[patch V2 0/8] exec/exit: POSIX timer related bugfixes and related cleanups

[patch V2 0/8] exec/exit: POSIX timer related bugfixes and related cleanups

[Thomas Gleixner]

This is a follow up to V1 which can be found here:

  https://lore.kernel.org/20260904112100.683893401@kernel.org

Recent findings from Hyunwoo unearthed two bugs in handling POSIX timers on
exec().

The relevant patches, reports and discussions can be found here:

   https://patch.msgid.link/aok1rdkBgZsynHZB@v4bel
   https://patch.msgid.link/ao7Q8miiuLAPVnWv@v4bel

TLDR:

Both problems are related to non-leader exec(). POSIX CPU timers which are
targeted at tasks hold a pid reference of the target task, which is used to
look up the task in the related POSIX timer operations.

The non-leader exec() switches the TID of the old and the new leader, which
obviously invalidates these references for pid_task(PIDTYPE_PID) lookups.

This causes UAFs due to the resulting list corruptions or premature freeing
without removing the underlying POSIX CPU timers from the involved tasks.

The first issue which corrupts the signal pending list is solved by:

    - Preventing the queueing of per task signals on a task which has
      PF_EXITING set.

    - Protecting the unlocked setting of PF_EXITING in exit_signals() with
      sighand lock.

    - Flushing all per task signals right in exit_signals()

The second issue which keeps the POSIX CPU timers queued on the new leader
is solved by:

    - Moving the exec related POSIX timer cleanup right after de_thread()
      which ensures that the timers queued in new_leader::posix_cputimers
      are removed before the underlying POSIX timers are deleted.

After looking deeper at the exit() handling it turned out that the POSIX
timer cleanups can be done early in do_exit() instead of delaying them
until release_task().

The reason for this late cleanup is that POSIX CPU timers can be created,
rearmed and deleted as long as a task is visible, i.e. the pid is hashed
and sighand is not NULL. This allows to retrieve information from the timer
up to the point where the task is gone for real and that can't be changed
easily as that'd be a user visible change.

But once PF_EXITING is set on a task the task does not longer expire POSIX
CPU timers. So it makes no sense that the timers stay queued in
task::posix_cputimers after that point.

The only thing which needs to be prevented is that timers are requeued on
task::posix_cputimers once PF_EXITING is set or requeued on
signal::posix_cputimers when PF_EXITING is set and signal::live is zero,
which indicates that the thread group is dead.

With that solved the timers can be dequeued from task::posix_cputimer
pending when a task exits and from signal::posix_cputimer pending once the
threadgroup reaches the dead state, i.e. signal::live goes to zero in
do_exit().

The changes vs. V1:

    - Prevent requeuing POSIX timer signals in posixtimer_sig_unignore()
      when the target is exiting - Oleg, Frederic

    - Use signal::flags SIGNAL_GROUP_EXIT instead of signal::live to
      determine whether there is a group exit in progress - Eric

    - Refine existing and add new comments

The delta patch against V1 is below.

The series applies on 7.3-rc1 and is avalaible from git:

    git://git.kernel.org/pub/scm/linux/kernel/git/tglx/devel.git posix-timers

Thanks,

	tglx

---
diff --git a/fs/exec.c b/fs/exec.c
index caed7c3566ca..67418118df5a 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1156,9 +1156,9 @@ int begin_new_exec(struct linux_binprm * bprm)
 	/*
 	 * 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.
+	 * Otherwise in case of a TID switch the deletion of the related POSIX
+	 * timer would not remove an enqueued timer because the TID lookup
+	 * of the old TID fails.
 	 */
 	posixtimer_exec();
 
diff --git a/kernel/signal.c b/kernel/signal.c
index a3c6a47460af..f93d8f77ec1a 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1029,6 +1029,21 @@ static inline bool legacy_queue(struct sigpending *signals, int sig)
 	return (sig < SIGRTMIN) && sigismember(&signals->signal, sig);
 }
 
+/*
+ * When PF_EXITING is set the task is on the way out and has t::pending
+ * flushed already. Prevent queueing of PIDTYPE_PID signals as they would
+ * be leaked.
+ */
+static inline bool task_can_queue_signal(struct task_struct *t, enum pid_type type)
+{
+	lockdep_assert_held(&t->sighand->siglock);
+
+	if (!(t->flags & PF_EXITING))
+		return true;
+
+	return type != PIDTYPE_PID;
+}
+
 static int __send_signal_locked(int sig, struct kernel_siginfo *info,
 				struct task_struct *t, enum pid_type type, bool force)
 {
@@ -1041,7 +1056,7 @@ static int __send_signal_locked(int sig, struct kernel_siginfo *info,
 
 	result = TRACE_SIGNAL_IGNORED;
 
-	if (unlikely(type == PIDTYPE_PID && (t->flags & PF_EXITING)))
+	if (!task_can_queue_signal(t, type))
 		goto ret;
 
 	if (!prepare_signal(sig, t, force))
@@ -1982,11 +1997,25 @@ static inline struct task_struct *posixtimer_get_target(struct k_itimer *tmr)
 	struct task_struct *t = pid_task(tmr->it_pid, tmr->it_pid_type);
 
 	if (t && tmr->it_pid_type != PIDTYPE_PID &&
-	    same_thread_group(t, current) && !current->exit_state)
+	    same_thread_group(t, current) && !(current->flags & PF_EXITING))
 		t = current;
 	return t;
 }
 
+/*
+ * Find the target task for the POSIX timer signal and prevent that a
+ * PIDTYPE_PID signal is queued on a task which has PF_EXITING set.
+ */
+static inline struct task_struct *posixtimer_get_unignore_target(struct k_itimer *tmr)
+{
+	struct task_struct *t = posixtimer_get_target(tmr);
+
+	if (t && task_can_queue_signal(t, tmr->it_pid_type))
+		return t;
+
+	return NULL;
+}
+
 void posixtimer_send_sigqueue(struct k_itimer *tmr)
 {
 	struct sigqueue *q = &tmr->sigq;
@@ -2004,7 +2033,7 @@ void posixtimer_send_sigqueue(struct k_itimer *tmr)
 	if (!likely(lock_task_sighand(t, &flags)))
 		return;
 
-	if (unlikely(tmr->it_pid_type == PIDTYPE_PID && (t->flags & PF_EXITING)))
+	if (!task_can_queue_signal(t, tmr->it_pid_type))
 		goto unlock;
 
 	/*
@@ -2154,7 +2183,7 @@ static void posixtimer_sig_unignore(struct task_struct *tsk, int sig)
 		 * has exited by now, drop the reference count.
 		 */
 		guard(rcu)();
-		target = posixtimer_get_target(tmr);
+		target = posixtimer_get_unignore_target(tmr);
 		if (target)
 			posixtimer_queue_sigqueue(&tmr->sigq, target, tmr->it_pid_type);
 		else
diff --git a/kernel/time/posix-cpu-timers.c b/kernel/time/posix-cpu-timers.c
index 29ee485d4d0b..53e47c1b56c2 100644
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -686,13 +686,25 @@ void posix_cpu_timers_exit_group(void)
 	cleanup_timers(&current->signal->posix_cputimers);
 }
 
-static inline bool task_can_enqueue(struct k_itimer *timer, struct task_struct *p)
+/*
+ * This function validates that POSIX CPU timers can be safely enqueued on the
+ * target task.
+ *
+ * Enqueue is allowed when PF_EXITING is not set. If set then it is only allowed
+ * for process shared timers (type = PIDTYPE_TGID) as long as tsk::signal::flags
+ * does not have SIGNAL_GROUP_EXIT set. PIDTYPE_PID targets are not allowed at
+ * all when the task has PF_EXITING set.
+ *
+ * This guarantees that after the POSIX timer cleanup in posixtimer_exit() no
+ * POSIX CPU timers are queued on the task or in case of a group exit on the
+ * process.
+ */
+static inline bool task_can_enqueue_timer(struct task_struct *tsk, enum pid_type type)
 {
-	if (likely(!(p->flags & PF_EXITING)))
+	if (likely(!(tsk->flags & PF_EXITING)))
 		return true;
 
-	/* Allow TGID type unless the last thread is on the way out. */
-	return clock_pid_type(timer->it_clock) == PIDTYPE_TGID && atomic_read(&p->signal->live);
+	return type == PIDTYPE_TGID && !(tsk->signal->flags & SIGNAL_GROUP_EXIT);
 }
 
 /*
@@ -709,18 +721,7 @@ static void arm_timer(struct k_itimer *timer, struct task_struct *p)
 
 	timer->it_status = POSIX_TIMER_ARMED;
 
-	/*
-	 * Don't enqueue timers when the task or the group is exiting. That
-	 * ensures that timer operations are still succeeding as long as the
-	 * tasks are visible, but won't enqueue the timers on the task or
-	 * process. They won't expire anyway because run_posix_cpu_timers()
-	 * exits early when PF_EXITING is set.
-	 *
-	 * Enqueue is skipped if PF_EXITING is set when the timer is per task
-	 * and when the last thread decremented p::signal::live to zero also for
-	 * per process timers.
-	 */
-	if (unlikely(!task_can_enqueue(timer, p)))
+	if (unlikely(!task_can_enqueue_timer(p, clock_pid_type(timer->it_clock))))
 		return;
 
 	if (!cpu_timer_enqueue(&base->tqhead, ctmr))

[patch V2 1/8] signal: Prevent exec() race

[Thomas Gleixner]

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
---
V4: Prevent requeuing of signals which are unignored - Oleg, Eric

    Split out the decision into an inline which can be reused by the posix
    CPU timer follow up changes.

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 |  103 +++++++++++++++++++++++++++++++++++++++-----------------
 2 files changed, 78 insertions(+), 36 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.
  */
@@ -1019,6 +1029,21 @@ static inline bool legacy_queue(struct s
 	return (sig < SIGRTMIN) && sigismember(&signals->signal, sig);
 }
 
+/*
+ * When PF_EXITING is set the task is on the way out and has t::pending
+ * flushed already. Prevent queueing of PIDTYPE_PID signals as they would
+ * be leaked.
+ */
+static inline bool task_can_queue_signal(struct task_struct *t, enum pid_type type)
+{
+	lockdep_assert_held(&t->sighand->siglock);
+
+	if (!(t->flags & PF_EXITING))
+		return true;
+
+	return type != PIDTYPE_PID;
+}
+
 static int __send_signal_locked(int sig, struct kernel_siginfo *info,
 				struct task_struct *t, enum pid_type type, bool force)
 {
@@ -1030,6 +1055,10 @@ static int __send_signal_locked(int sig,
 	lockdep_assert_held(&t->sighand->siglock);
 
 	result = TRACE_SIGNAL_IGNORED;
+
+	if (!task_can_queue_signal(t, type))
+		goto ret;
+
 	if (!prepare_signal(sig, t, force))
 		goto ret;
 
@@ -1968,11 +1997,25 @@ static inline struct task_struct *posixt
 	struct task_struct *t = pid_task(tmr->it_pid, tmr->it_pid_type);
 
 	if (t && tmr->it_pid_type != PIDTYPE_PID &&
-	    same_thread_group(t, current) && !current->exit_state)
+	    same_thread_group(t, current) && !(current->flags & PF_EXITING))
 		t = current;
 	return t;
 }
 
+/*
+ * Find the target task for the POSIX timer signal and prevent that a
+ * PIDTYPE_PID signal is queued on a task which has PF_EXITING set.
+ */
+static inline struct task_struct *posixtimer_get_unignore_target(struct k_itimer *tmr)
+{
+	struct task_struct *t = posixtimer_get_target(tmr);
+
+	if (t && task_can_queue_signal(t, tmr->it_pid_type))
+		return t;
+
+	return NULL;
+}
+
 void posixtimer_send_sigqueue(struct k_itimer *tmr)
 {
 	struct sigqueue *q = &tmr->sigq;
@@ -1990,6 +2033,9 @@ void posixtimer_send_sigqueue(struct k_i
 	if (!likely(lock_task_sighand(t, &flags)))
 		return;
 
+	if (!task_can_queue_signal(t, tmr->it_pid_type))
+		goto unlock;
+
 	/*
 	 * Update @tmr::sigqueue_seq for posix timer signals with sighand
 	 * locked to prevent a race against dequeue_signal().
@@ -2081,6 +2127,7 @@ 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);
 }
 
@@ -2136,7 +2183,7 @@ static void posixtimer_sig_unignore(stru
 		 * has exited by now, drop the reference count.
 		 */
 		guard(rcu)();
-		target = posixtimer_get_target(tmr);
+		target = posixtimer_get_unignore_target(tmr);
 		if (target)
 			posixtimer_queue_sigqueue(&tmr->sigq, target, tmr->it_pid_type);
 		else
@@ -3120,42 +3167,36 @@ 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

[patch V2 2/8] exec: Cleanup POSIX timers right after de_thread()

[Thomas Gleixner]

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.
+	 * Otherwise in case of a TID switch the deletion of the related POSIX
+	 * timer would not remove an enqueued timer because the TID lookup
+	 * of the old TID fails.
+	 */
+	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.
 	 */

[patch V2 3/8] posix-timers: Move posixtimer_exec_cleanup() out of exec.c

[Thomas Gleixner]

Move it to the POSIX timer code and provide a proper stub when POSIX timers
are disabled in Kconfig.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 fs/exec.c                    |   13 +------------
 include/linux/posix-timers.h |    3 +++
 kernel/time/posix-timers.c   |    9 +++++++++
 3 files changed, 13 insertions(+), 12 deletions(-)

--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1115,17 +1115,6 @@ 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
@@ -1171,7 +1160,7 @@ int begin_new_exec(struct linux_binprm *
 	 * timer would not remove an enqueued timer because the TID lookup
 	 * of the old TID fails.
 	 */
-	posixtimer_exec(me);
+	posixtimer_exec();
 
 	/* see the comment in check_unsafe_exec() */
 	current->fs->in_exec = 0;
--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -232,6 +232,8 @@ void set_process_cpu_timer(struct task_s
 int update_rlimit_cpu(struct task_struct *task, unsigned long rlim_new);
 
 #ifdef CONFIG_POSIX_TIMERS
+void posixtimer_exec(void);
+
 static inline void posixtimer_putref(struct k_itimer *tmr)
 {
 	if (rcuref_put(&tmr->rcuref))
@@ -259,6 +261,7 @@ static inline bool posixtimer_valid(cons
 	return !(val & 0x1UL);
 }
 #else  /* CONFIG_POSIX_TIMERS */
+static inline void posixtimer_exec(void) { }
 static inline void posixtimer_sigqueue_getref(struct sigqueue *q) { }
 static inline void posixtimer_sigqueue_putref(struct sigqueue *q) { }
 #endif /* !CONFIG_POSIX_TIMERS */
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1120,6 +1120,15 @@ void exit_itimers(struct task_struct *ts
 	}
 }
 
+void posixtimer_exec(void)
+{
+	scoped_guard(spinlock_irq, &current->sighand->siglock)
+		posix_cpu_timers_exit(current);
+
+	exit_itimers(current);
+	flush_itimer_signals();
+}
+
 SYSCALL_DEFINE2(clock_settime, const clockid_t, which_clock,
 		const struct __kernel_timespec __user *, tp)
 {

[patch V2 4/8] posix-timers: Move POSIX timer group exit related code out of do_exit()

[Thomas Gleixner]

Move the POSIX timer group exit handling into the posix timer code and
provide a proper stub when POSIX timers are disabled in Kconfig.

No functional change.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 include/linux/posix-timers.h |    2 ++
 include/linux/sched/task.h   |    1 -
 kernel/exit.c                |    7 +++----
 kernel/time/posix-timers.c   |   16 +++++++++-------
 4 files changed, 14 insertions(+), 12 deletions(-)

--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -233,6 +233,7 @@ int update_rlimit_cpu(struct task_struct
 
 #ifdef CONFIG_POSIX_TIMERS
 void posixtimer_exec(void);
+void posixtimer_exit(void);
 
 static inline void posixtimer_putref(struct k_itimer *tmr)
 {
@@ -262,6 +263,7 @@ static inline bool posixtimer_valid(cons
 }
 #else  /* CONFIG_POSIX_TIMERS */
 static inline void posixtimer_exec(void) { }
+static inline void posixtimer_exit(void) { }
 static inline void posixtimer_sigqueue_getref(struct sigqueue *q) { }
 static inline void posixtimer_sigqueue_putref(struct sigqueue *q) { }
 #endif /* !CONFIG_POSIX_TIMERS */
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -94,7 +94,6 @@ static inline void exit_thread(struct ta
 extern __noreturn void do_group_exit(int);
 
 extern void exit_files(struct task_struct *);
-extern void exit_itimers(struct task_struct *);
 
 extern pid_t kernel_clone(struct kernel_clone_args *kargs);
 struct task_struct *copy_process(struct pid *pid, int trace, int node,
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -963,13 +963,12 @@ void __noreturn do_exit(long code)
 			panic("Attempted to kill init! exitcode=0x%08x\n",
 				tsk->signal->group_exit_code ?: (int)code);
 
-#ifdef CONFIG_POSIX_TIMERS
-		hrtimer_cancel(&tsk->signal->real_timer);
-		exit_itimers(tsk);
-#endif
+		posixtimer_exit();
+
 		if (tsk->mm)
 			setmax_mm_hiwater_rss(&tsk->signal->maxrss, tsk->mm);
 	}
+
 	acct_collect(code, group_dead);
 	if (group_dead)
 		tty_audit_exit();
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1077,13 +1077,9 @@ SYSCALL_DEFINE1(timer_delete, timer_t, t
 	return 0;
 }
 
-/*
- * Invoked from do_exit() when the last thread of a thread group exits.
- * At that point no other task can access the timers of the dying
- * task anymore.
- */
-void exit_itimers(struct task_struct *tsk)
+static void posixtimer_delete_timers(void)
 {
+	struct task_struct *tsk = current;
 	struct hlist_head timers;
 	struct hlist_node *next;
 	struct k_itimer *timer;
@@ -1120,12 +1116,18 @@ void exit_itimers(struct task_struct *ts
 	}
 }
 
+void posixtimer_exit(void)
+{
+	hrtimer_cancel(&current->signal->real_timer);
+	posixtimer_delete_timers();
+}
+
 void posixtimer_exec(void)
 {
 	scoped_guard(spinlock_irq, &current->sighand->siglock)
 		posix_cpu_timers_exit(current);
 
-	exit_itimers(current);
+	posixtimer_delete_timers();
 	flush_itimer_signals();
 }
 

[patch V2 5/8] posix-cpu-timers: Move inlines out of public header

[Thomas Gleixner]

They are only used in the POSIX CPU timer code. No point in exposing them
globally and parsing them for nothing.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 include/linux/posix-timers.h   |   32 --------------------------------
 kernel/time/posix-cpu-timers.c |   32 ++++++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 32 deletions(-)

--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -66,38 +66,6 @@ struct cpu_timer {
 	struct task_struct __rcu	*handling;
 };
 
-static inline bool cpu_timer_enqueue(struct timerqueue_head *head,
-				     struct cpu_timer *ctmr)
-{
-	ctmr->head = head;
-	return timerqueue_add(head, &ctmr->node);
-}
-
-static inline bool cpu_timer_queued(struct cpu_timer *ctmr)
-{
-	return !!ctmr->head;
-}
-
-static inline bool cpu_timer_dequeue(struct cpu_timer *ctmr)
-{
-	if (cpu_timer_queued(ctmr)) {
-		timerqueue_del(ctmr->head, &ctmr->node);
-		ctmr->head = NULL;
-		return true;
-	}
-	return false;
-}
-
-static inline u64 cpu_timer_getexpires(struct cpu_timer *ctmr)
-{
-	return ctmr->node.expires;
-}
-
-static inline void cpu_timer_setexpires(struct cpu_timer *ctmr, u64 exp)
-{
-	ctmr->node.expires = exp;
-}
-
 static inline void posix_cputimers_init(struct posix_cputimers *pct)
 {
 	memset(pct, 0, sizeof(*pct));
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -438,6 +438,38 @@ static void trigger_base_recalc_expires(
 	base->nextevt = 0;
 }
 
+static inline bool cpu_timer_enqueue(struct timerqueue_head *head,
+				     struct cpu_timer *ctmr)
+{
+	ctmr->head = head;
+	return timerqueue_add(head, &ctmr->node);
+}
+
+static inline bool cpu_timer_queued(struct cpu_timer *ctmr)
+{
+	return !!ctmr->head;
+}
+
+static inline bool cpu_timer_dequeue(struct cpu_timer *ctmr)
+{
+	if (cpu_timer_queued(ctmr)) {
+		timerqueue_del(ctmr->head, &ctmr->node);
+		ctmr->head = NULL;
+		return true;
+	}
+	return false;
+}
+
+static inline u64 cpu_timer_getexpires(struct cpu_timer *ctmr)
+{
+	return ctmr->node.expires;
+}
+
+static inline void cpu_timer_setexpires(struct cpu_timer *ctmr, u64 exp)
+{
+	ctmr->node.expires = exp;
+}
+
 /*
  * Dequeue the timer and reset the base if it was its earliest expiration.
  * It makes sure the next tick recalculates the base next expiration so we

[patch V2 6/8] posix-cpu-timers: Use PF_EXITING to indicate exit

[Thomas Gleixner]

Right now POSIX CPU timers use task::exit_state to check whether a task is
exiting. That works correctly, but exit_state is set later in do_exit() and
too late for allowing to cleanup POSIX timers earlier.

It does not matter in case of exit whether the cutoff is a bit earlier.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 kernel/time/posix-cpu-timers.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -1505,7 +1505,7 @@ void run_posix_cpu_timers(void)
 	 * posix_cpu_timer_del() may fail to lock_task_sighand(tsk) and
 	 * miss timer->it.cpu.firing != 0.
 	 */
-	if (tsk->exit_state)
+	if (tsk->flags & PF_EXITING)
 		return;
 
 	/*

[patch V2 7/8] posix-cpu-timers: Prevent enqueueing when PF_EXITING is set

[Thomas Gleixner]

To prepare for cleaning up POSIX CPU timers in do_exit(), prevent
enqueueing POSIX CPU timers on a task which has PF_EXITING set.  Queueing a
timer on such a task is pointless because the task won't expire the timer
anymore.

The same applies to process wide timers when tsk::signal::flags has
SIGNAL_GROUP_EXIT set.

Pretending that the timer is armed allows to keep the POSIX timer mechanism
"working" so that the timer stays accessible up to the point where a task
is unhashed.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 kernel/time/posix-cpu-timers.c |   28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -628,6 +628,7 @@ static int posix_cpu_timer_del(struct k_
 	}
 
 	if (!ret) {
+		WARN_ON_ONCE(cpu_timer_queued(&timer->it.cpu));
 		put_pid(timer->it.cpu.pid);
 		timer->it_status = POSIX_TIMER_DISARMED;
 	}
@@ -675,6 +676,27 @@ void posix_cpu_timers_exit_group(struct
 }
 
 /*
+ * This function validates that POSIX CPU timers can be safely enqueued on the
+ * target task.
+ *
+ * Enqueue is allowed when PF_EXITING is not set. If set then it is only allowed
+ * for process shared timers (type = PIDTYPE_TGID) as long as tsk::signal::flags
+ * does not have SIGNAL_GROUP_EXIT set. PIDTYPE_PID targets are not allowed at
+ * all when the task has PF_EXITING set.
+ *
+ * This guarantees that after the POSIX timer cleanup in posixtimer_exit() no
+ * POSIX CPU timers are queued on the task or in case of a group exit on the
+ * process.
+ */
+static inline bool task_can_enqueue_timer(struct task_struct *tsk, enum pid_type type)
+{
+	if (likely(!(tsk->flags & PF_EXITING)))
+		return true;
+
+	return type == PIDTYPE_TGID && !(tsk->signal->flags & SIGNAL_GROUP_EXIT);
+}
+
+/*
  * Insert the timer on the appropriate list before any timers that
  * expire later.  This must be called with the sighand lock held.
  */
@@ -684,7 +706,13 @@ static void arm_timer(struct k_itimer *t
 	struct cpu_timer *ctmr = &timer->it.cpu;
 	u64 newexp = cpu_timer_getexpires(ctmr);
 
+	lockdep_assert_held(&p->sighand->siglock);
+
 	timer->it_status = POSIX_TIMER_ARMED;
+
+	if (unlikely(!task_can_enqueue_timer(p, clock_pid_type(timer->it_clock))))
+		return;
+
 	if (!cpu_timer_enqueue(&base->tqhead, ctmr))
 		return;
 

[patch V2 8/8] posix-timers: Handle exit in do_exit() completely

[Thomas Gleixner]

Now that POSIX CPU timers cannot be enqueued on a task after PF_EXITING is
set and process wide timers cannot be enqueued when PF_EXITING is set and
the last thread in the group is exiting, it is possible to mop up POSIX
timers in do_exit() completely.

This requires to cancel an eventually pending POSIX CPU timer task work
right there because do_exit() invokes exit_task_work() later, which would
be acting on torn down data.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
---
 include/linux/posix-timers.h   |    6 ++----
 kernel/exit.c                  |   10 ++--------
 kernel/time/posix-cpu-timers.c |   38 +++++++++++++++++++++++++++++++-------
 kernel/time/posix-timers.c     |   15 +++++++++------
 kernel/time/posix-timers.h     |    3 +++
 5 files changed, 47 insertions(+), 25 deletions(-)

--- a/include/linux/posix-timers.h
+++ b/include/linux/posix-timers.h
@@ -192,8 +192,6 @@ struct k_itimer {
 } ____cacheline_aligned_in_smp;
 
 void run_posix_cpu_timers(void);
-void posix_cpu_timers_exit(struct task_struct *task);
-void posix_cpu_timers_exit_group(struct task_struct *task);
 void set_process_cpu_timer(struct task_struct *task, unsigned int clock_idx,
 			   u64 *newval, u64 *oldval);
 
@@ -201,7 +199,7 @@ int update_rlimit_cpu(struct task_struct
 
 #ifdef CONFIG_POSIX_TIMERS
 void posixtimer_exec(void);
-void posixtimer_exit(void);
+void posixtimer_exit(bool group_dead);
 
 static inline void posixtimer_putref(struct k_itimer *tmr)
 {
@@ -231,7 +229,7 @@ static inline bool posixtimer_valid(cons
 }
 #else  /* CONFIG_POSIX_TIMERS */
 static inline void posixtimer_exec(void) { }
-static inline void posixtimer_exit(void) { }
+static inline void posixtimer_exit(bool group_dead) { }
 static inline void posixtimer_sigqueue_getref(struct sigqueue *q) { }
 static inline void posixtimer_sigqueue_putref(struct sigqueue *q) { }
 #endif /* !CONFIG_POSIX_TIMERS */
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -167,12 +167,6 @@ static void __exit_signal(struct release
 					lockdep_tasklist_lock_is_held());
 	spin_lock(&sighand->siglock);
 
-#ifdef CONFIG_POSIX_TIMERS
-	posix_cpu_timers_exit(tsk);
-	if (group_dead)
-		posix_cpu_timers_exit_group(tsk);
-#endif
-
 	if (group_dead) {
 		tty = sig->tty;
 		sig->tty = NULL;
@@ -963,12 +957,12 @@ void __noreturn do_exit(long code)
 			panic("Attempted to kill init! exitcode=0x%08x\n",
 				tsk->signal->group_exit_code ?: (int)code);
 
-		posixtimer_exit();
-
 		if (tsk->mm)
 			setmax_mm_hiwater_rss(&tsk->signal->maxrss, tsk->mm);
 	}
 
+	posixtimer_exit(group_dead);
+
 	acct_collect(code, group_dead);
 	if (group_dead)
 		tty_audit_exit();
--- a/kernel/time/posix-cpu-timers.c
+++ b/kernel/time/posix-cpu-timers.c
@@ -661,18 +661,29 @@ static void cleanup_timers(struct posix_
 	cleanup_timerqueue(&pct->bases[CPUCLOCK_SCHED].tqhead);
 }
 
+static inline void posix_cpu_timers_exit_work(void);
+
 /*
- * These are both called with the siglock held, when the current thread
- * is being reaped.  When the final (leader) thread in the group is reaped,
- * posix_cpu_timers_exit_group will be called after posix_cpu_timers_exit.
+ * Invoked from posixtimer_exit_task() after PF_EXITING was set in tsk::flags or
+ * from posixtimer_exec_cleanup().
  */
-void posix_cpu_timers_exit(struct task_struct *tsk)
+void posix_cpu_timers_exit_task(void)
 {
-	cleanup_timers(&tsk->posix_cputimers);
+	posix_cpu_timers_exit_work();
+
+	guard(spinlock_irq)(&current->sighand->siglock);
+	cleanup_timers(&current->posix_cputimers);
 }
-void posix_cpu_timers_exit_group(struct task_struct *tsk)
+
+/*
+ * Invoked from posixtimer_exit_group() after PF_EXITING was set in tsk::flags.
+ */
+void posix_cpu_timers_exit_group(void)
 {
-	cleanup_timers(&tsk->signal->posix_cputimers);
+	posix_cpu_timers_exit_task();
+
+	guard(spinlock_irq)(&current->sighand->siglock);
+	cleanup_timers(&current->signal->posix_cputimers);
 }
 
 /*
@@ -1257,6 +1268,17 @@ static void posix_cpu_timers_work(struct
 	mutex_unlock(&cw->mutex);
 }
 
+static inline void posix_cpu_timers_exit_work(void)
+{
+	/*
+	 * current->flags has PF_EXITING set so this can be done lockless and
+	 * with interrupts enabled as PF_EXITING prevents the interrupt from
+	 * scheduling the work.
+	 */
+	if (current->posix_cputimers_work.scheduled)
+		task_work_cancel(current, &current->posix_cputimers_work.work);
+}
+
 /*
  * Invoked from the posix-timer core when a cancel operation failed because
  * the timer is marked firing. The caller holds rcu_read_lock(), which
@@ -1387,6 +1409,8 @@ static inline void __run_posix_cpu_timer
 	lockdep_posixtimer_exit();
 }
 
+static inline void posix_cpu_timers_exit_work(void) { }
+
 static void posix_cpu_timer_wait_running(struct k_itimer *timr)
 {
 	cpu_relax();
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -1116,17 +1116,20 @@ static void posixtimer_delete_timers(voi
 	}
 }
 
-void posixtimer_exit(void)
+void posixtimer_exit(bool group_dead)
 {
-	hrtimer_cancel(&current->signal->real_timer);
-	posixtimer_delete_timers();
+	if (group_dead) {
+		hrtimer_cancel(&current->signal->real_timer);
+		posix_cpu_timers_exit_group();
+		posixtimer_delete_timers();
+	} else {
+		posix_cpu_timers_exit_task();
+	}
 }
 
 void posixtimer_exec(void)
 {
-	scoped_guard(spinlock_irq, &current->sighand->siglock)
-		posix_cpu_timers_exit(current);
-
+	posix_cpu_timers_exit_task();
 	posixtimer_delete_timers();
 	flush_itimer_signals();
 }
--- a/kernel/time/posix-timers.h
+++ b/kernel/time/posix-timers.h
@@ -51,3 +51,6 @@ int common_timer_set(struct k_itimer *ti
 		     struct itimerspec64 *old_setting);
 void posix_timer_set_common(struct k_itimer *timer, struct itimerspec64 *new_setting);
 int common_timer_del(struct k_itimer *timer);
+
+void posix_cpu_timers_exit_task(void);
+void posix_cpu_timers_exit_group(void);