pids: sys_getsid: fix unsafe *pid usage, fix possible 0 instead of -ESRCH

1. sys_getsid() needs rcu_read_lock() to derive the session _nr, even if
   the task is current, otherwise we can race with another thread which
   does sys_setsid().

2. The task can exit between find_task_by_vpid() and task_session_vnr(),
   in that unlikely case sys_getsid() returns 0 instead of -ESRCH.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/sys.c

@@ -1022,23 +1022,30 @@ asmlinkage long sys_getpgrp(void)
 
 asmlinkage long sys_getsid(pid_t pid)
 {
-	if (!pid)
-		return task_session_vnr(current);
-	else {
-		int retval;
 	struct task_struct *p;
+	struct pid *sid;
+	int retval;
 
 	rcu_read_lock();
-		p = find_task_by_vpid(pid);
+	if (!pid)
+		sid = task_session(current);
+	else {
 		retval = -ESRCH;
-		if (p) {
+		p = find_task_by_vpid(pid);
+		if (!p)
+			goto out;
+		sid = task_session(p);
+		if (!sid)
+			goto out;
+
 		retval = security_task_getsid(p);
-			if (!retval)
-				retval = task_session_vnr(p);
+		if (retval)
+			goto out;
 	}
+	retval = pid_vnr(sid);
+out:
 	rcu_read_unlock();
 	return retval;
-	}
 }
 
 asmlinkage long sys_setsid(void)