NVD - CVE-2026-64560
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Information Technology Laboratory
National Vulnerability Database
National Vulnerability Database
NVD
Vulnerabilities
CVE-2026-64560<br>Detail
Received
This CVE record has recently been published to the CVE List and has been included within the NVD dataset.
Description
In the Linux kernel, the following vulnerability has been resolved:
posix-cpu-timers: Prevent UAF caused by non-leader exec() race
Wongi and Jungwoo decoded and reported a non-leader exec() related race<br>which can result in an UAF:
sys_timer_delete() exec()<br>posix_cpu_timer_del()<br>// Observes old leader<br>p = pid_task(pid, pid_type); de_thread()<br>switch_leader();<br>release_task(old_leader)<br>__exit_signal(old_leader)<br>sighand = lock(old_leader, sighand);<br>posix_cpu_timers*_exit();<br>sighand = lock_task_sighand(p) unhash_task(old_leader);<br>sh = lock(p, sighand) old_leader->sighand = NULL;<br>unlock(sighand);<br>(p->sighand == NULL)<br>unlock(sh)<br>return NULL;
// Returns without action<br>if(!sighand)<br>return 0;<br>free_posix_timer();
This is "harmless" unless the deleted timer was armed and enqueued in<br>p->signal because on exec() a TGID targeted timer is inherited.
As sys_timer_delete() freed the underlying posix timer object<br>run_posix_cpu_timers() or any timerqueue related add/delete operations on<br>other timers will access the freed object's timerqueue node, which results<br>in an UAF.
There is a similar problem vs. posix_cpu_timer_set(). For regular posix<br>timers it just transiently returns -ESRCH to user space, but for the use<br>case in do_cpu_nanosleep() it's the same UAF just that the k_itimer is<br>allocated on the stack.
Also posix_cpu_timer_rearm() fails to rearm the timer, which means it stops<br>to expire.
While debating solutions Frederic pointed out another problem:
posix_cpu_timer_del(tmr)<br>__exit_signal(p)<br>posix_cpu_timers*_exit(p);<br>unhash_task(p);<br>p->sighand = NULL;<br>sh = lock_task_sighand(p)<br>sighand = p->sighand;<br>if (!sighand)<br>return NULL;<br>lock(sighand);
if (!sh)<br>WARN_ON_ONCE(timer_queued(tmr));
On weakly ordered architectures it is not guaranteed that<br>posix_cpu_timer_del() will observe the stores in posix_cpu_timers*_exit()<br>when p->sighand is observed as NULL, which means the WARN() can be a false<br>positive.
Solve these issues by:
1) Changing the store in __exit_signal() to smp_store_release().
2) Adding a smp_acquire__after_ctrl_dep() into the !sighand path<br>of lock_task_sighand().
3) Creating a helper function for looking up the task and locking sighand<br>which does not return when sighand == NULL. Instead it retries the<br>task lookup and only if that fails it gives up.
4) Using that helper in the three affected functions.
#1/#2 ensures that the reader side which observes sighand == NULL also<br>observes all preceeding stores, i.e. the stores in posix_cpu_timers*_exit()<br>and the ones in unhash_task().
#3 ensures that the above described non-leader exec() situation is handled<br>gracefully. When the task lookup returns the old leader, but sighand ==<br>NULL then it retries. In the non-leader exec() case the subsequent task<br>lookup will observe the new leader due to #1/#2. In normal exit() scenarios<br>the subsequent lookup fails.
When the task lookup fails, the function also checks whether the timer is<br>still enqueued and issues a warning if that's the case. Unfortunately there<br>is nothing which can be done about it, but as the task is already not<br>longer visible the timer should not be accessed anymore. This check also<br>requires memory ordering, which is not provided when the first lookup<br>fails. To achieve that the check is preceeded by a smp_rmb() which pairs<br>with the smp_wmb() in write_seqlock() in __exit_signal(). That ensures that<br>the stores in posix_cpu_timers*_exit() are visible.
The history of the non-leader exec() issue goes back to the early days of<br>posix CPU timers, which stored a pointer to the group leader task in the<br>timer. That obviously fails when a non-leader exec() switches the leader.<br>commit e0a70217107e ("posix-cpu-timers: workaround to suppress the problems<br>with mt exec") added a temporary workaround for that in 2010 which surv<br>---truncated---
Metrics
 
CVSS Version 4.0
CVSS Version 3.x
CVSS Version 2.0
NVD enrichment efforts reference publicly available information to associate<br>vector strings. CVSS information contributed by other sources is also<br>displayed.
CVSS 4.0 Severity and Vector Strings:
NIST: NVD
N/A
NVD assessment<br>not yet provided.
CVSS 3.x Severity and Vector Strings:
NIST: NVD
Base<br>Score: N/A
NVD assessment<br>not yet provided.
CNA: kernel.org
Base<br>Score: 7.8 HIGH
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H<br>CVSS v3.1 Severity and Metrics: Base Score: 7.8 HIGH Vector: AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H Impact...