CVE-2026-98163

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

cgroup: Avoid iteration of dying tasks with zero refcount

The commit 260fbcb92bbea (cgroup: Move dying_tasks cleanup from cgroup_task_release() to cgroup_task_free()) extended the lifetime of tasks on the dying_tasks list. The iterators have provision to go through dying_tasks because of dying threadgroup leaders or explicit CSS_TASK_ITER_WITH_DEAD, however, it was expected that such tasks can obtain a new reference (that is possible before cgroup_task_release()/put_task_struct_rcu_user()). The tasks after cgroup_task_release() and before cgroup_task_free() are subject to race when they may or may not have ->usage count > 0.

The race window is between css_task_iter_next() invocations when css_set_lock is released and we may arrive at a new ->task_pos. The iterator should not attempt to resurrect tasks whose ->usage count dropped to zero. (When that happens, __put_task_struct_rcu_cb() is already imminent and the returned task_struct would could be used after free.)

As for the fix, we cannot simply check the signal->live count of a task on the dying list because that wont distinguish regular zombies waiting to be reaped from RCU remnant tasks that are going to be freed. Therefore add an extra check to rule out ->usage==0 tasks from any iteration.

The repeat: loop in css_task_iter_advance() doesnt consider ->usage count, so add a new loop to css_task_iter_next() to skip de-used tasks on the dying_list.

Rough illustration of the possible race

R (reader of cgroup.procs) T (thread) L (group leader)

                                                                  L exits, signal->live > 0
                                                                  cgroup_task_dead(L)
                                                                    css_set_skip_task_iters() // skips only cset->tasks
                                                                    list_add_tail(&L->cg_list, &cset->dying_tasks)

css_task_iter_next() take css_set_lock css_task_iter_advance() leader && signal->live != 0 => it->task_pos = &L->cg_list release css_set_lock T exits –signal->live == 0 cgroup_task_dead(T) // css_set_lock release_task(T) cgroup_task_release(T) release_task(L) // zap_leader cgroup_task_release(L) put_task_struct_rcu_user(L) …RCU… put_task_struct(L) L->usage = 0 /* L still on dying_tasks */ …RCU… __put_task_struct(L) css_task_iter_next() // another iteration take css_set_lock it->task_pos = &L->cg_list get_task_struct(L) => addition on 0 drop css_set_lock cgroup_task_free(L) css_set_skip_task_iters() // dying skip comes too late free_task(L) cgroup_procs_show() task_pid_vnr(L)

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/057dac23d329d5c5ed62352f2659a39fd46c6d4a
• https://git.kernel.org/stable/c/828938118d6c2bb711301748c3e39e4bed6a62f5