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SubjectRE: [PATCH v7 1/3] Drivers: hv: vmbus: Add vmbus_requestor data structure for VMBus hardening
Date
From: Andrea Parri (Microsoft) <parri.andrea@gmail.com> Sent: Monday, September 7, 2020 9:19 AM
>
> From: Andres Beltran <lkmlabelt@gmail.com>
>
> 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@gmail.com>
> Co-developed-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
> ---

[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;
> }

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