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Tue, 20 Oct 2026 09:54:22 -0700 (PDT) Received: from localhost ([2a03:2880:2ff:51::]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2ea3b308afasm1724805ad.4.2026.10.20.09.54.21 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 20 Oct 2026 09:54:21 -0700 (PDT) Date: Tue, 20 Oct 2026 09:54:16 -0700 From: Stanislav Fomichev To: Kaifeng Wang Cc: netdev@vger.kernel.org, edumazet@google.com, ncardwell@google.com, kuniyu@google.com, davem@davemloft.net, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, asml.silence@gmail.com, almasrymina@google.com, willemb@google.com, kaiyuanz@google.com, sdf@fomichev.me, linux-kernel@vger.kernel.org Subject: Re: [PATCH net v2] tcp: reject devmem tx with fastopen and repair Message-ID: References: <20261019174522.55644-1-kaifengw@google.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20261019174522.55644-1-kaifengw@google.com> On 10/19, 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 the caller's memory > at those addresses 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 inside the Fast Open and TCP repair branches when a devmem > binding is present. Checking inside the tp->repair branch also covers > the case where repair mode is enabled while sk_stream_wait_connect() > drops the socket lock. > > Additionally, drop the !sockc_err gate from the devmem validation check > and return sockc_err if set. Otherwise, if a valid SCM_DEVMEM_DMABUF is > followed by a failing cmsg, sockc.dmabuf_id remains set while binding is > not acquired, allowing the Fast Open and TCP_RECV_QUEUE repair paths to > proceed and return before sockc_err is checked. > > 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") > Reviewed-by: Mina Almasry > Signed-off-by: Kaifeng Wang > --- > v1 -> v2: > - Move the devmem rejection checks into the Fast Open and TCP repair > branches directly, also covering the case where tp->repair is enabled > while sk_stream_wait_connect() drops the socket lock. > - Drop the !sockc_err gate from the devmem validation check and return > sockc_err if set, preventing a valid SCM_DEVMEM_DMABUF followed by a > failing cmsg from bypassing the check. Acked-by: Stanislav Fomichev