[PATCH 1/3] proc: first_tid: fix the potential use-after-free

[Oleg Nesterov <oleg@redhat.com>]

proc_task_readdir() verifies that the result of get_proc_task()
is pid_alive() and thus its ->group_leader is fine too. However
this is not necessarily true after rcu_read_unlock(), we need
to recheck this after first_tid() does rcu_read_lock() again.

The race is subtle and unlikely, but still it is possible afaics.
To simplify lets ignore the "likely" case when tid != 0, f_version
can be cleared by proc_task_operations->llseek().

Suppose we have a main thread M and its subthread T. Suppose that
f_pos == 3, iow first_tid() should return T. Now suppose that the
following happens between rcu_read_unlock() and rcu_read_lock():

	1. T execs and becomes the new leader. This removes M from
	    ->thread_group but next_thread(M) is still T.

	2. T creates another thread X which does exec as well, T
	   goes away.

	3. X creates another subthread, this increments nr_threads.

	4. first_tid() does next_thread(M) and returns the already
	   dead T.

Note that we need 2. and 3. only because of get_nr_threads() check,
and this check was supposed to be optimization only.

Note: I think that proc_task_readdir/first_tid interaction can be
simplified, but this needs another patch. proc_task_readdir() should
not play with ->group_leader at all. See the next patches.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 fs/proc/base.c |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index dd51e50..c939c9f 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -3186,10 +3186,13 @@ static struct task_struct *first_tid(struct task_struct *leader,
 			goto found;
 	}
 
-	/* If nr exceeds the number of threads there is nothing todo */
 	pos = NULL;
+	/* If nr exceeds the number of threads there is nothing todo */
 	if (nr && nr >= get_nr_threads(leader))
 		goto out;
+	/* It could be unhashed before we take rcu lock */
+	if (!pid_alive(leader))
+		goto out;
 
 	/* If we haven't found our starting place yet start
 	 * with the leader and walk nr threads forward.
-- 
1.5.5.1