[PATCH 2/6] Drivers: hv: vmbus: Avoid double fetch of msgtype in vmbus_on_msg_dpc()

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

vmbus_on_msg_dpc() double fetches from msgtype.  The double fetch can
lead to an out-of-bound access when accessing the channel_message_table
array.  In turn, the use of the out-of-bound entry could lead to code
execution primitive (entry->message_handler()).  Avoid the double fetch
by saving the value of msgtype 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 | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 0a2711aa63a15..82b23baa446d7 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1057,6 +1057,7 @@ void vmbus_on_msg_dpc(unsigned long data)
 	struct hv_message *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;
@@ -1072,12 +1073,19 @@ void vmbus_on_msg_dpc(unsigned long data)
 		/* no msg */
 		return;
 
+	/*
+	 * The hv_message object is in memory shared with the host.  The host
+	 * could erroneously or maliciously modify such object.  Make sure to
+	 * validate its fields and avoid double fetches whenever feasible.
+	 */
+
 	hdr = (struct vmbus_channel_message_header *)msg->u.payload;
+	msgtype = hdr->msgtype;
 
 	trace_vmbus_on_msg_dpc(hdr);
 
-	if (hdr->msgtype >= CHANNELMSG_COUNT) {
-		WARN_ONCE(1, "unknown msgtype=%d\n", hdr->msgtype);
+	if (msgtype >= CHANNELMSG_COUNT) {
+		WARN_ONCE(1, "unknown msgtype=%d\n", msgtype);
 		goto msg_handled;
 	}
 
@@ -1087,14 +1095,14 @@ void vmbus_on_msg_dpc(unsigned long data)
 		goto msg_handled;
 	}
 
-	entry = &channel_message_table[hdr->msgtype];
+	entry = &channel_message_table[msgtype];
 
 	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",
-			  hdr->msgtype, msg->header.payload_size);
+			  msgtype, msg->header.payload_size);
 		goto msg_handled;
 	}
 
@@ -1115,7 +1123,7 @@ void vmbus_on_msg_dpc(unsigned long data)
 		 * by offer_in_progress and by channel_mutex.  See also the
 		 * inline comments in vmbus_onoffer_rescind().
 		 */
-		switch (hdr->msgtype) {
+		switch (msgtype) {
 		case CHANNELMSG_RESCIND_CHANNELOFFER:
 			/*
 			 * If we are handling the rescind message;
-- 
2.25.1