CVE-2024-35804

In the Linux kernel, the following vulnerability has been resolved:

KVM: x86: Mark target gfn of emulated atomic instruction as dirty

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.

base-commit: 6769ea8da8a93ed4630f1ce64df6aafcaabfce64