pidns: fix NULL dereference in __task_pid_nr_ns()

I got a crash during a "perf top" session that was caused by a race in
__task_pid_nr_ns() :

pid_nr_ns() was inlined, but apparently compiler chose to read
task->pids[type].pid twice, and the pid->level dereference crashed
because we got a NULL pointer at the second read :

    if (pid && ns->level <= pid->level) { // CRASH

Just use RCU API properly to solve this race, and not worry about "perf
top" crashing hosts :(

get_task_pid() can benefit from same fix.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/kernel/pid.c b/kernel/pid.c
index ca368793808e..78b3d9f80d44 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -467,7 +467,7 @@ struct pid *get_task_pid(struct task_struct *task, enum pid_type type)
         rcu_read_lock();
         if (type != PIDTYPE_PID)
                 task = task->group_leader;
-        pid = get_pid(task->pids[type].pid);
+        pid = get_pid(rcu_dereference(task->pids[type].pid));
         rcu_read_unlock();
         return pid;
 }
@@ -528,7 +528,7 @@ pid_t __task_pid_nr_ns(struct task_struct *task, enum pid_type type,
         if (likely(pid_alive(task))) {
                 if (type != PIDTYPE_PID)
                         task = task->group_leader;
-                nr = pid_nr_ns(task->pids[type].pid, ns);
+                nr = pid_nr_ns(rcu_dereference(task->pids[type].pid), ns);
         }
         rcu_read_unlock();