CVE-2026-46113

Status: UPDATED  |  Advisory ID: CVE-2026-46113

Key Details

Affected Products, Subsystems & Sectors

What to Know

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

KVM: x86: Fix shadow paging use-after-free due to unexpected GFN

The shadow MMU computes GFNs for direct shadow pages using sp->gfn plus

the SPTE index. This assumption breaks for shadow paging if the guest

page tables are modified between VM entries (similar to commit

aad885e77496, "KVM: x86/mmu: Drop/zap existing present SPTE even

when creating an MMIO SPTE", 2026-03-27). The flow is as follows:

- a PDE is installed for a 2MB mapping, and a page in that area is

accessed. KVM creates a kvm_mmu_page consisting of 512 4KB pages;

the kvm_mmu_page is marked by FNAME(fetch) as direct-mapped because

the guest's mapping is a huge page (and thus contiguous).

- the PDE mapping is changed from outside the guest.

- the guest accesses another page in the same 2MB area. KVM installs

a new leaf SPTE and rmap entry; the SPTE uses the "correct" GFN

(i.e. based on the new mapping, as changed in the previous step) but

that GFN is outside of the [sp->gfn, sp->gfn + 511] range; therefore

the rmap entry cannot be found and removed when the kvm_mmu_page

is zapped.

- the memslot that covers the first 2MB mapping is deleted, and the

kvm_mmu_page for the now-invalid GPA is zapped. However, rmap_remove()

only looks at the [sp->gfn, sp->gfn + 511] range established in step 1,

and fails to find the rmap entry that was recorded by step 3.

- any operation that causes an rmap walk for the same page accessed

by step 3 then walks a stale rmap and dereferences a freed kvm_mmu_page.

This includes dirty logging or MMU notifier invalidations (e.g., from

MADV_DONTNEED).

The underlying issue is that KVM's walking of shadow PTEs assumes that

if a SPTE is present when KVM wants to install a non-leaf SPTE, then the

existing kvm_mmu_page must be for the correct gfn. Because the only way

for the gfn to be wrong is if KVM messed up and failed to zap a SPTE...

which shouldn't happen, but *actually* only happens in response to a

guest write.

That bug dates back literally forever, as even the first version of KVM

assumes that the GFN matches and walks into the "wrong" shadow page.

However, that was only an imprecision until 2032a93d66fa ("KVM: MMU:

Don't allocate gfns page for direct mmu pages") came along.

Fix it by checking for a target gfn mismatch and zapping the existing

SPTE. That way the old SP and rmap entries are gone, KVM installs

the rmap in the right location, and everyone is happy. (NVD)

What to Do

Monitor Linux's web page for any future patch releases.

References