Upstream commit 910c57dfa4d113aae6571c2a8b9ae8c430975902.
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 base-commit: 6769ea8da8a93ed4630f1ce64df6aafcaabfce64 Reviewed-by: Jim Mattson jmattson@google.com Link: https://lore.kernel.org/r/20240215010004.1456078-2-seanjc@google.com Signed-off-by: Sean Christopherson seanjc@google.com --- arch/x86/kvm/x86.c | 10 ++++++++++ 1 file changed, 10 insertions(+)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index a9c26397dcfd..dc0a7b9469e3 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -7106,6 +7106,16 @@ static int emulator_cmpxchg_emulated(struct x86_emulate_ctxt *ctxt,
if (r < 0) return X86EMUL_UNHANDLEABLE; + + /* + * Mark the page dirty _before_ checking whether or not the CMPXCHG was + * successful, as the old value is written back on failure. Note, for + * live migration, this is unnecessarily conservative as CMPXCHG writes + * back the original value and the access is atomic, but KVM's ABI is + * that all writes are dirty logged, regardless of the value written. + */ + kvm_vcpu_mark_page_dirty(vcpu, gpa_to_gfn(gpa)); + if (r) return X86EMUL_CMPXCHG_FAILED;