linux-kernel - [RFC PATCH 1/3] Fix: sched: task_rcu_dereference: check probe_kernel

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Date:   Tue,  3 Sep 2019 12:00:34 -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 1/3] Fix: sched: task_rcu_dereference: check probe_kernel_address return value

probe_kernel_address can return -EFAULT on error, which leads to use of
an uninitialized or partially initialized sighand variable.

There is ongoing discussion on removing task_rcu_dereference altogether,
which seems like a nice way forward. This patch is submitted as a fix
aiming to be backported to prior stable kernel releases.

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/exit.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index 5b4a5dcce8f8..b1c3e1ba501c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -249,7 +249,8 @@ struct task_struct *task_rcu_dereference(struct task_struct **ptask)
 	if (!task)
 		return NULL;
 
-	probe_kernel_address(&task->sighand, sighand);
+	if (probe_kernel_address(&task->sighand, sighand))
+		sighand = NULL;
 
 	/*
 	 * Pairs with atomic_dec_and_test() in put_task_struct(). If this task
-- 
2.17.1