* [patch 1/8] signal: Prevent exec() race
[not found] <20260904112100.683893401@kernel.org>
@ 2026-09-04 11:22 ` Thomas Gleixner
2026-09-04 11:35 ` Oleg Nesterov
2026-09-04 11:22 ` [patch 2/8] exec: Cleanup POSIX timers right after de_thread() Thomas Gleixner
1 sibling, 1 reply; 6+ messages in thread
From: Thomas Gleixner @ 2026-09-04 11:22 UTC (permalink / raw)
To: LKML
Cc: Hyunwoo Kim, Oleg Nesterov, Frederic Weisbecker,
Christian Brauner, Peter Zijlstra, John Stultz, Ingo Molnar,
Alexander Viro, Eric W. Biederman, stable
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.
Fixes: fb3bbcfe344e ("exit: change the release_task() paths to call flush_sigqueue() lockless")
Reported-by: Hyunwoo Kim <imv4bel@gmail.com>
Debugged-by: Hyunwoo Kim <imv4bel@gmail.com>
Suggested-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Closes: https://patch.msgid.link/aok1rdkBgZsynHZB@v4bel
---
V3: Restructure code and fix the missing unlock - Oleg
V2: Don't flush w/o sighand lock held - Oleg
Move the while pending list under the lock and free it lockless
---
kernel/exit.c | 11 ++++----
kernel/signal.c | 70 ++++++++++++++++++++++++++++++++------------------------
2 files changed, 47 insertions(+), 34 deletions(-)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -299,12 +299,13 @@ void release_task(struct task_struct *p)
free_pids(post.pids);
release_thread(p);
/*
- * This task was already removed from the process/thread/pid lists
- * and lock_task_sighand(p) can't succeed. Nobody else can touch
- * ->pending or, if group dead, signal->shared_pending. We can call
- * flush_sigqueue() lockless.
+ * This task was already removed from the process/thread/pid lists and
+ * lock_task_sighand(p) can't succeed. If it's the group leader then
+ * flush tsk->signal->shared_pending. tsk->pending has been flushed
+ * already in exit_signals(). Nothing else can touch
+ * signal->shared_pending anymore, so flush_sigqueue() can be invoked
+ * lockless.
*/
- flush_sigqueue(&p->pending);
if (thread_group_leader(p))
flush_sigqueue(&p->signal->shared_pending);
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -457,18 +457,28 @@ static void __sigqueue_free(struct sigqu
kmem_cache_free(sigqueue_cachep, q);
}
-void flush_sigqueue(struct sigpending *queue)
+static void flush_sigqueue_list(struct list_head *head)
{
- struct sigqueue *q;
+ struct sigqueue *q, *tmp;
- sigemptyset(&queue->signal);
- while (!list_empty(&queue->list)) {
- q = list_entry(queue->list.next, struct sigqueue , list);
+ list_for_each_entry_safe(q, tmp, head, list) {
list_del_init(&q->list);
__sigqueue_free(q);
}
}
+void flush_sigqueue(struct sigpending *queue)
+{
+ sigemptyset(&queue->signal);
+ flush_sigqueue_list(&queue->list);
+}
+
+static void sigqueue_dequeue_pending(struct sigpending *queue, struct list_head *head)
+{
+ sigemptyset(&queue->signal);
+ list_splice_init(&queue->list, head);
+}
+
/*
* Flush all pending signals for this kthread.
*/
@@ -1030,6 +1040,10 @@ static int __send_signal_locked(int sig,
lockdep_assert_held(&t->sighand->siglock);
result = TRACE_SIGNAL_IGNORED;
+
+ if (unlikely(type == PIDTYPE_PID && (t->flags & PF_EXITING)))
+ goto ret;
+
if (!prepare_signal(sig, t, force))
goto ret;
@@ -1990,6 +2004,9 @@ void posixtimer_send_sigqueue(struct k_i
if (!likely(lock_task_sighand(t, &flags)))
return;
+ if (unlikely(tmr->it_pid_type == PIDTYPE_PID && (t->flags & PF_EXITING)))
+ goto unlock;
+
/*
* Update @tmr::sigqueue_seq for posix timer signals with sighand
* locked to prevent a race against dequeue_signal().
@@ -2081,6 +2098,7 @@ void posixtimer_send_sigqueue(struct k_i
result = TRACE_SIGNAL_DELIVERED;
out:
trace_signal_generate(sig, &q->info, t, tmr->it_pid_type != PIDTYPE_PID, result);
+unlock:
unlock_task_sighand(t, &flags);
}
@@ -3120,42 +3138,36 @@ static void retarget_shared_pending(stru
void exit_signals(struct task_struct *tsk)
{
+ LIST_HEAD(sigq_list);
int group_stop = 0;
- sigset_t unblocked;
/*
* @tsk is about to have PF_EXITING set - lock out users which
- * expect stable threadgroup.
+ * expect a stable threadgroup.
*/
cgroup_threadgroup_change_begin(tsk);
- if (thread_group_empty(tsk) || (tsk->signal->flags & SIGNAL_GROUP_EXIT)) {
+ scoped_guard(spinlock_irq, &tsk->sighand->siglock) {
tsk->flags |= PF_EXITING;
- cgroup_threadgroup_change_end(tsk);
- return;
- }
- spin_lock_irq(&tsk->sighand->siglock);
- /*
- * From now this task is not visible for group-wide signals,
- * see wants_signal(), do_signal_stop().
- */
- tsk->flags |= PF_EXITING;
+ sigqueue_dequeue_pending(&tsk->pending, &sigq_list);
- cgroup_threadgroup_change_end(tsk);
+ if (task_sigpending(tsk) && !thread_group_empty(tsk) &&
+ !(tsk->signal->flags & SIGNAL_GROUP_EXIT)) {
+ sigset_t unblocked = tsk->blocked;
+
+ signotset(&unblocked);
+ retarget_shared_pending(tsk, &unblocked);
+
+ if (unlikely(tsk->jobctl & JOBCTL_STOP_PENDING) &&
+ task_participate_group_stop(tsk))
+ group_stop = CLD_STOPPED;
+ }
+ }
- if (!task_sigpending(tsk))
- goto out;
+ cgroup_threadgroup_change_end(tsk);
- unblocked = tsk->blocked;
- signotset(&unblocked);
- retarget_shared_pending(tsk, &unblocked);
-
- if (unlikely(tsk->jobctl & JOBCTL_STOP_PENDING) &&
- task_participate_group_stop(tsk))
- group_stop = CLD_STOPPED;
-out:
- spin_unlock_irq(&tsk->sighand->siglock);
+ flush_sigqueue_list(&sigq_list);
/*
* If group stop has completed, deliver the notification. This
^ permalink raw reply [flat|nested] 6+ messages in thread* [patch 2/8] exec: Cleanup POSIX timers right after de_thread()
[not found] <20260904112100.683893401@kernel.org>
2026-09-04 11:22 ` [patch 1/8] signal: Prevent exec() race Thomas Gleixner
@ 2026-09-04 11:22 ` Thomas Gleixner
1 sibling, 0 replies; 6+ messages in thread
From: Thomas Gleixner @ 2026-09-04 11:22 UTC (permalink / raw)
To: LKML
Cc: Hyunwoo Kim, Oleg Nesterov, Frederic Weisbecker,
Christian Brauner, Peter Zijlstra, John Stultz, Ingo Molnar,
Alexander Viro, Eric W. Biederman, stable
From: Hyunwoo Kim <imv4bel@gmail.com>
A per-thread CPU timer holds a reference to the PID of the thread it is
attached to and, while it is armed, its node is queued in that thread's
posix_cputimers. The task is looked up by that PID.
When a non-leader thread exec()s, de_thread() changes which task owns
that PID. pid_task(timer->it.cpu.pid, PIDTYPE_PID) then returns NULL,
but the node is still queued on tsk, which is alive. timer_lock_sighand()
takes a failed lookup to mean that the node is already dequeued, so it
has nothing to undo.
begin_new_exec() calls posix_cpu_timers_exit(me) right after
exec_task_namespaces() and that removes the leftover node, so the state
normally stays invisible. But bprm->point_of_no_return is set before
de_thread(), so if unshare_files(), set_mm_exe_file(), exec_mmap() or
exec_task_namespaces() fails, the task dies before it gets there.
exit_itimers() then frees the k_itimer while its node is still queued,
and reaping tsk later erases that freed node from the rbtree.
In short:
the non-leader thread B the parent
timer_create(CLOCK_THREAD_CPUTIME_ID)
timer_settime()
arm_timer() // the node is queued on B
execve()
de_thread(B)
exchange_tids(B, leader) // B's PID now belongs to the leader
release_task(leader)
__exit_signal(leader)
posix_cpu_timers_exit(leader) // cleans leader's queue, not B's
__unhash_process(leader) // that PID has no task anymore
exec_mmap()
mmap_read_lock_killable(old_mm)
kill(B, SIGKILL)
// -EINTR
get_signal()
do_exit()
exit_itimers()
posix_timer_delete()
posix_cpu_timer_del()
posix_timer_unhash_and_free() // freed while still queued
wait4()
release_task(B)
posix_cpu_timers_exit(B)
cleanup_timerqueue()
timerqueue_del() // use-after-free
Move the POSIX timer cleanup right after de_thread() before any of the
later failure conditions brings the task into do_exit().
[ tglx: Move the cleanup right after de_thread() ]
Fixes: 55e8c8eb2c7b ("posix-cpu-timers: Store a reference to a pid not a task")
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Link: https://patch.msgid.link/ao7Q8miiuLAPVnWv@v4bel
---
Changes in v3:
Move the cleanup right after de_thread() - Oleg
Rework change log
Changes in v2:
- Add the trigger sequence and the KASAN log to the commit message.
- v1: https://lore.kernel.org/all/anfgrsPlUdwBhdrp@v4bel/
---
fs/exec.c | 29 +++++++++++++++++++++--------
1 file changed, 21 insertions(+), 8 deletions(-)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1115,6 +1115,17 @@ static struct file *bprm_identity_file(c
return bprm->file;
}
+static void posixtimer_exec(struct task_struct *me)
+{
+#ifdef CONFIG_POSIX_TIMERS
+ spin_lock_irq(&me->sighand->siglock);
+ posix_cpu_timers_exit(me);
+ spin_unlock_irq(&me->sighand->siglock);
+ exit_itimers(me);
+ flush_itimer_signals();
+#endif
+}
+
/*
* Calling this is the point of no return. None of the failures will be
* seen by userspace since either the process is already taking a fatal
@@ -1152,6 +1163,16 @@ int begin_new_exec(struct linux_binprm *
retval = de_thread(me);
if (retval)
goto out;
+
+ /*
+ * This must be done here to ensure that POSIX CPU timers which were
+ * armed on the current task are dequeued from me::posix_cputimers.
+ * That ensures that in case of a TID switch the deletion of the related
+ * POSIX timer will not free an enqueued timer because the TID lookup
+ * failed as the original target TID was the old leader.
+ */
+ posixtimer_exec(me);
+
/* see the comment in check_unsafe_exec() */
current->fs->in_exec = 0;
/*
@@ -1192,14 +1213,6 @@ int begin_new_exec(struct linux_binprm *
if (retval)
goto out_unlock;
-#ifdef CONFIG_POSIX_TIMERS
- spin_lock_irq(&me->sighand->siglock);
- posix_cpu_timers_exit(me);
- spin_unlock_irq(&me->sighand->siglock);
- exit_itimers(me);
- flush_itimer_signals();
-#endif
-
/*
* Make the signal table private.
*/
^ permalink raw reply [flat|nested] 6+ messages in thread