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

[Michael Kelley]

From: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx> Sent: Wednesday, November 18, 2020 6:37 AM
> 
> 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.

Similar comment here about providing some brief context in the commit
message on the problem that we are guarding against by removing the
double fetch.

I could see combining this patch with the previous one since the
motivation and pattern of the changes are exactly the same, just for
two different fields.

Michael

> 
> Reported-by: Juan Vazquez <juvazq@xxxxxxxxxxxxx>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
> ---
>  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