[RFC PATCH 1/3] Fix: sched: task_rcu_dereference: check probe_kernel_address return value

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

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@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Russell King - ARM Linux admin <linux@armlinux.org.uk>
Cc: Chris Metcalf <cmetcalf@ezchip.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Kirill Tkhai <tkhai@yandex.ru>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@kernel.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