Commit dfccbb5e authored Apr 07, 2014 by Oleg Nesterov Committed by Linus Torvalds Apr 07, 2014

wait: fix reparent_leader() vs EXIT_DEAD->EXIT_ZOMBIE race



wait_task_zombie() first does EXIT_ZOMBIE->EXIT_DEAD transition and
drops tasklist_lock.  If this task is not the natural child and it is
traced, we change its state back to EXIT_ZOMBIE for ->real_parent.

The last transition is racy, this is even documented in 50b8d257
"ptrace: partially fix the do_wait(WEXITED) vs EXIT_DEAD->EXIT_ZOMBIE
race".  wait_consider_task() tries to detect this transition and clear
->notask_error but we can't rely on ptrace_reparented(), debugger can
exit and do ptrace_unlink() before its sub-thread sets EXIT_ZOMBIE.

And there is another problem which were missed before: this transition
can also race with reparent_leader() which doesn't reset >exit_signal if
EXIT_DEAD, assuming that this task must be reaped by someone else.  So
the tracee can be re-parented with ->exit_signal != SIGCHLD, and if
/sbin/init doesn't use __WALL it becomes unreapable.

Change reparent_leader() to update ->exit_signal even if EXIT_DEAD.
Note: this is the simple temporary hack for -stable, it doesn't try to
solve all problems, it will be reverted by the next changes.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reported-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Reported-by: Michal Schmidt <mschmidt@redhat.com>
Tested-by: Michal Schmidt <mschmidt@redhat.com>
Cc: Al Viro <viro@ZenIV.linux.org.uk>
Cc: Lennart Poettering <lpoetter@redhat.com>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 23aebe16

kernel/exit.c

+11 −4

Original line number	Diff line number	Diff line
		@@ -560,9 +560,6 @@ static void reparent_leader(struct task_struct father, struct task_struct p,
		struct list_head *dead)
		{
		list_move_tail(&p->sibling, &p->real_parent->children);

		if (p->exit_state == EXIT_DEAD)
		return;
		/*
		* If this is a threaded reparent there is no need to
		* notify anyone anything has happened.
		@@ -570,9 +567,19 @@ static void reparent_leader(struct task_struct father, struct task_struct p,
		if (same_thread_group(p->real_parent, father))
		return;

		/* We don't want people slaying init. */
		/*
		* We don't want people slaying init.
		*
		* Note: we do this even if it is EXIT_DEAD, wait_task_zombie()
		* can change ->exit_state to EXIT_ZOMBIE. If this is the final
		* state, do_notify_parent() was already called and ->exit_signal
		* doesn't matter.
		*/
		p->exit_signal = SIGCHLD;

		if (p->exit_state == EXIT_DEAD)
		return;

		/* If it has exited notify the new parent about this child's death. */
		if (!p->ptrace &&
		p->exit_state == EXIT_ZOMBIE && thread_group_empty(p)) {

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