Drivers: hv: vmbus: Resolve race condition in vmbus_onoffer_rescind()

An erroneous or malicious host could send multiple rescind messages for
a same channel.  In vmbus_onoffer_rescind(), the guest maps the channel
ID to obtain a pointer to the channel object and it eventually releases
such object and associated data.  The host could time rescind messages
and lead to an use-after-free.  Add a new flag to the channel structure
to make sure that only one instance of vmbus_onoffer_rescind() can get
the reference to the channel object.
Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20201209070827.29335-6-parri.andrea@gmail.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/channel_mgmt.c

@@ -1063,6 +1063,18 @@ static void vmbus_onoffer_rescind(struct vmbus_channel_message_header *hdr)
 
 	mutex_lock(&vmbus_connection.channel_mutex);
 	channel = relid2channel(rescind->child_relid);
+	if (channel != NULL) {
+		/*
+		 * Guarantee that no other instance of vmbus_onoffer_rescind()
+		 * has got a reference to the channel object.  Synchronize on
+		 * &vmbus_connection.channel_mutex.
+		 */
+		if (channel->rescind_ref) {
+			mutex_unlock(&vmbus_connection.channel_mutex);
+			return;
+		}
+		channel->rescind_ref = true;
+	}
 	mutex_unlock(&vmbus_connection.channel_mutex);
 
 	if (channel == NULL) {

include/linux/hyperv.h

@@ -809,6 +809,7 @@ struct vmbus_channel {
 	u8 monitor_bit;
 
 	bool rescind; /* got rescind msg */
+	bool rescind_ref; /* got rescind msg, got channel reference */
 	struct completion rescind_event;
 
 	u32 ringbuffer_gpadlhandle;