Re: [PATCH net] tun: free page on short-frame rejection in tun_xdp_one()

[Dongli Zhang]

On 2026-05-21 9:44 AM, Weiming Shi wrote:
> On 26-05-20 20:58, Willem de Bruijn wrote:
>> Dongli Zhang wrote:
>>>
>>>
>>> On 2026-05-20 5:05 PM, Willem de Bruijn wrote:
>>>> Weiming Shi wrote:
>>>>> tun_xdp_one() returns -EINVAL on a frame shorter than ETH_HLEN without
>>>>> freeing the page that vhost_net_build_xdp() allocated for it.
>>>>> tun_sendmsg() discards that -EINVAL and still returns total_len, so
>>>>> vhost_tx_batch() takes the success path and never frees the page; each
>>>>> short frame in a batch leaks one page-frag chunk.
>>>>>
>>>>> A local process that can open /dev/net/tun and /dev/vhost-net can hit
>>>>> this path: it attaches a tun/tap device as the vhost-net backend and
>>>>> feeds TX descriptors whose length minus the virtio-net header is below
>>>>> ETH_HLEN. Each kick leaks the page-frag chunks for that batch, and a
>>>>> tight submission loop exhausts host memory and triggers an OOM panic.
>>>>> Free the page before returning -EINVAL, matching the XDP-program error
>>>>> path in the same function.
>>>>>
>>>>> Fixes: 049584807f1d ("tun: add missing verification for short frame")
>>>>> Reported-by: Xiang Mei <xmei5@xxxxxxx>
>>>>> Assisted-by: Claude:claude-opus-4-7
>>>>> Signed-off-by: Weiming Shi <bestswngs@xxxxxxxxx>
>>>>> ---
>>>>>  drivers/net/tun.c | 4 +++-
>>>>>  1 file changed, 3 insertions(+), 1 deletion(-)
>>>>>
>>>>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
>>>>> index b183189f1853..f594360d66d6 100644
>>>>> --- a/drivers/net/tun.c
>>>>> +++ b/drivers/net/tun.c
>>>>> @@ -2394,8 +2394,10 @@ static int tun_xdp_one(struct tun_struct *tun,
>>>>>  	bool skb_xdp = false;
>>>>>  	struct page *page;
>>>>>  
>>>>> -	if (unlikely(datasize < ETH_HLEN))
>>>>> +	if (unlikely(datasize < ETH_HLEN)) {
>>>>> +		put_page(virt_to_head_page(xdp->data));
>>>>>  		return -EINVAL;
>>>>> +	}
>>>>
>>>> Make sense, thanks.
>>>>
>>>> The error path from tun_xdp_act does the same. And the default: label
>>>> used to too, before a batching optimization was introduced.
>>>>
>>>> Is the same then also missing if build_skb fails?
>>>>
>>>
>>> I also agree that we may need to handle build_skb() failure.
>>
>> Thanks. Fine to defer to a separate patch btw. Either way.
>>  
>>> In addition, I think we may need this fix for tap_get_user_xdp() as well.
>>>
>>> Thank you very much!
>>>
>>> Dongli Zhang
>>
>>
> Hi,
> 
> I have sent a patch to fix the same issue  and the patch is under review now. 
> Please take a look if you have time, thanks!
> 
> https://urldefense.com/v3/__https://lore.kernel.org/all/20260521163230.1478627-2-bestswngs@xxxxxxxxx/__;!!ACWV5N9M2RV99hQ!IDJKNE6zGJJ7hjjCFU8OviGWfh4A_mNZAhrTowR0Kb4p5bm4JmoCmEI6_GfOXFjcxd2ctJ3pbcJypErFgC4-$ 
> https://urldefense.com/v3/__https://lore.kernel.org/all/20260521163312.1479805-2-bestswngs@xxxxxxxxx/__;!!ACWV5N9M2RV99hQ!IDJKNE6zGJJ7hjjCFU8OviGWfh4A_mNZAhrTowR0Kb4p5bm4JmoCmEI6_GfOXFjcxd2ctJ3pbcJypMyOq4Op$ 
> 

The bug fix applies to both TAP and TUN.

TAP has a single patch, while TUN has two separate patches. I'm not sure which
approach is better.

[PATCH net] tap: free page on error paths in tap_get_user_xdp()
https://lore.kernel.org/netdev/20260521163230.1478627-2-bestswngs@xxxxxxxxx

[PATCH net] tun: free page on short-frame rejection in tun_xdp_one()
https://lore.kernel.org/all/20260520160020.375349-2-bestswngs@xxxxxxxxx

[PATCH net] tun: free page on build_skb failure in tun_xdp_one()
https://lore.kernel.org/netdev/20260521163312.1479805-2-bestswngs@xxxxxxxxx/


The bug fix looks good to me.

Reviewed-by: Dongli Zhang <dongli.zhang@xxxxxxxxxx>

+ Michael for FYI regarding vhost as well.

Thank you very much!

Dongli Zhang