Re: [PATCH 1/2] KVM: x86: Mark target gfn of emulated atomic instruction as dirty

[David Matlack]

On Fri, Feb 16, 2024 at 9:10 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Thu, Feb 15, 2024, Sean Christopherson wrote:
> > On Thu, Feb 15, 2024, David Matlack wrote:
> > > On Wed, Feb 14, 2024 at 5:00 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > >
> > > > When emulating an atomic access on behalf of the guest, mark the target
> > > > 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 emulated
> > > > CMPXCHG was converted to do a user access.  Before that, KVM explicitly
> > > > mapped the guest page into kernel memory, and marked the page dirty during
> > > > the unmap phase.
> > > >
> > > > Mark the page dirty even if the CMPXCHG fails, as the old data is written
> > > > back on failure, i.e. the page is still written.  The value written is
> > > > 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 atomic accesses")
> > >
> > > I'm only half serious but... Should we just revert this commit?
> >
> > No.
>
> David, any objection to this patch?  I'd like to get this on its way to Paolo
> asap, but also want to make sure we all agree this is the right solution before
> doing so.

Sorry for the late response. No objection to this patch. I'd like a
better story for KVM code that interacts directly with user pointers,
but I have no objection to fixing forward for this case.