pids: improve get_task_pid() to fix the unsafe sys_wait4()->task_pgrp()

sys_wait4() does get_pid(task_pgrp(current)), this is not safe.  We can
add rcu lock/unlock around, but we already have get_task_pid() which can
be improved to handle the special pids in more reliable manner.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Louis Rilling <Louis.Rilling@kerlabs.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Cc: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Cc: Roland McGrath <roland@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index 1b3586fe753a..6628abcc520e 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -403,6 +403,8 @@ struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
 {
         struct pid *pid;
         rcu_read_lock();
+        if (type != PIDTYPE_PID)
+                task = task->group_leader;
         pid = get_pid(task->pids[type].pid);
         rcu_read_unlock();
         return pid;