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

[Michael Kelley <mikelley@microsoft.com>]

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

The commit message is missing some context.  Why is a double fetch a
problem?  The comments below in the code explain why, but the why
should also be briefly explained in the commit message.

> 
> 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.

The "when feasible" phrase sounds like not doing double fetches is optional in
some circumstances.  But I think we always have to protect against modification
of memory shared with the host.  So perhaps the comment should be more
precise.

> +	 */
> +
>  	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