Vulnerability CVE-2024-35804
Published: 2024-05-17

Description:
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

 References:
https://git.kernel.org/stable/c/a9bd6bb6f02bf7132c1ab192ba62bbfa52df7d66
https://git.kernel.org/stable/c/726374dde5d608b15b9756bd52b6fc283fda7a06
https://git.kernel.org/stable/c/9d1b22e573a3789ed1f32033ee709106993ba551
https://git.kernel.org/stable/c/225d587a073584946c05c9b7651d637bd45c0c71
https://git.kernel.org/stable/c/910c57dfa4d113aae6571c2a8b9ae8c430975902
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