CVE-2026-52991 sched/psi: fix race between file release and pressure write
A race condition in the Linux kernel's PSI subsystem allows a use-after-free, potentially enabling local attackers to execute code or escalate privileges.
- 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-2026-52991?
CVE-2026-52991 is a high-severity use after free vulnerability in Linux kernel, published on 24 Jun 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:
sched/psi: fix race between file release and pressure write
A potential race condition exists between pressure write and cgroup file release regarding the priv member of struct kernfs_open_file, which triggers the uaf reported in [1].
Consider the following scenario involving execution on two separate CPUs:
CPU0 CPU1 ==== ==== vfs_rmdir() kernfs_iop_rmdir() cgroup_rmdir() cgroup_kn_lock_live() cgroup_destroy_locked() cgroup_addrm_files() cgroup_rm_file() kernfs_remove_by_name() kernfs_remove_by_name_ns() vfs_write() __kernfs_remove() new_sync_write() kernfs_drain() kernfs_fop_write_iter() kernfs_drain_open_files() cgroup_file_write() kernfs_release_file() pressure_write() cgroup_file_release() ctx = of->priv; kfree(ctx); of->priv = NULL; cgroup_kn_unlock() cgroup_kn_lock_live() cgroup_get(cgrp) cgroup_kn_unlock() if (ctx->psi.trigger) // here, trigger uaf for ctx, that is of->priv
The cgroup_rmdir() is protected by the cgroup_mutex, it also safeguards the memory deallocation of of->priv performed within cgroup_file_release(). However, the operations involving of->priv executed within pressure_write() are not entirely covered by the protection of cgroup_mutex. Consequently, if the code in pressure_write(), specifically the section handling the ctx variable executes after cgroup_file_release() has completed, a uaf vulnerability involving of->priv is triggered.
Therefore, the issue can be resolved by extending the scope of the cgroup_mutex lock within pressure_write() to encompass all code paths involving of->priv, thereby properly synchronizing the race condition occurring between cgroup_file_release() and pressure_write().
And, if an live kn lock can be successfully acquired while executing the pressure write operation, it indicates that the cgroup deletion process has not yet reached its final stage; consequently, the priv pointer within open_file cannot be NULL. Therefore, the operation to retrieve the ctx value must be moved to a point *after* the live kn lock has been successfully acquired.
In another situation, specifically after entering cgroup_kn_lock_live() but before acquiring cgroup_mutex, there exists a different class of race condition:
CPU0: write memory.pressure CPU1: write cgroup.pressure=0 =========================== =============================
kernfs_fop_write_iter() kernfs_get_active_of(of) pressure_write() cgroup_kn_lock_live(memory.pressure) cgroup_tryget(cgrp) kernfs_break_active_protection(kn) ... blocks on cgroup_mutex
cgroup_pressure_write() cgroup_kn_lock_live(cgroup.pressure) cgroup_file_show(memory.pressure, false) kernfs_show(false) kernfs_drain_open_files() cgroup_file_release(of) kfree(ctx) of->priv = NULL cgroup_kn_unlock()
... acquires cgroup_mutex ctx = of->priv; // may now be NULL if (ctx->psi.trigger) // NULL dereference
Consequently, there is a possibility that of->priv is NULL, the pressure write needs to check for this.
Now that the scope of the cgroup_mutex has been expanded, the original explicit cgroup_get/put operations are no longer necessary, this is because acquiring/releasing the live kn lock inherently executes a cgroup get/put operation.
[1] BUG: KASAN: slab-use-after-free in pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011 Call Trace: pressure_write+0xa4/0x210 kernel/cgroup/cgroup.c:4011 cgroup_file_write+0x36f/0x790 kernel/cgroup/cgroup.c:43 ---truncated---
Which versions are affected by CVE-2026-52991?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-52991?
Apply the vendor's patch.
Upgrade to the fixed version. These bugs are found with fuzzing and sanitisers and avoided by memory-safe languages. More about Use after free.
Is CVE-2026-52991 being exploited?
There is no known exploitation of CVE-2026-52991 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-90497, BELL-CVE-2026-52991, DEBIAN-CVE-2026-52991, GHSA-3v2r-c9x3-5fgr, ROOT-OS-DEBIAN-11-CVE-2026-52991, ROOT-OS-DEBIAN-12-CVE-2026-52991 and 4 more.