From 45c18b0bb579b5c1b89f8c99f1b6ffa4c586ba08 Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sat, 4 Nov 2006 10:06:02 -0800 Subject: [PATCH] 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 Cc: Oleg Nesterov Cc: Andrew Morton Cc: Ingo Molnar Signed-off-by: Linus Torvalds --- kernel/user.c | 11 +++++++++++ 1 file changed, 11 insertions(+) diff --git a/kernel/user.c b/kernel/user.c index 6408c042429..220e586127a 100644 --- a/kernel/user.c +++ b/kernel/user.c @@ -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(¤t->sighand->siglock); + free_uid(old_user); suid_keys(current); } -- 2.41.1