netdev - Re: [PATCH bpf 1/2] xdp: update xdp_rxq_info's mem type in XDP generic hook

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Message-ID: <CAADnVQLGocfOT224=9_nJZ6093QDh1M_EDLQ3cNVQZKEDnjwog@mail.gmail.com>
Date: Fri, 3 Oct 2025 10:29:08 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc: bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	Ilias Apalodimas <ilias.apalodimas@...aro.org>, Toke Høiland-Jørgensen <toke@...hat.com>, 
	Lorenzo Bianconi <lorenzo@...nel.org>, Network Development <netdev@...r.kernel.org>, 
	"Karlsson, Magnus" <magnus.karlsson@...el.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Stanislav Fomichev <stfomichev@...il.com>, Alexander Lobakin <aleksander.lobakin@...el.com>, 
	syzbot+ff145014d6b0ce64a173@...kaller.appspotmail.com, 
	Ihor Solodrai <ihor.solodrai@...ux.dev>, Octavian Purdila <tavip@...gle.com>
Subject: Re: [PATCH bpf 1/2] xdp: update xdp_rxq_info's mem type in XDP
 generic hook

On Fri, Oct 3, 2025 at 7:03 AM Maciej Fijalkowski
<maciej.fijalkowski@...el.com> wrote:
>
> Currently, generic XDP hook uses xdp_rxq_info from netstack Rx queues
> which do not have its XDP memory model registered. There is a case when
> XDP program calls bpf_xdp_adjust_tail() BPF helper that releases
> underlying memory. This happens when it consumes enough amount of bytes
> and when XDP buffer has fragments. For this action the memory model
> knowledge passed to XDP program is crucial so that core can call
> suitable function for freeing/recycling the page.
>
> For netstack queues it defaults to MEM_TYPE_PAGE_SHARED (0) due to lack
> of mem model registration. The problem we're fixing here is when kernel
> copied the skb to new buffer backed by system's page_pool and XDP buffer
> is built around it. Then when bpf_xdp_adjust_tail() calls
> __xdp_return(), it acts incorrectly due to mem type not being set to
> MEM_TYPE_PAGE_POOL and causes a page leak.
>
> For this purpose introduce a small helper, xdp_update_mem_type(), that
> could be used on other callsites such as veth which are open to this
> problem as well. Here we call it right before executing XDP program in
> generic XDP hook.
>
> This problem was triggered by syzbot as well as AF_XDP test suite which
> is about to be integrated to BPF CI.
>
> Reported-by: syzbot+ff145014d6b0ce64a173@...kaller.appspotmail.com
> Closes: https://lore.kernel.org/netdev/6756c37b.050a0220.a30f1.019a.GAE@google.com/
> Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in generic mode")
> Tested-by: Ihor Solodrai <ihor.solodrai@...ux.dev>
> Co-developed-by: Octavian Purdila <tavip@...gle.com>
> Signed-off-by: Octavian Purdila <tavip@...gle.com> # whole analysis, testing, initiating a fix
> Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@...el.com> # commit msg and proposed more robust fix
> ---
>  include/net/xdp.h | 7 +++++++
>  net/core/dev.c    | 2 ++
>  2 files changed, 9 insertions(+)
>
> diff --git a/include/net/xdp.h b/include/net/xdp.h
> index f288c348a6c1..5568e41cc191 100644
> --- a/include/net/xdp.h
> +++ b/include/net/xdp.h
> @@ -336,6 +336,13 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
>         skb->pfmemalloc |= pfmemalloc;
>  }
>
> +static inline void
> +xdp_update_mem_type(struct xdp_buff *xdp)
> +{
> +       xdp->rxq->mem.type = page_pool_page_is_pp(virt_to_page(xdp->data)) ?
> +               MEM_TYPE_PAGE_POOL : MEM_TYPE_PAGE_SHARED;
> +}

AI says that it's racy and I think it's onto something.
See
https://github.com/kernel-patches/bpf/actions/runs/18224704286/job/51892959919
and
https://github.com/kernel-patches/bpf/actions/runs/18224704286/job/51892959937

click on 'Check review-inline.txt'