crypto: cryptd - Protect per-CPU resource by disabling BH.

The access to cryptd_queue::cpu_queue is synchronized by disabling
preemption in cryptd_enqueue_request() and disabling BH in
cryptd_queue_worker(). This implies that access is allowed from BH.

If cryptd_enqueue_request() is invoked from preemptible context _and_
soft interrupt then this can lead to list corruption since
cryptd_enqueue_request() is not protected against access from
soft interrupt.

Replace get_cpu() in cryptd_enqueue_request() with local_bh_disable()
to ensure BH is always disabled.
Remove preempt_disable() from cryptd_queue_worker() since it is not
needed because local_bh_disable() ensures synchronisation.

Fixes: 254eff771441 ("crypto: cryptd - Per-CPU thread implementation...")
Signed-off-by: Sebastian Andrzej Siewior <[e-mail]>
Signed-off-by: Herbert Xu <[e-mail]>

crypto/cryptd.c

···
 };
 
 struct cryptd_queue {
+	/*
+	 * Protected by disabling BH to allow enqueueing from softinterrupt and
+	 * dequeuing from kworker (cryptd_queue_worker()).
+	 */
 	struct cryptd_cpu_queue __percpu *cpu_queue;
 };
 
···
 static int cryptd_enqueue_request(struct cryptd_queue *queue,
 				  struct crypto_async_request *request)
 {
-	int cpu, err;
+	int err;
 	struct cryptd_cpu_queue *cpu_queue;
 	refcount_t *refcnt;
 
-	cpu = get_cpu();
+	local_bh_disable();
 	cpu_queue = this_cpu_ptr(queue->cpu_queue);
 	err = crypto_enqueue_request(&cpu_queue->queue, request);
 
 	refcnt = crypto_tfm_ctx(request->tfm);
 
 	if (err == -ENOSPC)
-		goto out_put_cpu;
+		goto out;
 
-	queue_work_on(cpu, cryptd_wq, &cpu_queue->work);
+	queue_work_on(smp_processor_id(), cryptd_wq, &cpu_queue->work);
 
 	if (!refcount_read(refcnt))
-		goto out_put_cpu;
+		goto out;
 
 	refcount_inc(refcnt);
 
-out_put_cpu:
-	put_cpu();
+out:
+	local_bh_enable();
 
 	return err;
 }
···
 	cpu_queue = container_of(work, struct cryptd_cpu_queue, work);
 	/*
 	 * Only handle one request at a time to avoid hogging crypto workqueue.
-	 * preempt_disable/enable is used to prevent being preempted by
-	 * cryptd_enqueue_request(). local_bh_disable/enable is used to prevent
-	 * cryptd_enqueue_request() being accessed from software interrupts.
 	 */
 	local_bh_disable();
-	preempt_disable();
 	backlog = crypto_get_backlog(&cpu_queue->queue);
 	req = crypto_dequeue_request(&cpu_queue->queue);
-	preempt_enable();
 	local_bh_enable();
 
 	if (!req)