[PATCH 3/6] Drivers: hv: vmbus: Avoid double fetch of payload_size in vmbus_on_msg_dpc()

["Andrea Parri (Microsoft)" <parri.andrea@gmail.com>]

vmbus_on_msg_dpc() double fetches from payload_size.  The double fetch
can lead to a buffer overflow when (mem)copying the hv_message object.
Avoid the double fetch by saving the value of payload_size into a local
variable.

Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
 drivers/hv/vmbus_drv.c | 17 +++++++----------
 1 file changed, 7 insertions(+), 10 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 82b23baa446d7..0e39f1d6182e9 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1056,6 +1056,7 @@ void vmbus_on_msg_dpc(unsigned long data)
 	void *page_addr = hv_cpu->synic_message_page;
 	struct hv_message *msg = (struct hv_message *)page_addr +
 				  VMBUS_MESSAGE_SINT;
+	__u8 payload_size = msg->header.payload_size;
 	struct vmbus_channel_message_header *hdr;
 	enum vmbus_channel_message_type msgtype;
 	const struct vmbus_channel_message_table_entry *entry;
@@ -1089,9 +1090,8 @@ void vmbus_on_msg_dpc(unsigned long data)
 		goto msg_handled;
 	}
 
-	if (msg->header.payload_size > HV_MESSAGE_PAYLOAD_BYTE_COUNT) {
-		WARN_ONCE(1, "payload size is too large (%d)\n",
-			  msg->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;
 	}
 
@@ -1100,21 +1100,18 @@ void vmbus_on_msg_dpc(unsigned long data)
 	if (!entry->message_handler)
 		goto msg_handled;
 
-	if (msg->header.payload_size < entry->min_payload_len) {
-		WARN_ONCE(1, "message too short: msgtype=%d len=%d\n",
-			  msgtype, msg->header.payload_size);
+	if (payload_size < entry->min_payload_len) {
+		WARN_ONCE(1, "message too short: msgtype=%d len=%d\n", msgtype, payload_size);
 		goto msg_handled;
 	}
 
 	if (entry->handler_type	== VMHT_BLOCKING) {
-		ctx = kmalloc(sizeof(*ctx) + msg->header.payload_size,
-			      GFP_ATOMIC);
+		ctx = kmalloc(sizeof(*ctx) + payload_size, GFP_ATOMIC);
 		if (ctx == NULL)
 			return;
 
 		INIT_WORK(&ctx->work, vmbus_onmessage_work);
-		memcpy(&ctx->msg, msg, sizeof(msg->header) +
-		       msg->header.payload_size);
+		memcpy(&ctx->msg, msg, sizeof(msg->header) + payload_size);
 
 		/*
 		 * The host can generate a rescind message while we
-- 
2.25.1