linux-kernel - [RFC PATCH 2/3] Fix: sched/membarrier: READ_ONCE p->mm in membarrier_global

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Message-Id: <20190903160036.2400-3-mathieu.desnoyers@efficios.com>
Date:   Tue,  3 Sep 2019 12:00:35 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Russell King - ARM Linux admin <linux@...linux.org.uk>,
        Chris Metcalf <cmetcalf@...hip.com>,
        Christoph Lameter <cl@...ux.com>,
        Kirill Tkhai <tkhai@...dex.ru>, Mike Galbraith <efault@....de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: [RFC PATCH 2/3] Fix: sched/membarrier: READ_ONCE p->mm in membarrier_global_expedited

Due to the lack of READ_ONCE() on p->mm, this code can in fact turn into
a NULL deref when we hit do_exit() around exit_mm(). The first p->mm
read is before and sees !NULL, the second is after and does observe
NULL, which triggers a null pointer dereference.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Oleg Nesterov <oleg@...hat.com>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc: Chris Metcalf <cmetcalf@...hip.com>
Cc: Christoph Lameter <cl@...ux.com>
Cc: Kirill Tkhai <tkhai@...dex.ru>
Cc: Mike Galbraith <efault@....de>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/membarrier.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/membarrier.c b/kernel/sched/membarrier.c
index aa8d75804108..02feb7c8da4f 100644
--- a/kernel/sched/membarrier.c
+++ b/kernel/sched/membarrier.c
@@ -72,12 +72,16 @@ static int membarrier_global_expedited(void)
 
 		rcu_read_lock();
 		p = task_rcu_dereference(&cpu_rq(cpu)->curr);
-		if (p && p->mm && (atomic_read(&p->mm->membarrier_state) &
+		if (p) {
+			struct mm_struct *mm = READ_ONCE(p->mm);
+
+			if (mm && (atomic_read(&mm->membarrier_state) &
 				   MEMBARRIER_STATE_GLOBAL_EXPEDITED)) {
-			if (!fallback)
-				__cpumask_set_cpu(cpu, tmpmask);
-			else
-				smp_call_function_single(cpu, ipi_mb, NULL, 1);
+				if (!fallback)
+					__cpumask_set_cpu(cpu, tmpmask);
+				else
+					smp_call_function_single(cpu, ipi_mb, NULL, 1);
+			}
 		}
 		rcu_read_unlock();
 	}
-- 
2.17.1