Re: [PATCH 2/2] vsock/test: Add test for a linear and non-linear skb getting coalesced

[Michal Luczaj <mhal@rbox.co>]

On 1/13/26 10:36, Stefano Garzarella wrote:
> On Mon, Jan 12, 2026 at 10:20:50PM +0100, Michal Luczaj wrote:
>> On 1/12/26 17:48, Stefano Garzarella wrote:
>>>>>>>> diff --git a/tools/testing/vsock/vsock_test.c b/tools/testing/vsock/vsock_test.c
>>>>>>>> index bbe3723babdc..21c8616100f1 100644
>>>>>>>> --- a/tools/testing/vsock/vsock_test.c
>>>>>>>> +++ b/tools/testing/vsock/vsock_test.c
>>>>>>>> @@ -2403,6 +2403,11 @@ static struct test_case test_cases[] = {
>>>>>>>> 		.run_client = test_stream_accepted_setsockopt_client,
>>>>>>>> 		.run_server = test_stream_accepted_setsockopt_server,
>>>>>>>> 	},
>>>>>>>> +	{
>>>>>>>> +		.name = "SOCK_STREAM MSG_ZEROCOPY coalescence corruption",
>>>>>>>
>>>>>>> This is essentially a regression test for virtio transport, so I'd add
>>>>>>> virtio in the test name.
>>>>>>
>>>>>> Isn't virtio transport unaffected? It's about loopback transport (that
>>>>>> shares common code with virtio transport).
>>>>>
>>>>> Why virtio transport is not affected?
>>>>
>>>> With the usual caveat that I may be completely missing something, aren't
>>>> all virtio-transport's rx skbs linear? See virtio_vsock_alloc_linear_skb()
>>>> in virtio_vsock_rx_fill().
>>>>
>>>
>>> True, but what about drivers/vhost/vsock.c ?
>>>
>>> IIUC in vhost_vsock_handle_tx_kick() we call vhost_vsock_alloc_skb(),
>>> that calls virtio_vsock_alloc_skb() and pass that skb to
>>> virtio_transport_recv_pkt(). So, it's also affected right?
>>
>> virtio_vsock_alloc_skb() returns a non-linear skb only if size >
>> SKB_WITH_OVERHEAD(PAGE_SIZE << PAGE_ALLOC_COSTLY_ORDER)). And that is way
>> more than GOOD_COPY_LEN, so we're good.
>>
>> At least until someone increases GOOD_COPY_LEN and/or reduces the size
>> condition for non-linear allocation. So, yeah, a bit brittle.
> 
> I see, thanks for clarify. So please add all of this conclusions in the 
> patch 1 description to make it clear that only loopback is affected, so 
> no guest/host attack is possible. (not really severe CVE)

OK, here's v2:
https://lore.kernel.org/netdev/20260113-vsock-recv-coalescence-v2-0-552b17837cf4@rbox.co/