Messages in this thread | | | From | Jim Mattson <> | Date | Thu, 15 Feb 2024 09:13:36 -0800 | Subject | Re: [PATCH 1/2] KVM: x86: Mark target gfn of emulated atomic instruction as dirty |
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On Wed, Feb 14, 2024 at 5:00 PM Sean Christopherson <seanjc@google.com> 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@vger.kernel.org > Cc: David Matlack <dmatlack@google.com> > Cc: Pasha Tatashin <tatashin@google.com> > Cc: Michael Krebs <mkrebs@google.com> > Signed-off-by: Sean Christopherson <seanjc@google.com>
Let's try this again, gmail...
Reviewed-by: Jim Mattson <jmattson@google.com>
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