CRED: Fix RCU warning due to previous patch fixing __task_cred()'s checks

Commit 8f92054e ("CRED: Fix __task_cred()'s lockdep check and banner
comment") fixed the lockdep checks on __task_cred().  This has shown up
a place in the signalling code where a lock should be held - namely that
check_kill_permission() requires its callers to hold the RCU lock.

Fix group_send_sig_info() to get the RCU read lock around its call to
check_kill_permission().

Without this patch, the following warning can occur:

  ===================================================
  [ INFO: suspicious rcu_dereference_check() usage. ]
  ---------------------------------------------------
  kernel/signal.c:660 invoked rcu_dereference_check() without protection!
  ...
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -637,7 +637,7 @@ static inline bool si_fromuser(const struct siginfo *info)
 
 /*
  * Bad permissions for sending the signal
- * - the caller must hold at least the RCU read lock
+ * - the caller must hold the RCU read lock
  */
 static int check_kill_permission(int sig, struct siginfo *info,
 				 struct task_struct *t)
@@ -1127,11 +1127,14 @@ struct sighand_struct *lock_task_sighand(struct task_struct *tsk, unsigned long
 
 /*
  * send signal info to all the members of a group
- * - the caller must hold the RCU read lock at least
  */
 int group_send_sig_info(int sig, struct siginfo *info, struct task_struct *p)
 {
-	int ret = check_kill_permission(sig, info, p);
+	int ret;
+
+	rcu_read_lock();
+	ret = check_kill_permission(sig, info, p);
+	rcu_read_unlock();
 
 	if (!ret && sig)
 		ret = do_send_sig_info(sig, info, p, true);