rcu_read_lock/unlock protect find_task_by_vpid call in check_clock

[Dave Jones <davej@redhat.com>]

===================================================
[ 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 scrashme/13382:
 #0:  (tasklist_lock){.?.?..}, at: [<ffffffff8106ddea>] check_clock+0x46/0x9a

stack backtrace:
Pid: 13382, comm: scrashme Not tainted 2.6.37-rc3+ #8
Call Trace:
 [<ffffffff8107cfe1>] lockdep_rcu_dereference+0x9d/0xa5
 [<ffffffff81069d08>] find_task_by_pid_ns+0x44/0x5d
 [<ffffffff81069d43>] find_task_by_vpid+0x22/0x24
 [<ffffffff8106ddf2>] check_clock+0x4e/0x9a
 [<ffffffff8106deac>] posix_cpu_clock_getres+0x16/0x41
 [<ffffffff8106be74>] sys_clock_getres+0x39/0xa0
 [<ffffffff81009cb2>] system_call_fastpath+0x16/0x1b

Signed-off-by: Dave Jones <davej@redhat.com>

diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c
index 6842eeb..4bef9aa 100644
--- a/kernel/posix-cpu-timers.c
+++ b/kernel/posix-cpu-timers.c
@@ -38,11 +38,13 @@ static int check_clock(const clockid_t which_clock)
 		return 0;
 
 	read_lock(&tasklist_lock);
+	rcu_read_lock();
 	p = find_task_by_vpid(pid);
 	if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ?
 		   same_thread_group(p, current) : thread_group_leader(p))) {
 		error = -EINVAL;
 	}
+	rcu_read_unlock();
 	read_unlock(&tasklist_lock);
 
 	return error;