Description

In the Linux kernel, the following vulnerability has been resolved: signal: Prevent exec() race Hyunwoo debugged the following KASAN UAF splat: BUG: KASAN: slab-use-after-free in __send_signal_locked+0xb27/0xba0 Write of size 8 at addr ffff888007ed80c8 by task poc/79 ... Call Trace: __send_signal_locked+0xb27/0xba0 do_send_sig_info+0xa7/0x160 do_send_specific+0x76/0xa0 __x64_sys_tgkill+0x193/0x270 ... Allocated by task 80: do_timer_create+0x1a4/0x1030 __x64_sys_timer_create+0x145/0x190 ... Freed by task 12: kmem_cache_free_bulk+0x1f8/0x4a0 kvfree_rcu_bulk+0x14f/0x1c0 kfree_rcu_work+0x128/0x1a0 ... Last potentially related work creation: kvfree_call_rcu+0x39/0x390 __flush_itimer_signals+0x211/0x320 flush_itimer_signals+0x47/0x90 begin_new_exec+0xa6b/0x28c0 It turned out that this happens with a non-leader exec() as Hyunwoo explained: de_thread() calls exchange_tids() before release_task(leader), so the struct pid held by a SIGEV_THREAD_ID timer created against the leader's tid now points to the thread which called execve(). pid_task() returns that thread and lock_task_sighand() on it succeeds. If the timer signal is blocked, its sigqueue stays queued on the leader's task::pending. The next expiry of that timer can then run while release_task() flushes the queue. posixtimer_send_sigqueue() checks whether the sigqueue is already queued with a plain list_empty(), which only reads list_head::next. list_del_init() is not atomic and INIT_LIST_HEAD() stores list_head::next before list_head::prev, so the check can pass in between. list_add_tail() queues the entry on the task::pending of the live thread, and the list_head::prev store from the flush then overwrites the list_head::prev link that list_add_tail() has just set. __flush_itimer_signals() does not undo that either. With list_head::prev pointing at the entry itself, its list_del_init() only stores the same values again, so the entry is not removed from the list. It is still there after the last reference is dropped and the timer is freed by RCU, and the list_add_tail() of a later tgkill() follows that list_head::prev into the freed timer. This problem surfaced with the recent commit which moved the sigqueue flush out of the sighand lock held region. Hyonwoo proposed to fix this by using list_del_init_careful(), but that just papers over the problem. After some disucssions and various attempts to solve it, Eric pointed out that there is no reason to flush task::pending late in release_task() and it should be done in exit_signals() already. As nothing can collect and deliver signals which are queued in a dying task's pending queue, there is no reason to delay it further. But it has to be ensured that no signals can be queued into it after that point. exit_signals() sets PF_EXITING in task::flags, which can be used as an indicator for this. Cure it by: - Preventing signal queueing for task private signals (PIDTYPE_PID) when the task has PF_EXITING set in __send_signal_locked() and in posixtimer_send_sigqueue(). - Protecting the unlocked setting of PF_EXITING in exit_signals() for the task group empty and the group exit case with sighand lock - Flushing task::pending signals right there. Optimize that by moving the whole pending list to an on-stack list head under sighand lock and free the signals without the lock held. There has been quite some discussion about the lockless flush and the non-leader exec case on weakly ordered systems. The problem is that a third party which tries to send a posix timer signal relies on the PID lookup to find the target task and that lookup might result in the new leader when the signal was originaly directed to the old leader. In case that the signal was queued on the old leader then the lockless flush raised a concern over the following situation: old_leader new_leader third party A: flush_list() // list_del_in ---truncated---

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.8",
  "pubDate": "2026-10-06T09:18:14.373Z",
  "pubdate": "2026-10-06T09:18:14.373Z",
  "executiveSummary": "This vulnerability is a Use-After-Free (UAF) flaw residing in the Linux kernel's signal handling subsystem during the process execution (exec) lifecycle.\nThe issue arises from a race condition between thread migration during exec() and POSIX timer signal delivery, leading to memory corruption.\nAffected systems include Linux kernel environments where multi-threaded processes utilize SIGEV_THREAD_ID POSIX timers.\nAn attacker capable of triggering specific timing conditions during process execution can potentially cause a kernel panic, memory corruption, or local privilege escalation.\nExploitation requires the ability to spawn processes and manipulate POSIX timers to target a non-leader thread that is undergoing an execve() system call, making it a sophisticated local attack vector.",
  "technicalDetails": "The root cause of this vulnerability is a race condition during the exec() process, specifically involving the transition of thread identities and the flushing of signal queues. When a non-leader thread calls execve(), the kernel invokes de_thread(), which performs an exchange_tids() operation. This updates the process structure such that a PID previously associated with the old leader is now associated with the new executor.\nIf a SIGEV_THREAD_ID timer is active and targeted at the old leader, the timer’s signal queue can become associated with the task structure in a way that creates a window for UAF. The vulnerability occurs because the signal queue flushing mechanism (specifically __flush_itimer_signals()) does not guarantee atomicity when interacting with concurrent signal queuing via list_add_tail().\nThe list_del_init() operation used to remove queued signals is not atomic. If an interrupt or concurrent signal delivery (e.g., via tgkill()) occurs during the removal process, the kernel can perform a list_add_tail() on a list entry that is simultaneously being modified by the flush operation. Because the flush operation overwrites list pointers, a list_head::prev pointer may end up pointing to the signal entry itself, even after the timer structure has been freed via RCU (Read-Copy-Update).\nA subsequent tgkill() call follows these corrupted list pointers into the freed timer memory, resulting in a slab-use-after-free write operation (size 8).\nThe exploitation flow proceeds as follows: 1) An attacker creates a timer targeting a thread group leader; 2) The leader thread is subjected to an execve() call; 3) The kernel reassigns PIDs; 4) The attacker triggers the race by timing a signal queue flush against a tgkill() call; 5) The kernel operates on an improperly cleaned-up list entry that references memory already freed via kfree_rcu_work. This triggers the KASAN splat and memory corruption.\nThe vulnerability highlights the risks associated with moving sigqueue flushes out of the sighand lock-protected critical section, as the lack of mutual exclusion allows for race conditions between pending signal delivery and process exit signaling."
}