sys_ioprio_get() accesses __task_cred() without being in a RCU read
side critical section. tasklist_lock is not protecting that when
CONFIG_TREE_PREEMPT_RCU=y.

Add a rcu_read_lock/unlock() section around the code which accesses
__task_cred().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: David Howells <dhowells@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: linux-security-module@vger.kernel.org
Cc: linux-fsdevel@vger.kernel.org
Cc: Al Viro <viro@zeniv.linux.org.uk>
---
 fs/ioprio.c |    2 ++
 1 file changed, 2 insertions(+)

Index: linux-2.6-tip/fs/ioprio.c
===================================================================
--- linux-2.6-tip.orig/fs/ioprio.c
+++ linux-2.6-tip/fs/ioprio.c
@@ -230,6 +230,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, 
 			if (!user)
 				break;
 
+			rcu_read_lock();
 			do_each_thread(g, p) {
 				if (__task_cred(p)->uid != user->uid)
 					continue;
@@ -241,6 +242,7 @@ SYSCALL_DEFINE2(ioprio_get, int, which, 
 				else
 					ret = ioprio_best(ret, tmpio);
 			} while_each_thread(g, p);
+			rcu_read_unlock();
 
 			if (who)
 				free_uid(user);