[PATCH v2 4/7] Drivers: hv: vmbus: Copy the hv_message object in vmbus_on_msg_dpc()

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

The hv_message object is in memory shared with the host.  To prevent
an erroneous or a malicious host from 'corrupting' such object, copy
the object into private memory.

Suggested-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
---
 drivers/hv/vmbus_drv.c | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
index 0e39f1d6182e9..796202aa7f9b4 100644
--- a/drivers/hv/vmbus_drv.c
+++ b/drivers/hv/vmbus_drv.c
@@ -1054,14 +1054,24 @@ 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;
-	__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;
 	struct onmessage_work_context *ctx;
-	u32 message_type = msg->header.message_type;
+	__u8 payload_size;
+	u32 message_type;
+
+	/*
+	 * The hv_message object is in memory shared with the host.  Prevent an
+	 * erroneous or malicious host from 'corrupting' such object by copying
+	 * the object to private memory.
+	 */
+	memcpy(&msg_copy, msg, sizeof(struct hv_message));
+
+	payload_size = msg_copy.header.payload_size;
+	message_type = msg_copy.header.message_type;
 
 	/*
 	 * 'enum vmbus_channel_message_type' is supposed to always be 'u32' as
@@ -1074,13 +1084,7 @@ 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;
+	hdr = (struct vmbus_channel_message_header *)msg_copy.u.payload;
 	msgtype = hdr->msgtype;
 
 	trace_vmbus_on_msg_dpc(hdr);
@@ -1111,7 +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_copy.header) + payload_size);
 
 		/*
 		 * The host can generate a rescind message while we
-- 
2.25.1