Drivers: hv: vmbus: Copy the hv_message in vmbus_on_msg_dpc()

Since the message is in memory shared with the host, an erroneous or a
malicious Hyper-V could 'corrupt' the message while vmbus_on_msg_dpc()
or individual message handlers are executing.  To prevent it, copy the
message into private memory.
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/20201209070827.29335-4-parri.andrea@gmail.comSigned-off-by: Wei Liu <wei.liu@kernel.org>

drivers/hv/vmbus_drv.c

@@ -1054,14 +1054,14 @@ void vmbus_on_msg_dpc(unsigned long data)
 {
 	struct hv_per_cpu_context *hv_cpu = (void *)data;
 	void *page_addr = hv_cpu->synic_message_page;
-	struct hv_message *msg = (struct hv_message *)page_addr +
+	struct hv_message msg_copy, *msg = (struct hv_message *)page_addr +
 				  VMBUS_MESSAGE_SINT;
 	struct vmbus_channel_message_header *hdr;
 	enum vmbus_channel_message_type msgtype;
 	const struct vmbus_channel_message_table_entry *entry;
 	struct onmessage_work_context *ctx;
-	u32 message_type = msg->header.message_type;
 	__u8 payload_size;
+	u32 message_type;
 
 	/*
 	 * 'enum vmbus_channel_message_type' is supposed to always be 'u32' as
@@ -1070,11 +1070,20 @@ void vmbus_on_msg_dpc(unsigned long data)
 	 */
 	BUILD_BUG_ON(sizeof(enum vmbus_channel_message_type) != sizeof(u32));
 
+	/*
+	 * Since the message is in memory shared with the host, an erroneous or
+	 * malicious Hyper-V could modify the message while vmbus_on_msg_dpc()
+	 * or individual message handlers are executing; to prevent this, copy
+	 * the message into private memory.
+	 */
+	memcpy(&msg_copy, msg, sizeof(struct hv_message));
+
+	message_type = msg_copy.header.message_type;
 	if (message_type == HVMSG_NONE)
 		/* no msg */
 		return;
 
-	hdr = (struct vmbus_channel_message_header *)msg->u.payload;
+	hdr = (struct vmbus_channel_message_header *)msg_copy.u.payload;
 	msgtype = hdr->msgtype;
 
 	trace_vmbus_on_msg_dpc(hdr);
@@ -1084,7 +1093,7 @@ void vmbus_on_msg_dpc(unsigned long data)
 		goto msg_handled;
 	}
 
-	payload_size = msg->header.payload_size;
+	payload_size = msg_copy.header.payload_size;
 	if (payload_size > HV_MESSAGE_PAYLOAD_BYTE_COUNT) {
 		WARN_ONCE(1, "payload size is too large (%d)\n", payload_size);
 		goto msg_handled;
@@ -1106,7 +1115,7 @@ void vmbus_on_msg_dpc(unsigned long data)
 			return;
 
 		INIT_WORK(&ctx->work, vmbus_onmessage_work);
-		memcpy(&ctx->msg, msg, sizeof(msg->header) + payload_size);
+		memcpy(&ctx->msg, &msg_copy, sizeof(msg->header) + payload_size);
 
 		/*
 		 * The host can generate a rescind message while we