Re: [PATCH v2 3/3] hv_netvsc: Leak send/recv buffers if GPADL teardown fails

[Simon Horman]

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Full review at: https://sashiko.dev/#/patchset/20260907214902.9046-2-mhklinux@xxxxxxxxxxx?part=3
---
hv_netvsc: Leak send/recv buffers if GPADL teardown fails

This commit addresses a bug where send/recv buffers were improperly freed if
GPADL teardown failed. It introduces calls to vmbus_leak_buffer() to safely
leak the buffers instead of freeing them when they might still be accessed by
the host.

Closes: https://lore.kernel.org/linux-hyperv/20260731201210.3653C1F00AC4@xxxxxxxxxxxxxxx/

> diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c
> index 5cd084e5696cd..e9292c3fac925 100644
> --- a/drivers/net/hyperv/netvsc.c
> +++ b/drivers/net/hyperv/netvsc.c
> @@ -316,9 +316,11 @@ static void netvsc_teardown_recv_gpadl(struct hv_device *device,
>  		 * rather than continue and a bugchk
>  		 */
>  		if (ret != 0) {
> +			vmbus_leak_buffer(&net_device->recv_buf,
> +					  &net_device->recv_buf_chunks,
> +					  &net_device->recv_buf_chunk_cnt);

[Severity: High]
Could this lead to a NULL pointer dereference if NAPI is still polling
concurrently?

In netvsc_device_add(), NAPI is enabled and the VMBus channel is opened. If a
subsequent initialization step fails (like send_buf setup), the error path
calls netvsc_teardown_recv_gpadl().

If the GPADL teardown fails, vmbus_leak_buffer() will set
net_device->recv_buf to NULL. Because NAPI is not synchronized with this
teardown and is disabled later in the error path, a concurrent softirq could
execute this path in netvsc_receive():

    char *recv_buf = net_device->recv_buf;
    ...
    data = recv_buf + offset;
    ...
    ret = rndis_filter_receive(ndev, net_device, nvchan, data, buflen);

Since the code does not check for NULL, it passes the faulty data pointer
(NULL + offset) to rndis_filter_receive().

This would result in a kernel panic when trying to read from the pointer:

    memcpy(rndis_msg, data, RNDIS_HEADER_SIZE);

Is there a way to defer setting recv_buf to NULL until after NAPI has been
disabled?

>  			netdev_err(ndev,
>  				   "unable to teardown receive buffer's gpadl\n");
> -			return;
>  		}
>  	}
>  }