Christopherson wrote:
> On Thu, Feb 15, 2024, David Matlack wrote:
> > On Wed, Feb 14, 2024 at 5:00=E2=80=AFPM Sean Christopherson <seanjc@goo=
gle.com> wrote:
> > >
> > > When emulating an atomic access on behalf of the guest, mark the targ=
et
> > > gfn dirty if the CMPXCHG by KVM is attempted and doesn't fault.  This
> > > fixes a bug where KVM effectively corrupts guest memory during live
> > > migration by writing to guest memory without informing userspace that=
 the
> > > page is dirty.
> > >
> > > Marking the page dirty got unintentionally dropped when KVM's emulate=
d
> > > CMPXCHG was converted to do a user access.  Before that, KVM explicit=
ly
> > > mapped the guest page into kernel memory, and marked the page dirty d=
uring
> > > the unmap phase.
> > >
> > > Mark the page dirty even if the CMPXCHG fails, as the old data is wri=
tten
> > > back on failure, i.e. the page is still written.  The value written i=
s
> > > guaranteed to be the same because the operation is atomic, but KVM's =
ABI
> > > is that all writes are dirty logged regardless of the value written. =
 And
> > > more importantly, that's what KVM did before the buggy commit.
> > >
> > > Huge kudos to the folks on the Cc list (and many others), who did all=
 the
> > > actual work of triaging and debugging.
> > >
> > > Fixes: 1c2361f667f3 ("KVM: x86: Use __try_cmpxchg_user() to emulate a=
tomic accesses")
> >=20
> > I'm only half serious but... Should we just revert this commit?
>=20
> No.

David, any objection to this patch?  I'd like to get this on its way to Pao=
lo
asap, but also want to make sure we all agree this is the right solution be=
fore
doing so.