CVE-2026-52969: KVM: Reject wrapped offset in kvm_reset_dirty_gfn()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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

KVM: Reject wrapped offset in kvm_reset_dirty_gfn()

kvm_reset_dirty_gfn() guards the gfn range with

	if (!memslot || (offset + __fls(mask)) >= memslot->npages)
		return;

but offset is u64 and the addition is unchecked.  The check can be
silently bypassed by a u64 wrap.

The dirty ring backing those entries is MAP_SHARED at
KVM_DIRTY_LOG_PAGE_OFFSET of the vcpu fd, so the VMM can rewrite the
slot and offset fields of any entry between when the kernel pushes
them and when KVM_RESET_DIRTY_RINGS consumes them.  On reset,
kvm_dirty_ring_reset() re-reads the values via READ_ONCE() and feeds
them straight back into this check; only the flags handshake is
treated as the handover, the slot/offset payload is taken on trust.

Crafting two entries

	entry[i].offset   = 0xffffffffffffffc1
	entry[i+1].offset = 0

makes the coalescing loop in kvm_dirty_ring_reset() compute

	delta = (s64)(0 - 0xffffffffffffffc1) = 63

which falls in [0, BITS_PER_LONG), so it folds entry[i+1] into the
existing mask by setting bit 63.  The trailing kvm_reset_dirty_gfn()
call then sees offset = 0xffffffffffffffc1 and __fls(mask) = 63;
the sum is 0 in u64 and the bounds check passes.

That offset propagates into kvm_arch_mmu_enable_log_dirty_pt_masked()
unchanged.  On the legacy MMU path -- kvm_memslots_have_rmaps() ==
true, i.e. shadow paging, any VM that has allocated shadow roots, or
a write-tracked slot -- it reaches gfn_to_rmap(), which indexes
slot->arch.rmap[0][] with a near-U64_MAX gfn.  That is an
out-of-bounds load of a kvm_rmap_head, followed by a conditional
clear of PT_WRITABLE_MASK in whatever the loaded pointer points at.
The path is reachable from any process holding /dev/kvm.

Range-check offset on its own first, so the addition cannot wrap.
memslot->npages is bounded well below U64_MAX, so once offset <
npages holds, offset + __fls(mask) (with __fls(mask) < BITS_PER_LONG)
stays in range.

The Linux kernel CVE team has assigned CVE-2026-52969 to this issue.


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

	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 5.15.209 with commit 74f1a22f7a80f03d28ad8551a2d25d563433addf
	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 6.1.175 with commit 0eb281eb95b2d4eea4db1da5fe91023aecc97095
	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 6.6.141 with commit 01b71b930f15728aa8599478a7ce90c19dcd9fc2
	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 6.12.91 with commit b315b033a877b1ee6d827810b5d7bb4392ffcf8d
	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 6.18.33 with commit 0d419c23bb11b5c9664de777c47c1f04a235882d
	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 7.0.10 with commit ecf9b3ea7847fe14f34b8c41f00de1eb95c747da
	Issue introduced in 5.11 with commit fb04a1eddb1a65b6588a021bdc132270d5ae48bb and fixed in 7.1 with commit 577a8d3bae0531f0e5ccfac919cd8192f920a804

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-2026-52969
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:
	virt/kvm/dirty_ring.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/74f1a22f7a80f03d28ad8551a2d25d563433addf
	https://git.kernel.org/stable/c/0eb281eb95b2d4eea4db1da5fe91023aecc97095
	https://git.kernel.org/stable/c/01b71b930f15728aa8599478a7ce90c19dcd9fc2
	https://git.kernel.org/stable/c/b315b033a877b1ee6d827810b5d7bb4392ffcf8d
	https://git.kernel.org/stable/c/0d419c23bb11b5c9664de777c47c1f04a235882d
	https://git.kernel.org/stable/c/ecf9b3ea7847fe14f34b8c41f00de1eb95c747da
	https://git.kernel.org/stable/c/577a8d3bae0531f0e5ccfac919cd8192f920a804