netdev - Re: [PATCH net] xdp: use multi-buff only if receive queue supports page pool

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Message-ID: <aNUObDuryXVFJ1T9@boxer>
Date: Thu, 25 Sep 2025 11:42:04 +0200
From: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To: Octavian Purdila <tavip@...gle.com>
CC: Jakub Kicinski <kuba@...nel.org>, <davem@...emloft.net>,
	<edumazet@...gle.com>, <pabeni@...hat.com>, <horms@...nel.org>,
	<ast@...nel.org>, <daniel@...earbox.net>, <hawk@...nel.org>,
	<john.fastabend@...il.com>, <sdf@...ichev.me>, <ahmed.zaki@...el.com>,
	<aleksander.lobakin@...el.com>, <toke@...hat.com>, <lorenzo@...nel.org>,
	<netdev@...r.kernel.org>, <bpf@...r.kernel.org>,
	<syzbot+ff145014d6b0ce64a173@...kaller.appspotmail.com>, Kuniyuki Iwashima
	<kuniyu@...gle.com>
Subject: Re: [PATCH net] xdp: use multi-buff only if receive queue supports
 page pool

On Thu, Sep 25, 2025 at 12:53:53AM -0700, Octavian Purdila wrote:
> On Wed, Sep 24, 2025 at 5:09 PM Jakub Kicinski <kuba@...nel.org> wrote:
> >
> > On Wed, 24 Sep 2025 06:08:42 +0000 Octavian Purdila wrote:
> > > When a BPF program that supports BPF_F_XDP_HAS_FRAGS is issuing
> > > bpf_xdp_adjust_tail and a large packet is injected via /dev/net/tun a
> > > crash occurs due to detecting a bad page state (page_pool leak).
> > >
> > > This is because xdp_buff does not record the type of memory and
> > > instead relies on the netdev receive queue xdp info. Since the TUN/TAP
> > > driver is using a MEM_TYPE_PAGE_SHARED memory model buffer, shrinking
> > > will eventually call page_frag_free. But with current multi-buff
> > > support for BPF_F_XDP_HAS_FRAGS programs buffers are allocated via the
> > > page pool.
> > >
> > > To fix this issue check that the receive queue memory mode is of
> > > MEM_TYPE_PAGE_POOL before using multi-buffs.
> >
> > This can also happen on veth, right? And veth re-stamps the Rx queues.

What do you mean by 're-stamps' in this case?

> 
> I am not sure if re-stamps will have ill effects.
> 
> The allocation and deallocation for this issue happens while
> processing the same packet (receive skb -> skb_pp_cow_data ->
> page_pool alloc ... __bpf_prog_run ->  bpf_xdp_adjust_tail).
> 
> IIUC, if the veth re-stamps the RX queue to MEM_TYPE_PAGE_POOL
> skb_pp_cow_data will proceed to allocate from page_pool and
> bpf_xdp_adjust_tail will correctly free from page_pool.

netif_get_rxqueue() gives you a pointer the netstack queue, not the driver
one. Then you take the xdp_rxq from there. Do we even register memory
model for these queues? Or am I missing something here.

We're in generic XDP hook where driver specifics should not matter here
IMHO.

>