Subject: CVE-2024-35804: KVM: x86: Mark target gfn of emulated atomic instruction as dirty

From: Greg Kroah-Hartman <[email-masked]>

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

The Linux kernel CVE team has assigned CVE-2024-35804 to this issue.

Affected and fixed versions
===========================

	Issue introduced in 5.15.58 with commit d97c0667c1e6 and fixed in 5.15.154 with commit a9bd6bb6f02b
	Issue introduced in 5.19 with commit 1c2361f667f3 and fixed in 6.1.84 with commit 726374dde5d6
	Issue introduced in 5.19 with commit 1c2361f667f3 and fixed in 6.6.24 with commit 9d1b22e573a3
	Issue introduced in 5.19 with commit 1c2361f667f3 and fixed in 6.7.12 with commit 225d587a0735
	Issue introduced in 5.19 with commit 1c2361f667f3 and fixed in 6.8 with commit 910c57dfa4d1
	Issue introduced in 5.17.13 with commit b0f294103f4c
	Issue introduced in 5.18.2 with commit e964665cc7ca

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2024-35804
will be updated if fixes are backported, please check that for the most
up to date information about this issue.

Affected files
==============

The file(s) affected by this issue are:
	arch/x86/kvm/x86.c

Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	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