net/tls: Fix race in TLS device down flow

Socket destruction flow and tls_device_down function sync against each
other using tls_device_lock and the context refcount, to guarantee the
device resources are freed via tls_dev_del() by the end of
tls_device_down.

In the following unfortunate flow, this won't happen:
- refcount is decreased to zero in tls_device_sk_destruct.
- tls_device_down starts, skips the context as refcount is zero, going
  all the way until it flushes the gc work, and returns without freeing
  the device resources.
- only then, tls_device_queue_ctx_destruction is called, queues the gc
  work and frees the context's device resources.

Solve it by decreasing the refcount in the socket's destruction flow
under the tls_device_lock, for perfect synchronization.  This does not
slow down the common likely destructor flow, in which both the refcount
is decreased and the spinlock is acquired, anyway.

Fixes: e8f6979 ("net/tls: Add generic NIC offload infrastructure")
Reviewed-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/tls/tls_device.c

@@ -97,13 +97,16 @@ static void tls_device_queue_ctx_destruction(struct tls_context *ctx)
 	unsigned long flags;
 
 	spin_lock_irqsave(&tls_device_lock, flags);
+	if (unlikely(!refcount_dec_and_test(&ctx->refcount)))
+		goto unlock;
+
 	list_move_tail(&ctx->list, &tls_device_gc_list);
 
 	/* schedule_work inside the spinlock
 	 * to make sure tls_device_down waits for that work.
 	 */
 	schedule_work(&tls_device_gc_work);
-
+unlock:
 	spin_unlock_irqrestore(&tls_device_lock, flags);
 }
 
@@ -194,8 +197,7 @@ void tls_device_sk_destruct(struct sock *sk)
 		clean_acked_data_disable(inet_csk(sk));
 	}
 
-	if (refcount_dec_and_test(&tls_ctx->refcount))
-		tls_device_queue_ctx_destruction(tls_ctx);
+	tls_device_queue_ctx_destruction(tls_ctx);
 }
 EXPORT_SYMBOL_GPL(tls_device_sk_destruct);