ioprio: fix RCU locking around task dereference
commit1447399b3e34af016c368b4178db7ef0e04e15b0
authorDaniel J Blueman <daniel.blueman@gmail.com>
Tue, 9 Nov 2010 20:33:02 +0000 (9 21:33 +0100)
committerJens Axboe <jaxboe@fusionio.com>
Wed, 10 Nov 2010 13:40:53 +0000 (10 14:40 +0100)
treee5de50c5520b2c20359c2afbf903e052b88bb995
parenta014741c0adfb8fb79952939ca087cf03d272bb9
ioprio: fix RCU locking around task dereference

With 2.6.37-rc1, I observe sys_ioprio_set not taking the RCU lock [1]
across access to the task credentials.

Inspecting the code in fs/ioprio.c, the tasklist_lock is held for read
across the __task_cred call, which is presumably sufficient to prevent
the task credentials becoming stale.

===================================================

[ INFO: suspicious rcu_dereference_check() usage. ]

---------------------------------------------------

kernel/pid.c:419 invoked rcu_dereference_check() without protection!

other info that might help us debug this:

rcu_scheduler_active = 1, debug_locks = 1

1 lock held by start-stop-daem/2246:

 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff811a2dfa>]
sys_ioprio_set+0x8a/0x400

stack backtrace:

Pid: 2246, comm: start-stop-daem Not tainted 2.6.37-rc1-330cd+ #2

Call Trace:

 [<ffffffff8109f5f4>] lockdep_rcu_dereference+0xa4/0xc0

 [<ffffffff81085651>] find_task_by_pid_ns+0x81/0x90

 [<ffffffff8108567d>] find_task_by_vpid+0x1d/0x20

 [<ffffffff811a3160>] sys_ioprio_set+0x3f0/0x400

 [<ffffffff816efa79>] ? trace_hardirqs_on_thunk+0x3a/0x3f

 [<ffffffff81003482>] system_call_fastpath+0x16/0x1b

Take the RCU lock for read across acquiring the pointer to the task
credentials and dereferencing it.

Signed-off-by: Daniel J Blueman <daniel.blueman@gmail.com>
Fixed up by Jens to fix missing rcu_read_unlock() on mismatches.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
fs/ioprio.c