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Commit 3245d6ac authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Linus Torvalds
Browse files

exit: fix race between wait_consider_task() and wait_task_zombie() 



wait_consider_task() checks EXIT_ZOMBIE after EXIT_DEAD/EXIT_TRACE and
both checks can fail if we race with EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE
change in between, gcc needs to reload p->exit_state after
security_task_wait().  In this case ->notask_error will be wrongly
cleared and do_wait() can hang forever if it was the last eligible
child.

Many thanks to Arne who carefully investigated the problem.

Note: this bug is very old but it was pure theoretical until commit
b3ab0316 ("wait: completely ignore the EXIT_DEAD tasks").  Before
this commit "-O2" was probably enough to guarantee that compiler won't
read ->exit_state twice.

Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Reported-by: default avatarArne Goedeke <el@laramies.com>
Tested-by: default avatarArne Goedeke <el@laramies.com>
Cc: <stable@vger.kernel.org>	[3.15+]
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent eb4f73b4

kernel/exit.c

+9 −3
Original line number Diff line number Diff line
@@ -1287,9 +1287,15 @@ static int wait_task_continued(struct wait_opts *wo, struct task_struct *p) 
static int wait_consider_task(struct wait_opts *wo, int ptrace,

				struct task_struct *p)

{

	/*

	 * We can race with wait_task_zombie() from another thread.

	 * Ensure that EXIT_ZOMBIE -> EXIT_DEAD/EXIT_TRACE transition

	 * can't confuse the checks below.

	 */

	int exit_state = ACCESS_ONCE(p->exit_state);

	int ret;



	if (unlikely(p->exit_state == EXIT_DEAD))

	if (unlikely(exit_state == EXIT_DEAD))

		return 0;



	ret = eligible_child(wo, p);
@@ -1310,7 +1316,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, 
		return 0;

	}



	if (unlikely(p->exit_state == EXIT_TRACE)) {

	if (unlikely(exit_state == EXIT_TRACE)) {

		/*

		 * ptrace == 0 means we are the natural parent. In this case

		 * we should clear notask_error, debugger will notify us.
@@ -1337,7 +1343,7 @@ static int wait_consider_task(struct wait_opts *wo, int ptrace, 
	}



	/* slay zombie? */

	if (p->exit_state == EXIT_ZOMBIE) {

	if (exit_state == EXIT_ZOMBIE) {

		/* we don't reap group leaders with subthreads */

		if (!delay_group_leader(p)) {

			/*