RE: [PATCH v7 1/3] Drivers: hv: vmbus: Add vmbus_requestor data structure for VMBus hardening

[Michael Kelley]

From: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx> Sent: Monday, September 7, 2020 9:19 AM
> 
> From: Andres Beltran <lkmlabelt@xxxxxxxxx>
> 
> Currently, VMbus drivers use pointers into guest memory as request IDs
> for interactions with Hyper-V. To be more robust in the face of errors
> or malicious behavior from a compromised Hyper-V, avoid exposing
> guest memory addresses to Hyper-V. Also avoid Hyper-V giving back a
> bad request ID that is then treated as the address of a guest data
> structure with no validation. Instead, encapsulate these memory
> addresses and provide small integers as request IDs.
> 
> Signed-off-by: Andres Beltran <lkmlabelt@xxxxxxxxx>
> Co-developed-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@xxxxxxxxx>
> ---

[snip]

> --- a/drivers/hv/ring_buffer.c
> +++ b/drivers/hv/ring_buffer.c
> @@ -248,7 +248,8 @@ void hv_ringbuffer_cleanup(struct hv_ring_buffer_info *ring_info)
> 
>  /* Write to the ring buffer. */
>  int hv_ringbuffer_write(struct vmbus_channel *channel,
> -			const struct kvec *kv_list, u32 kv_count)
> +			const struct kvec *kv_list, u32 kv_count,
> +			u64 requestid)
>  {
>  	int i;
>  	u32 bytes_avail_towrite;
> @@ -258,6 +259,8 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
>  	u64 prev_indices;
>  	unsigned long flags;
>  	struct hv_ring_buffer_info *outring_info = &channel->outbound;
> +	struct vmpacket_descriptor *desc = kv_list[0].iov_base;
> +	u64 rqst_id = VMBUS_NO_RQSTOR;
> 
>  	if (channel->rescind)
>  		return -ENODEV;
> @@ -300,6 +303,22 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
>  						     kv_list[i].iov_len);
>  	}
> 
> +	/*
> +	 * Allocate the request ID after the data has been copied into the
> +	 * ring buffer.  Once this request ID is allocated, the completion
> +	 * path could find the data and free it.
> +	 */
> +
> +	if (desc->flags == VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED) {
> +		rqst_id = vmbus_next_request_id(&channel->requestor, requestid);
> +		if (rqst_id == VMBUS_RQST_ERROR) {
> +			pr_err("No request id available\n");
> +			return -EAGAIN;
> +		}
> +	}
> +	desc = hv_get_ring_buffer(outring_info) + old_write;
> +	desc->trans_id = (rqst_id == VMBUS_NO_RQSTOR) ? requestid : rqst_id;
> +

This is a nit, but the above would be clearer to me if written like this:

	flags = desc->flags;
	if (flags == VMBUS_DATA_PACKET_FLAG_COMPLETION_REQUESTED) {
		rqst_id = vmbus_next_request_id(&channel->requestor, requestid);
		if (rqst_id == VMBUS_RQST_ERROR) {
			pr_err("No request id available\n");
			return -EAGAIN;
		}
	} else {
		rqst_id = requestid;
	}
	desc = hv_get_ring_buffer(outring_info) + old_write;
	desc->trans_id = rqst_id;

The value of the flags field controls what will be used as the value for the
rqst_id.  Having another test to see which value will be used as the trans_id
somehow feels a bit redundant.  And then rqst_id doesn't have to be initialized.

>  	/* Set previous packet start */
>  	prev_indices = hv_get_ring_bufferindices(outring_info);
> 
> @@ -319,8 +338,13 @@ int hv_ringbuffer_write(struct vmbus_channel *channel,
> 
>  	hv_signal_on_write(old_write, channel);
> 
> -	if (channel->rescind)
> +	if (channel->rescind) {
> +		if (rqst_id != VMBUS_NO_RQSTOR) {

Of course, with my proposed change, the above test would also have to be for
the value of the flags field, which actually makes the code a bit more consistent.

Michael

> +			/* Reclaim request ID to avoid leak of IDs */
> +			vmbus_request_addr(&channel->requestor, rqst_id);
> +		}
>  		return -ENODEV;
> +	}
> 
>  	return 0;
>  }