CVE-2026-64514 userfaultfd: gate must_wait writability check on pte_present()
A local attacker can cause a denial of service by triggering a race condition in userfaultfd that leaves threads asleep indefinitely due to incorrect PTE checks.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-64514?
CVE-2026-64514 is a denial of service vulnerability in Linux, published on 25 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
userfaultfd: gate must_wait writability check on pte_present()
userfaultfd_must_wait() and userfaultfd_huge_must_wait() read the PTE without taking the page table lock and then apply pte_write() / huge_pte_write() to it. Those accessors decode bits from the present encoding only; on a swap or migration entry they read the offset bits that happen to share the same position and return an undefined result.
The intent of the check is "is this fault still WP-blocked?". A non-marker swap entry means the page is in transit -- the userfault context the original fault delivered against is no longer the same, and the swap-in or migration completion path will re-deliver a fresh fault if userspace still needs to handle it. Worst case under the current code the garbage write bit says "wait", and the thread stays asleep until a UFFDIO_WAKE that may never arrive.
Gate the writability check on pte_present() so the lockless re-check only inspects present-PTE bits when the entry is actually present. The non-present, non-marker case returns "don't wait" and lets the fault path retry.
Which versions are affected by CVE-2026-64514?
How do I fix CVE-2026-64514?
Apply the vendor's patch.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-64514 being exploited?
There is no known exploitation of CVE-2026-64514 in the wild and no public exploit code.
Sources
Also known as AZL-93608, BELL-CVE-2026-64514, DEBIAN-CVE-2026-64514, GHSA-r2gx-xm5r-33f5, ROOT-OS-UBUNTU-2204-CVE-2026-64514, UBUNTU-CVE-2026-64514.