[PATCH RFC] pid: make setpgid() system call use RCU read-side critical section

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

[   23.584719]
[   23.584720] ===================================================
[   23.585059] [ INFO: suspicious rcu_dereference_check() usage. ]
[   23.585176] ---------------------------------------------------
[   23.585176] kernel/pid.c:419 invoked rcu_dereference_check() without protection!
[   23.585176]
[   23.585176] other info that might help us debug this:
[   23.585176]
[   23.585176]
[   23.585176] rcu_scheduler_active = 1, debug_locks = 1
[   23.585176] 1 lock held by rc.sysinit/728:
[   23.585176]  #0:  (tasklist_lock){.+.+..}, at: [<ffffffff8104771f>] sys_setpgid+0x5f/0x193
[   23.585176]
[   23.585176] stack backtrace:
[   23.585176] Pid: 728, comm: rc.sysinit Not tainted 2.6.36-rc2 #2
[   23.585176] Call Trace:
[   23.585176]  [<ffffffff8105b436>] lockdep_rcu_dereference+0x99/0xa2
[   23.585176]  [<ffffffff8104c324>] find_task_by_pid_ns+0x50/0x6a
[   23.585176]  [<ffffffff8104c35b>] find_task_by_vpid+0x1d/0x1f
[   23.585176]  [<ffffffff81047727>] sys_setpgid+0x67/0x193
[   23.585176]  [<ffffffff810029eb>] system_call_fastpath+0x16/0x1b
[   24.959669] type=1400 audit(1282938522.956:4): avc:  denied  { module_request } for  pid=766 comm="hwclock" kmod="char-major-10-135" scontext=system_u:system_r:hwclock_t:s0 tcontext=system_u:system_r:kernel_t:s0 tclas

It turns out that the setpgid() system call fails to enter an RCU
read-side critical section before doing a PID-to-task_struct translation.
This commit therefore does rcu_read_lock() before the translation, and
also does rcu_read_unlock() after the last use of the returned pointer.

Located-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---

 sys.c |    2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/sys.c b/kernel/sys.c
index e9ad444..05a4b0c 100644
--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -938,6 +938,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 	write_lock_irq(&tasklist_lock);
 
 	err = -ESRCH;
+	rcu_read_lock();
 	p = find_task_by_vpid(pid);
 	if (!p)
 		goto out;
@@ -983,6 +984,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid_t, pgid)
 	err = 0;
 out:
 	/* All paths lead to here, thus we are safe. -DaveM */
+	rcu_read_unlock();
 	write_unlock_irq(&tasklist_lock);
 	return err;
 }