pids: sys_getpgid: fix unsafe *pid usage, s/tasklist/rcu/

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

2. Use rcu_read_lock() instead of tasklist_lock when pid != 0, make sure
   that we don't use the NULL pid if the task exits right after successful
   find_task_by_vpid().
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

@@ -991,31 +991,37 @@ out:
 
 asmlinkage long sys_getpgid(pid_t pid)
 {
-	if (!pid)
-		return task_pgrp_vnr(current);
-	else {
-		int retval;
 	struct task_struct *p;
+	struct pid *grp;
+	int retval;
 
-		read_lock(&tasklist_lock);
-		p = find_task_by_vpid(pid);
+	rcu_read_lock();
+	if (!pid)
+		grp = task_pgrp(current);
+	else {
 		retval = -ESRCH;
-		if (p) {
+		p = find_task_by_vpid(pid);
+		if (!p)
+			goto out;
+		grp = task_pgrp(p);
+		if (!grp)
+			goto out;
+
 		retval = security_task_getpgid(p);
-			if (!retval)
-				retval = task_pgrp_vnr(p);
+		if (retval)
+			goto out;
 	}
-		read_unlock(&tasklist_lock);
+	retval = pid_vnr(grp);
+out:
+	rcu_read_unlock();
 	return retval;
-	}
 }
 
 #ifdef __ARCH_WANT_SYS_GETPGRP
 
 asmlinkage long sys_getpgrp(void)
 {
-	/* SMP - assuming writes are word atomic this is fine */
-	return task_pgrp_vnr(current);
+	return sys_getpgid(0);
 }
 
 #endif