Re: [PATCH] signal: Prevent exec() race

[David Laight]

On Mon, 31 Aug 2026 12:50:46 +0200
Thomas Gleixner <tglx@xxxxxxxxxx> wrote:

> 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 is due to a recent commit which moved the sigqueue flush
> out of the sighand lock held region. Before that it was properly
> serialized.
> 
> Hyonwoo proposed to fix this by using list_del_init_careful(), but that
> just papers over the underlying 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.
> 
>     This can be done unlocked because PF_EXITING prevents further signals
>     to be queued and there is no other code which accesses task::pending.
> 
> Fixes: fb3bbcfe344e ("exit: change the release_task() paths to call flush_sigqueue() lockless")
> Reported-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
> Debugged-by: Hyunwoo Kim <imv4bel@xxxxxxxxx>
> Suggested-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Closes: https://patch.msgid.link/aok1rdkBgZsynHZB@v4bel
> ---
>  kernel/exit.c   |   11 ++++++-----
>  kernel/signal.c |   23 ++++++++++++++++++++++-
>  2 files changed, 28 insertions(+), 6 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
> @@ -1030,6 +1030,10 @@ 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;

Is that unlikely() going do the right thing?
Pretty much the only way to avoid a branch in the 'usual' path is to test
PF_EXITING first.
So you could do:
	if (unlikely(t->flags & PF_EXITING) && type == PIDTYPE_PID)
		goto ret;
(assuming t->flags is unlikely to be a cache miss).
Or, if you can persuade the compiler not to use a branch for the ?:
	if (unlikely(t->flags & (type == PIDTYPE_PID ? PF_EXITING : 0)))
		goto ret;

David

> +
>  	if (!prepare_signal(sig, t, force))
>  		goto ret;
>  
> @@ -1990,6 +1994,9 @@ 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)))
> +		return;
> +
>  	/*
>  	 * Update @tmr::sigqueue_seq for posix timer signals with sighand
>  	 * locked to prevent a race against dequeue_signal().
> @@ -3118,6 +3125,16 @@ static void retarget_shared_pending(stru
>  	}
>  }
>  
> +/*
> + * tsk::flags has PF_EXITING set which prevents signals to be queued on
> + * tsk::pending. Nothing else can touch tsk::pending anymore so it can be
> + * flushed lockless.
> + */
> +static inline void flush_pending_unlocked(struct task_struct *tsk)
> +{
> +	flush_sigqueue(&tsk->pending);
> +}
> +
>  void exit_signals(struct task_struct *tsk)
>  {
>  	int group_stop = 0;
> @@ -3130,8 +3147,10 @@ void exit_signals(struct task_struct *ts
>  	cgroup_threadgroup_change_begin(tsk);
>  
>  	if (thread_group_empty(tsk) || (tsk->signal->flags & SIGNAL_GROUP_EXIT)) {
> -		tsk->flags |= PF_EXITING;
> +		scoped_guard(spinlock_irq, &tsk->sighand->siglock)
> +			tsk->flags |= PF_EXITING;
>  		cgroup_threadgroup_change_end(tsk);
> +		flush_pending_unlocked(tsk);
>  		return;
>  	}
>  
> @@ -3157,6 +3176,8 @@ void exit_signals(struct task_struct *ts
>  out:
>  	spin_unlock_irq(&tsk->sighand->siglock);
>  
> +	flush_pending_unlocked(tsk);
> +
>  	/*
>  	 * If group stop has completed, deliver the notification.  This
>  	 * should always go to the real parent of the group leader.
>