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

[Dongli Zhang]

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.

In addition, I think we may need this fix for tap_get_user_xdp() as well.

Thank you very much!

Dongli Zhang