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([147.46.174.222]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2c164f88473sm26810795ad.25.2026.06.03.05.26.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 03 Jun 2026 05:26:52 -0700 (PDT) From: Sechang Lim To: willemdebruijn.kernel@gmail.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com Cc: horms@kernel.org, daniel@iogearbox.net, cong.wang@bytedance.com, john.fastabend@gmail.com, jakub@cloudflare.com, netdev@vger.kernel.org, bpf@vger.kernel.org, linux-kernel@vger.kernel.org Subject: [PATCH net] udp: clear skb->dev before running a sockmap verdict Date: Wed, 3 Jun 2026 12:26:37 +0000 Message-ID: <20260603122639.615994-1-rhkrqnwk98@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" On the UDP receive path skb->dev is repurposed as dev_scratch (the truesize/state cache set by udp_set_dev_scratch()), through the union { struct net_device *dev; unsigned long dev_scratch; } in sk_buff. When a UDP socket is in a sockmap, sk_data_ready is sk_psock_verdict_data_ready(), which calls udp_read_skb() -> recv_actor() (sk_psock_verdict_recv) to run the attached SK_SKB verdict program in softi= rq. If that program calls a socket-lookup helper (bpf_sk_lookup_tcp/udp, bpf_skc_lookup_tcp), bpf_skc_lookup() does: if (skb->dev) caller_net =3D dev_net(skb->dev); skb->dev still holds the dev_scratch value (a non-NULL integer), so dev_net= () dereferences it as a struct net_device * and the kernel takes a general protection fault on a non-canonical address in softirq: Oops: general protection fault, probably for non-canonical address 0x1010= 000800004a0 CPU: 1 UID: 0 PID: 1406 Comm: syz.2.19 Not tainted 7.1.0-rc6 #1 PREEMPT(f= ull) RIP: 0010:bpf_skc_lookup net/core/filter.c:7033 [inline] RIP: 0010:bpf_sk_lookup+0x45/0x160 net/core/filter.c:7047 Call Trace: bpf_prog_4675cb904b7071f8+0x12e/0x14e bpf_prog_run_pin_on_cpu+0xc6/0x1f0 sk_psock_verdict_recv+0x1ba/0x350 udp_read_skb+0x31a/0x370 sk_psock_verdict_data_ready+0x2e3/0x600 __udp_enqueue_schedule_skb+0x4c8/0x650 udpv6_queue_rcv_one_skb+0x3ec/0x740 udp6_unicast_rcv_skb+0x11d/0x140 ip6_protocol_deliver_rcu+0x61e/0x950 ip6_input_finish+0xa9/0x150 NF_HOOK+0x286/0x2f0 ip6_input+0x117/0x220 NF_HOOK+0x286/0x2f0 __netif_receive_skb+0x85/0x200 process_backlog+0x374/0x9a0 __napi_poll+0x4f/0x1c0 net_rx_action+0x3b0/0x770 handle_softirqs+0x15a/0x460 do_softirq+0x57/0x80 The rmem charge that dev_scratch accounted for is released by skb_recv_udp(= ) on dequeue, just above, so the scratch is dead by the time recv_actor() runs. = Clear skb->dev so bpf_skc_lookup() falls back to sock_net(skb->sk), which skb_set_owner_sk_safe() set just above. Fixes: 965b57b469a5 ("net: Introduce a new proto_ops ->read_skb()") Signed-off-by: Sechang Lim --- net/ipv4/udp.c | 5 +++++ 1 file changed, 5 insertions(+) diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c index 0ac2bf4f8759..f5ba36e721cb 100644 --- a/net/ipv4/udp.c +++ b/net/ipv4/udp.c @@ -2011,6 +2011,11 @@ int udp_read_skb(struct sock *sk, skb_read_actor_t r= ecv_actor) } =20 WARN_ON_ONCE(!skb_set_owner_sk_safe(skb, sk)); + /* skb->dev still aliases the UDP rx dev_scratch (its charge was freed + * on dequeue above); a sockmap verdict program may deref it via + * bpf_sk_lookup_*(), so clear it -> bpf_skc_lookup() uses skb->sk + */ + skb->dev =3D NULL; return recv_actor(sk, skb); } =20 --=20 2.43.0