Re: [PATCH bpf v2 1/2] bpf, veth: xdp: fix page_pool page leak on skb-backed XDP

[Jiayuan Chen <jiayuan.chen@linux.dev>]

On 8/24/26 10:50 PM, Lorenzo Bianconi wrote:
>> On 8/24/26 6:31 PM, Lorenzo Bianconi wrote:
>>>> bpf_xdp_shrink_data() frees a released frag via __xdp_return() using
>>>> xdp->rxq->mem.type, but that type is wrong for skb-backed XDP: the skb is
>>>> cow'd into page_pool memory while the rxq still says MEM_TYPE_PAGE_SHARED,
>>>> so the page_pool page is freed with page_frag_free() and we hit
>>>> "Bad page state ... page_pool leak".
>>>>
>>>> Both generic XDP and veth are affected. A non-linear skb is cow'd into
>>>> page_pool memory (skb_cow_data_for_xdp() -> skb_pp_cow_data() for generic
>>>> XDP, veth_convert_skb_to_xdp_buff() for veth), so its frags become
>>>> page_pool pages while the rxq keeps MEM_TYPE_PAGE_SHARED.
>>>>
>>>> We can't just fix rxq->mem.type in place:
>>>> - generic XDP: xdp->rxq is dev->_rx[queue].xdp_rxq (see
>>>>     bpf_prog_run_generic_xdp()), a shared rxq that other CPUs may access in
>>>>     parallel, so we must not write to it.
>>>> - veth: rq->xdp_rxq.mem is shared per-queue state that veth resets on XDP
>>>>     teardown, and with GRO that reset runs without stopping in-flight NAPI,
>>>>     so a type stashed there can be clobbered under a packet still in flight.
>>>>
>>>> Adding a check in __xdp_return() or bpf_xdp_shrink_data() itself is not an
>>>> option either: without recording it somewhere, both can only guess the
>>>> frag's memory type, which quickly gets confusing.
>>>>
>>>> So record it in the xdp_buff. Add a XDP_FLAGS_FRAGS_PAGE_POOL flag; the two
>>>> skb-cow sites set it, and bpf_xdp_shrink_data() frees the frag to the
>>>> page_pool when it is set, otherwise it keeps falling back to
>>>> xdp->rxq->mem.type unchanged. No other path changes behaviour.
>>>>
>>>> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
>>>> Fixes: 0ebab78cbcbf ("net: veth: add page_pool for page recycling")
>>> Hi Jiayuan Chen,
>>>
>>> thx for fixing it. Can we do something like the patch below instead?
>>>
>>> Regards,
>>> Lorenzo
>>>
>>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>>> index 1d679e8fd649..4ed659b58141 100644
>>> --- a/net/core/xdp.c
>>> +++ b/net/core/xdp.c
>>> @@ -433,16 +433,16 @@ EXPORT_SYMBOL_GPL(xdp_rxq_info_attach_page_pool);
>>>    void __xdp_return(netmem_ref netmem, enum xdp_mem_type mem_type,
>>>    		  bool napi_direct, struct xdp_buff *xdp)
>>>    {
>>> +	netmem_ref head_netmem = netmem_compound_head(netmem);
>>> +	if (netmem_is_pp(head_netmem))
>>> +		mem_type = MEM_TYPE_PAGE_POOL;
>>> +
>>>    	switch (mem_type) {
>>>    	case MEM_TYPE_PAGE_POOL:
>>> -		netmem = netmem_compound_head(netmem);
>>>    		if (napi_direct && xdp_return_frame_no_direct())
>>>    			napi_direct = false;
>>> -		/* No need to check netmem_is_pp() as mem->type knows this a
>>> -		 * page_pool page
>>> -		 */
>>> -		page_pool_put_full_netmem(netmem_get_pp(netmem), netmem,
>>> -					  napi_direct);
>>> +		page_pool_put_full_netmem(netmem_get_pp(head_netmem),
>>> +					  head_netmem, napi_direct);
>>>    		break;
>>>    	case MEM_TYPE_PAGE_SHARED:
>>>    		page_frag_free(__netmem_address(netmem));
>>
>> Hi Lorenzo,
>>
>> I tried this, but it regresses the bpf selftest with a page_pool ref
>> underflow (0 warns on master, 45 with the patch):
>>
>>      WARNING: include/net/page_pool/helpers.h:297 at
>> page_pool_alloc_frag_netmem
>>      skb_pp_cow_data
>>      veth_xdp_rcv_skb
>>
>>
>> On XDP_TX/XDP_REDIRECT veth has to take plain page refs via get_page()
>> (veth_xdp_get()) and then
>> consume_skb(): the skb itself must be freed while the data pages stay alive
>> for the frame. consume_skb()
>> already returns the skb's page_pool ref, so what the frame holds afterwards
>> is a plain page ref, to be
>> dropped with page_frag_free().
>>
>> netmem_is_pp() can't see that: it only says the page still belongs to a pool
>> (other users may still hold pool refs on the same page),
>> not what kind of ref we're dropping. So __xdp_return() turns those plain-ref
>> drops into a second pool
>> put and pp_ref goes negative.
>>
>> That's why I kept the type in the xdp_buff and only override it in the
>> shrink path, where we know the
>> frag ref is the cow'd page_pool one.
>>
>> Regards,
>> Jiayuan
>>
> Right. I can see the point now :). IIUC we are currently able to trigger
> the issue just on xdp fragments running bpf_xdp_shrink_data() but the problem
> theoretically occurs even for the xdp->data, right? (it is rallocated using the
> page_pool in skb_pp_cow_data()). Is it better to always set this new flag when
> the buffers are reallocated via skb_pp_cow_data()? (Maybe renaming it in
> something like XDP_FLAGS_DATA_FROM_PP).
>
> Regards,
> Lorenzo


Hi Lorenzo,

Theoretically you are right, the linear part has the same issue.
But in practice the head is always freed via consume_skb()/kfree_skb(), 
which are pp-aware.
If we really want to go that way, we'd probably have to touch quite a 
few function signatures.
As a fix that also needs backporting, I think the current change is the 
minimal one that covers
all the reachable cases(happy to hear a better idea.).