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

Commit 8f92054e7ca1 ("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 <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Acked-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/signal.c

···
 
 /*
  * 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)
···
 
 /*
  * 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);