Commit 45c18b0b authored Nov 04, 2006 by Linus Torvalds

Fix unlikely (but possible) race condition on task->user access

There's a possible race condition when doing a "switch_uid()" from one
user to another, which could race with another thread doing a signal
allocation and looking at the old thread ->user pointer as it is freed.

This explains an oops reported by Lukasz Trabinski:
	http://permalink.gmane.org/gmane.linux.kernel/462241



We fix this by delaying the (reference-counted) freeing of the user
structure until the thread signal handler lock has been released, so
that we know that the signal allocation has either seen the new value or
has properly incremented the reference count of the old one.

Race identified by Oleg Nesterov.

Cc: Lukasz Trabinski <lukasz@wsisiz.edu.pl>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Andrew Morton <akpm@osdl.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

parent 80491eb9

kernel/user.c

+11 −0

Original line number	Diff line number	Diff line
		@@ -187,6 +187,17 @@ void switch_uid(struct user_struct *new_user)
		atomic_dec(&old_user->processes);
		switch_uid_keyring(new_user);
		current->user = new_user;

		/*
		* We need to synchronize with __sigqueue_alloc()
		* doing a get_uid(p->user).. If that saw the old
		* user value, we need to wait until it has exited
		* its critical region before we can free the old
		* structure.
		*/
		smp_mb();
		spin_unlock_wait(&current->sighand->siglock);

		free_uid(old_user);
		suid_keys(current);
		}