CVE-2026-52969

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.

Affected Software

References

• https://git.kernel.org/stable/c/01b71b930f15728aa8599478a7ce90c19dcd9fc2
• https://git.kernel.org/stable/c/0d419c23bb11b5c9664de777c47c1f04a235882d
• https://git.kernel.org/stable/c/0eb281eb95b2d4eea4db1da5fe91023aecc97095
• https://git.kernel.org/stable/c/577a8d3bae0531f0e5ccfac919cd8192f920a804
• https://git.kernel.org/stable/c/74f1a22f7a80f03d28ad8551a2d25d563433addf
• https://git.kernel.org/stable/c/b315b033a877b1ee6d827810b5d7bb4392ffcf8d
• https://git.kernel.org/stable/c/ecf9b3ea7847fe14f34b8c41f00de1eb95c747da