CVE-2026-52973 Overview
CVE-2026-52973 is a Linux kernel vulnerability in the futex subsystem affecting the private default hash allocation logic. The flaw resides in need_futex_hash_allocate_default(), which relied on strict pthread semantics by checking for CLONE_THREAD. This assumption breaks down when an mm_struct is shared through other CLONE_VM paths, leading to a use-after-free condition on the per-CPU mm->futex_ref counter. KASAN reports confirm a slab-use-after-free in futex_hash_put where the reference bias lands on a percpu counter that mm->futex_ref no longer points at. The issue has been resolved upstream by loosening the check to cover any CLONE_VM clone except vfork().
Critical Impact
A use-after-free in the futex subsystem can corrupt kernel memory and may be leveraged for local privilege escalation or denial of service on systems sharing mm_struct via non-CLONE_THREAD paths.
Affected Products
- Linux kernel versions containing the mm->futex_ref percpu allocation logic prior to commits 1dcd3642, 974ac49a, and ee9dce44
- Linux distributions shipping affected stable kernels
- Workloads using CLONE_VM without CLONE_THREAD (for example, certain language runtimes and sandboxing layers)
Discovery Timeline
- 2026-06-24 - CVE-2026-52973 published to NVD
- 2026-06-24 - Last updated in NVD database
Technical Details for CVE-2026-52973
Vulnerability Analysis
The futex (fast userspace mutex) subsystem maintains a private default hash table tied to the memory descriptor mm_struct. The kernel tracks references on this hash through mm->futex_ref, a per-CPU counter allocated lazily on first use. The function need_futex_hash_allocate_default() decides whether the allocation must occur for a newly created task.
The original implementation gated allocation on the CLONE_THREAD flag, assuming only thread siblings share an mm. This ignored other clone paths that share an mm_struct without creating a POSIX thread group, such as bare CLONE_VM usage. When such clones race with futex operations, the +1 reference bias can be deposited on a per-CPU counter that mm->futex_ref has already been replaced or freed from, producing the slab-use-after-free observed by KASAN in futex_hash_put.
Root Cause
The root cause is an incorrect concurrency assumption [CWE-416: Use After Free]. The check CLONE_THREAD was used as a proxy for "this mm is shared," but CLONE_VM alone is sufficient to share an mm_struct. The mismatch lets two tasks operate on mm->futex_ref without the serialization the allocator relies on.
Attack Vector
Exploitation requires local code execution that issues a clone() syscall with CLONE_VM set and CLONE_THREAD cleared, then triggers futex operations from both the parent and child. The race window between the parent's pending hash allocation and the child's futex use causes the bias to be applied to a stale per-CPU counter. The patch restricts the optimization to vfork() (where the parent is suspended) and forces allocation for all other CLONE_VM clones.
No public proof-of-concept exploit is currently available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog. EPSS data places the exploit probability at 0.173%.
Detection Methods for CVE-2026-52973
Indicators of Compromise
- KASAN reports in dmesg showing slab-use-after-free in futex_hash_put or related futex call paths
- Kernel oops or panics referencing mm->futex_ref, futex_hash, or per-CPU reference counters
- Unexpected process crashes in workloads that heavily use clone(CLONE_VM) without CLONE_THREAD
Detection Strategies
- Enable KASAN on test and staging kernels to surface the use-after-free during fuzzing or regression runs
- Audit running kernel versions against the fixed commits 1dcd36420af2, 974ac49a9a06, and ee9dce44362b
- Correlate kernel ring buffer messages with process telemetry to identify tasks invoking clone() with CLONE_VM and no CLONE_THREAD
Monitoring Recommendations
- Forward /var/log/kern.log and journalctl -k output to a centralized logging or SIEM platform for pattern matching on futex_hash_put faults
- Track syscall telemetry for anomalous clone() flag combinations on production hosts
- Alert on repeated kernel taint events or process termination clusters tied to futex code paths
How to Mitigate CVE-2026-52973
Immediate Actions Required
- Inventory Linux hosts and identify kernels missing the upstream futex fix commits
- Schedule kernel updates from distribution vendors that include the patched futex hash allocation logic
- Restrict local access on multi-tenant systems until patched kernels are deployed
Patch Information
The fix has been merged upstream and is available in the following commits: Kernel Git Commit 1dcd364, Kernel Git Commit 974ac49, and Kernel Git Commit ee9dce4. The patches loosen the need_futex_hash_allocate_default() check to allocate for any CLONE_VM clone except vfork(), eliminating the race on mm->futex_ref.
Workarounds
- Apply the upstream patches or rebuild kernels with the fix backported when distribution updates are not yet available
- Limit untrusted local code execution on shared systems to reduce the chance of a malicious CLONE_VM race
- Reboot into the patched kernel after installation — live kernel patching may be required for environments that cannot tolerate downtime
# Verify the running kernel and check for the fix after updating
uname -r
grep -E 'futex_hash|need_futex_hash_allocate_default' /proc/kallsyms | head
# Apply distribution updates
sudo apt-get update && sudo apt-get install --only-upgrade linux-image-$(uname -r)
# or, on RHEL-family systems
sudo dnf update kernel
sudo reboot
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
