[PATCH 2/3] seccomp: release task filters when the task exits

Andrei Vagin
2024-05-23T01:45:55Z

Previously, seccomp filters were released in release_task(), which
required the process to exit and its zombie to be collected. However,
exited threads/processes can't trigger any seccomp events, making it
more logical to release filters upon task exits.

This adjustment simplifies scenarios where a parent is tracing its child
process. The parent process can now handle all events from a seccomp
listening descriptor and then call wait to collect a child zombie.

seccomp_filter_release takes the siglock to avoid races with
seccomp_sync_threads. There was an idea to bypass taking the lock by
checking PF_EXITING, but it can be set without holding siglock if
threads have SIGNAL_GROUP_EXIT. This means it can happen concurently
with seccomp_filter_release.

Signed-off-by: Andrei Vagin <avagin@google.com>
---
kernel/exit.c | 3 ++-
kernel/seccomp.c | 22 ++++++++++++++++------
2 files changed, 18 insertions(+), 7 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 41a12630cbbc..23439c021d8d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -278,7 +278,6 @@ void release_task(struct task_struct *p)
}

write_unlock_irq(&tasklist_lock);
- seccomp_filter_release(p);
proc_flush_pid(thread_pid);
put_pid(thread_pid);
release_thread(p);
@@ -836,6 +835,8 @@ void __noreturn do_exit(long code)
io_uring_files_cancel();
exit_signals(tsk); /* sets PF_EXITING */

+ seccomp_filter_release(tsk);
+
acct_update_integrals(tsk);
group_dead = atomic_dec_and_test(&tsk->signal->live);
if (group_dead) {
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index 35435e8f1035..67305e776dd3 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -502,6 +502,9 @@ static inline pid_t seccomp_can_sync_threads(void)
/* Skip current, since it is initiating the sync. */
if (thread == caller)
continue;
+ /* Skip exited threads. */
+ if (thread->flags & PF_EXITING)
+ continue;

if (thread->seccomp.mode == SECCOMP_MODE_DISABLED ||
(thread->seccomp.mode == SECCOMP_MODE_FILTER &&
@@ -563,18 +566,18 @@ static void __seccomp_filter_release(struct seccomp_filter *orig)
* @tsk: task the filter should be released from.
*
* This function should only be called when the task is exiting as
- * it detaches it from its filter tree. As such, READ_ONCE() and
- * barriers are not needed here, as would normally be needed.
+ * it detaches it from its filter tree. PF_EXITING has to be set
+ * for the task.
*/
void seccomp_filter_release(struct task_struct *tsk)
{
- struct seccomp_filter *orig = tsk->seccomp.filter;
-
- /* We are effectively holding the siglock by not having any sighand. */
- WARN_ON(tsk->sighand != NULL);
+ struct seccomp_filter *orig;

+ spin_lock_irq(&current->sighand->siglock);
+ orig = tsk->seccomp.filter;
/* Detach task from its filter tree. */
tsk->seccomp.filter = NULL;
+ spin_unlock_irq(&current->sighand->siglock);
__seccomp_filter_release(orig);
}

@@ -602,6 +605,13 @@ static inline void seccomp_sync_threads(unsigned long flags)
if (thread == caller)
continue;

+ /*
+ * Skip exited threads. seccomp_filter_release could have
+ * been already called for this task.
+ */
+ if (thread->flags & PF_EXITING)
+ continue;
+
/* Get a task reference for the new leaf node. */
get_seccomp_filter(caller);

--
2.45.1.288.g0e0cd299f1-goog