CVE-2026-64561

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

KVM: x86: Check for invalid/obsolete root after making MMU pages available

Check for a stale page fault, i.e. for an invalid and/or obsolete root, after making MMU pages available for the shadow MMU. If reclaiming shadow pages zaps an in-use root, i.e. marks it invalid, then KVM will attempt to map memory into an invalid root. On its own, populating an invalid root is fine, but because child shadow pages inherit their parents role, any children created during the map/fetch will be created as invalid pages, thus violating KVMs invariant that invalid pages are never on the list of active MMU pages.

Note, the underlying flaw has existed since KVM first started tracking invalid roots in 2008 (commit 2e53d63acba7, KVM: MMU: ignore zapped root pagetables), but the true badness only came along in 2020 (Linux 5.9) with the invariant that invalid shadow pages cant be on the list of active pages.

Note #2, inheriting role.invalid when creating child shadow pages is also far from ideal; that flaw will be addressed separately.

Weakness

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.

Affected Software

Extended Description

A race condition occurs within concurrent environments, and it is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• https://git.kernel.org/stable/c/0026dbb7de8ea76e97d6edf42fc3cc084564e2bf
• https://git.kernel.org/stable/c/2abd5287f08319fa35764566b15c6e22cb1068db
• https://git.kernel.org/stable/c/35e77467610c4a37cb0ff54ee56b85f73b1f5700
• https://git.kernel.org/stable/c/62ef67af1878fa2cd066642f2f59e33ade95f637
• https://git.kernel.org/stable/c/65c4f7a1028cf01a93a2762d679c289810ede990
• https://git.kernel.org/stable/c/bce0d3c26e2c761a4bf43c8949f333fc7374eb2d
• https://git.kernel.org/stable/c/f3477a6a4164f15287444eda685b5f6405dbd1e5
• https://github.com/V4bel/Zapscape