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([2a01:4b00:bd21:4f00:7cc6:d3ca:494:116c]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-c317c1c4f7csm220428966b.35.2026.10.08.05.53.59 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 08 Oct 2026 05:54:00 -0700 (PDT) Message-ID: <7f377853-ae56-450b-889b-688ec335e81c@gmail.com> Date: Thu, 8 Oct 2026 13:53:51 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH net] tcp: reject devmem tx with fastopen and repair To: Kaifeng Wang , netdev@vger.kernel.org Cc: edumazet@google.com, ncardwell@google.com, kuniyu@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, almasrymina@google.com, willemb@google.com, kaiyuanz@google.com, sdf@fomichev.me, linux-kernel@vger.kernel.org References: <20261005214002.3226574-1-kaifengw@google.com> Content-Language: en-US From: Pavel Begunkov In-Reply-To: <20261005214002.3226574-1-kaifengw@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 10/5/26 22:40, Kaifeng Wang wrote: > tcp_sendmsg_locked() enforces that devmem TX can only proceed if the > zero-copy path is active and a valid dmabuf binding exists. However, > subsequent branches in tcp_sendmsg_locked() can still intercept the > message before it reaches the devmem zero-copy loop: > > 1. TCP Fast Open (MSG_FASTOPEN or DEFER_CONNECT): > If TCP_FASTOPEN_CONNECT is set, the socket may have a valid dst with > NETIF_F_SG (so zc == MSG_ZEROCOPY and binding is present), but > tcp_sendmsg_fastopen() -> tcp_send_syn_data() will use > copy_page_from_iter() to byte-copy from the iterator. Since iov_base > represents dma-buf offsets rather than user virtual addresses, this > misinterprets offsets as user pointers and copies arbitrary user memory > into the SYN packet. > > 2. TCP repair mode: > If tp->repair is enabled with TCP_RECV_QUEUE, tcp_send_rcvq() similarly > calls skb_copy_datagram_from_iter(), byte-copying from the iterator. > > Neither path supports or makes sense for devmem transmission. Reject devmem > sends if Fast Open or repair mode is active. > > This pre-existing issue was identified by Sashiko AI review on commit > 125755776bc6 ("tcp: reject non zerocopy devmem tx") and has not been > hit in production. > > Fixes: 125755776bc6 ("tcp: reject non zerocopy devmem tx") > Fixes: bd61848900bf ("net: devmem: Implement TX path") > Signed-off-by: Kaifeng Wang > --- > net/ipv4/tcp.c | 4 +++- > 1 file changed, 3 insertions(+), 1 deletion(-) > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c > index 87ef6d5cbfeb..225e758c194a 100644 > --- a/net/ipv4/tcp.c > +++ b/net/ipv4/tcp.c > @@ -1169,7 +1169,9 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size) > zc = MSG_SPLICE_PAGES; > } > > - if (!sockc_err && sockc.dmabuf_id && (zc != MSG_ZEROCOPY || !binding)) { > + if (!sockc_err && sockc.dmabuf_id && > + (zc != MSG_ZEROCOPY || !binding || tp->repair || > + (flags & MSG_FASTOPEN) || inet_test_bit(DEFER_CONNECT, sk))) { It becomes unhandy, and taking into account the issue Stan commented about, I think it'd make sense to do the checks under the repair and fast open "ifs". if (((1 << sk->sk_state) & ~(TCPF_ESTABLISHED | TCPF_CLOSE_WAIT)) && !tcp_passive_fastopen(sk)) { if (binding) // fail; } if (unlikely(tp->repair)) { if (binding) // fail; } -- Pavel Begunkov