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Subject[32/80] pid: make setpgid() system call use RCU read-side critical section
2.6.35-stable review patch.  If anyone has any objections, please let us know.

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

From: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

commit 950eaaca681c44aab87a46225c9e44f902c080aa upstream.

[ 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.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: David Howells <dhowells@redhat.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

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

--- a/kernel/sys.c
+++ b/kernel/sys.c
@@ -931,6 +931,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid
pgid = pid;
if (pgid < 0)
return -EINVAL;
+ rcu_read_lock();

/* From this point forward we keep holding onto the tasklist lock
* so that our parent does not change from under us. -DaveM
@@ -984,6 +985,7 @@ SYSCALL_DEFINE2(setpgid, pid_t, pid, pid
out:
/* All paths lead to here, thus we are safe. -DaveM */
write_unlock_irq(&tasklist_lock);
+ rcu_read_unlock();
return err;
}




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