[Devel] [PATCH RHEL7 COMMIT] ms/pidns: fix NULL dereference in __task_pid_nr_ns()

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-693.1.1.vz7.37.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-693.1.1.vz7.37.16
------>
commit 4856f5d044f061c8c1491752d4dd31151b24d399
Author: Eric Dumazet <edumazet at google.com>
Date:   Thu Oct 19 12:01:00 2017 +0300

    ms/pidns: fix NULL dereference in __task_pid_nr_ns()
    
    commit 81b1a832d79749058863cffe2c0ed4ef40f6e6ec upstream.
    
    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 at google.com>
    Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
    
    https://jira.sw.ru/browse/PSBM-75247
    Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
---
 kernel/pid.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/pid.c b/kernel/pid.c
index 4f8d1d6..d1f9d4c 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -511,7 +511,7 @@ 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;
 }
@@ -572,7 +572,7 @@ 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();