[PATCH] dm-crypt: fix a possible race condition on exit

[Mikulas Patocka <mpatocka@xxxxxxxxxx>]

I found a possible race condition on exit in dm-crypt. The kernel thread
executes __set_current_state(TASK_INTERRUPTIBLE), __add_wait_queue,
spin_unlock_irq and then tests kthread_should_stop(). It is possible that
the processor reorders memory accesses so that kthread_should_stop() is
executed before __set_current_state(TASK_INTERRUPTIBLE). If such
reordering happens, there is a possible race on thread termination:

CPU 0:
calls kthread_should_stop()
	it tests KTHREAD_SHOULD_STOP bit, returns false
CPU 1:
calls kthread_stop(cc->write_thread)
	sets the KTHREAD_SHOULD_STOP bit
	calls wake_up_process on the kernel thread, that sets the thread
		state to TASK_RUNNING
CPU 0:
sets __set_current_state(TASK_INTERRUPTIBLE)
spin_unlock_irq(&cc->write_thread_wait.lock)
schedule() - and the process is stuck and never terminates, because the
	state is TASK_INTERRUPTIBLE and wake_up_process on CPU 1 already
	terminated

Fix this race condition by using a new flag DM_CRYPT_EXIT_THREAD to signal
that the kernel thread should exit. The flag is set and tested while
holding cc->write_thread_wait.lock, so there is no possibility of racy
access to the flag.

We also remove set_task_state(current, TASK_RUNNING) following the
schedule(), it is not needed. When the process was woken up, its state was
already set to TASK_RUNNING. Other kernel code also doesn't set the state
to TASK_RUNNING following schedule() (for example, do_wait_for_common in
completion.c doesn't do it).

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx	# v4.0+
Fixes: dc2676210c42 ("dm crypt: offload writes to thread")

---
 drivers/md/dm-crypt.c |   20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

Index: linux-4.4-rc1/drivers/md/dm-crypt.c
===================================================================
--- linux-4.4-rc1.orig/drivers/md/dm-crypt.c	2015-11-18 21:58:50.000000000 +0100
+++ linux-4.4-rc1/drivers/md/dm-crypt.c	2015-11-18 22:28:16.000000000 +0100
@@ -112,7 +112,8 @@ struct iv_tcw_private {
  * and encrypts / decrypts at the same time.
  */
 enum flags { DM_CRYPT_SUSPENDED, DM_CRYPT_KEY_VALID,
-	     DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD };
+	     DM_CRYPT_SAME_CPU, DM_CRYPT_NO_OFFLOAD,
+	     DM_CRYPT_EXIT_THREAD};
 
 /*
  * The fields in here must be read only after initialization.
@@ -1203,20 +1204,16 @@ continue_locked:
 		if (!RB_EMPTY_ROOT(&cc->write_tree))
 			goto pop_from_list;
 
+		if (unlikely(test_bit(DM_CRYPT_EXIT_THREAD, &cc->flags)))
+			break;
+
 		__set_current_state(TASK_INTERRUPTIBLE);
 		__add_wait_queue(&cc->write_thread_wait, &wait);
 
 		spin_unlock_irq(&cc->write_thread_wait.lock);
 
-		if (unlikely(kthread_should_stop())) {
-			set_task_state(current, TASK_RUNNING);
-			remove_wait_queue(&cc->write_thread_wait, &wait);
-			break;
-		}
-
 		schedule();
 
-		set_task_state(current, TASK_RUNNING);
 		spin_lock_irq(&cc->write_thread_wait.lock);
 		__remove_wait_queue(&cc->write_thread_wait, &wait);
 		goto continue_locked;
@@ -1531,8 +1528,13 @@ static void crypt_dtr(struct dm_target *
 	if (!cc)
 		return;
 
-	if (cc->write_thread)
+	if (cc->write_thread) {
+		spin_lock_irq(&cc->write_thread_wait.lock);
+		set_bit(DM_CRYPT_EXIT_THREAD, &cc->flags);
+		wake_up_locked(&cc->write_thread_wait);
+		spin_unlock_irq(&cc->write_thread_wait.lock);
 		kthread_stop(cc->write_thread);
+	}
 
 	if (cc->io_queue)
 		destroy_workqueue(cc->io_queue);

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