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Fri, 24 Oct 2025 02:25:31 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGlOwaUMR8VZbVJ4djy08L7hrr6l8ANdpbOoJ+KZ4mKPMSj4WRf9s7vqK0lEtnMuhu39rcxDw== X-Received: by 2002:a05:6402:13cc:b0:63b:dc7d:7308 with SMTP id 4fb4d7f45d1cf-63c1f6364d3mr25741560a12.5.1761297931165; Fri, 24 Oct 2025 02:25:31 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([45.145.92.2]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-63e3ebb2736sm3659474a12.6.2025.10.24.02.25.30 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 24 Oct 2025 02:25:30 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 6C4782EA50E; Fri, 24 Oct 2025 11:25:26 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Maciej Fijalkowski , bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, hawk@kernel.org Cc: netdev@vger.kernel.org, magnus.karlsson@intel.com, aleksander.lobakin@intel.com, ilias.apalodimas@linaro.org, lorenzo@kernel.org, kuba@kernel.org, Maciej Fijalkowski , syzbot+ff145014d6b0ce64a173@syzkaller.appspotmail.com, Ihor Solodrai , Octavian Purdila Subject: Re: [PATCH v3 bpf 1/2] xdp: introduce xdp_convert_skb_to_buff() In-Reply-To: <20251022125209.2649287-2-maciej.fijalkowski@intel.com> References: <20251022125209.2649287-1-maciej.fijalkowski@intel.com> <20251022125209.2649287-2-maciej.fijalkowski@intel.com> X-Clacks-Overhead: GNU Terry Pratchett Date: Fri, 24 Oct 2025 11:25:26 +0200 Message-ID: <87cy6cfy7t.fsf@toke.dk> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Maciej Fijalkowski writes: > 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, which in turn > 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. > > Pull out the existing code from bpf_prog_run_generic_xdp() that > init/prepares xdp_buff onto new helper xdp_convert_skb_to_buff() and > embed there rxq's mem_type initialization that is assigned to xdp_buff. > Make it agnostic to current skb->data position. > > 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@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/netdev/6756c37b.050a0220.a30f1.019a.GAE@g= oogle.com/ > Fixes: e6d5dbdd20aa ("xdp: add multi-buff support for xdp running in gene= ric mode") > Tested-by: Ihor Solodrai > Reviewed-by: Toke H=C3=B8iland-J=C3=B8rgensen > Co-developed-by: Octavian Purdila > Signed-off-by: Octavian Purdila # whole analysis, test= ing, initiating a fix > Signed-off-by: Maciej Fijalkowski # commit= msg and proposed more robust fix > --- > include/net/xdp.h | 27 +++++++++++++++++++++++++++ > net/core/dev.c | 25 ++++--------------------- > 2 files changed, 31 insertions(+), 21 deletions(-) > > diff --git a/include/net/xdp.h b/include/net/xdp.h > index aa742f413c35..cec43f56ae9a 100644 > --- a/include/net/xdp.h > +++ b/include/net/xdp.h > @@ -384,6 +384,33 @@ struct sk_buff *xdp_build_skb_from_frame(struct xdp_= frame *xdpf, > struct net_device *dev); > struct xdp_frame *xdpf_clone(struct xdp_frame *xdpf); >=20=20 > +static inline > +void xdp_convert_skb_to_buff(struct sk_buff *skb, struct xdp_buff *xdp, > + struct xdp_rxq_info *xdp_rxq) > +{ > + u32 frame_sz, pkt_len; > + > + /* SKB "head" area always have tailroom for skb_shared_info */ > + frame_sz =3D skb_end_pointer(skb) - skb->head; > + frame_sz +=3D SKB_DATA_ALIGN(sizeof(struct skb_shared_info)); > + > + DEBUG_NET_WARN_ON_ONCE(!skb_mac_header_was_set(skb)); > + pkt_len =3D skb->tail - skb->mac_header; Should probably just use the helpers here: pkt_len =3D skb_tail_pointer(skb) - skb_mac_header(skb); that way you don't have to open-code the WARN_ON_ONCE, and you get the right behaviour even when NET_SKBUFF_DATA_USES_OFFSET is set -Toke