linux-kernel - [PATCH] pidns: fix NULL dereference in __task_pid_nr

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Date:	Tue, 24 Nov 2015 11:39:54 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Stephane Eranian <eranian@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: [PATCH] pidns: fix NULL dereference in __task_pid_nr_ns()

From: Eric Dumazet <edumazet@...gle.com>

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@...gle.com>
---
 kernel/pid.c |    4 ++--
 1 file 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();
 


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