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

[Jim Mattson]

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")
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: David Matlack <dmatlack@xxxxxxxxxx>
> Cc: Pasha Tatashin <tatashin@xxxxxxxxxx>
> Cc: Michael Krebs <mkrebs@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Let's try this again, gmail...

Reviewed-by: Jim Mattson <jmattson@xxxxxxxxxx>