wait_task_stopped(): don't use task_pid_nr_ns() lockless

wait_task_stopped(WNOWAIT) does task_pid_nr_ns() without tasklist/rcu lock,
we can read an already freed memory.  Use the cached pid_t value.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Looks-good-to: Roland McGrath <roland@redhat.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 4 insertions, 5 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index cd0f1d4137a..0a4a382ecf2 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -1357,7 +1357,7 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
                              int __user *stat_addr, struct rusage __user *ru)
 {
         int retval, exit_code;
-        struct pid_namespace *ns;
+        pid_t pid;
 
         if (!p->exit_code)
                 return 0;
@@ -1376,12 +1376,11 @@ static int wait_task_stopped(struct task_struct *p, int delayed_group_leader,
          * keep holding onto the tasklist_lock while we call getrusage and
          * possibly take page faults for user memory.
          */
-        ns = current->nsproxy->pid_ns;
+        pid = task_pid_nr_ns(p, current->nsproxy->pid_ns);
         get_task_struct(p);
         read_unlock(&tasklist_lock);
 
         if (unlikely(noreap)) {
-                pid_t pid = task_pid_nr_ns(p, ns);
                 uid_t uid = p->uid;
                 int why = (p->ptrace & PT_PTRACED) ? CLD_TRAPPED : CLD_STOPPED;
 
@@ -1451,11 +1450,11 @@ bail_ref:
         if (!retval && infop)
                 retval = put_user(exit_code, &infop->si_status);
         if (!retval && infop)
-                retval = put_user(task_pid_nr_ns(p, ns), &infop->si_pid);
+                retval = put_user(pid, &infop->si_pid);
         if (!retval && infop)
                 retval = put_user(p->uid, &infop->si_uid);
         if (!retval)
-                retval = task_pid_nr_ns(p, ns);
+                retval = pid;
         put_task_struct(p);
 
         BUG_ON(!retval);