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([2405:201:682f:383f:5d62:8570:2345:8a42]) by smtp.gmail.com with ESMTPSA id 98e67ed59e1d1-364be00b175sm11838194a91.9.2026.05.03.17.32.00 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 03 May 2026 17:32:04 -0700 (PDT) From: Deepanshu Kartikey To: matttbe@kernel.org, martineau@kernel.org, geliang@kernel.org, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org Cc: cpaasch@apple.com, netdev@vger.kernel.org, mptcp@lists.linux.dev, linux-kernel@vger.kernel.org, Deepanshu Kartikey , syzbot+ff020673c5e3d94d9478@syzkaller.appspotmail.com Subject: [PATCH] mptcp: zero opts->ext_copy in mptcp_established_options_dss() Date: Mon, 4 May 2026 06:01:56 +0530 Message-ID: <20260504003156.11125-1-kartikey406@gmail.com> X-Mailer: git-send-email 2.43.0 Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" syzbot reported a KMSAN uninit-value in mptcp_write_data_fin() reached via tcp_send_ack() from the subflow shutdown path: BUG: KMSAN: uninit-value in mptcp_write_data_fin net/mptcp/options.c:542 mptcp_established_options_dss net/mptcp/options.c:590 mptcp_established_options net/mptcp/options.c:874 tcp_established_options net/ipv4/tcp_output.c:1192 __tcp_transmit_skb net/ipv4/tcp_output.c:1575 __tcp_send_ack net/ipv4/tcp_output.c:4499 tcp_send_ack net/ipv4/tcp_output.c:4505 mptcp_subflow_shutdown net/mptcp/protocol.c:3137 mptcp_check_send_data_fin net/mptcp/protocol.c:3218 __mptcp_wr_shutdown net/mptcp/protocol.c:3234 __mptcp_close net/mptcp/protocol.c:3313 Local variable opts created at: __tcp_transmit_skb net/ipv4/tcp_output.c:1536 __tcp_transmit_skb() partially clears its on-stack tcp_out_options via "memset(&opts.cleared, ...)" which does not cover opts.mptcp, so MPTCP is responsible for initializing opts->ext_copy before use. mptcp_established_options_dss() only writes opts->ext_copy when an MPTCP extension is attached to the skb ("opts->ext_copy =3D *mpext"). When tcp_send_ack() builds a fresh pure-ACK skb during subflow shutdown, the skb has no extension and that assignment is skipped, but the function still calls mptcp_write_data_fin(&opts->ext_copy) because snd_data_fin_enable is set. mptcp_write_data_fin() then reads ext->use_map to choose between writing a synthetic DATA_FIN mapping or extending an existing one, and that read hits uninitialized stack memory. Zero opts->ext_copy at the top of mptcp_established_options_dss() so mptcp_write_data_fin() takes its synthetic-mapping branch on clean memory. Fixes: 9c29e3615274 ("mptcp: fix DSS map generation on fin retransmission") Reported-by: syzbot+ff020673c5e3d94d9478@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3Dff020673c5e3d94d9478 Signed-off-by: Deepanshu Kartikey --- net/mptcp/options.c | 1 + 1 file changed, 1 insertion(+) diff --git a/net/mptcp/options.c b/net/mptcp/options.c index 8a1c5698983c..3fb914196987 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -572,6 +572,7 @@ static bool mptcp_established_options_dss(struct sock *= sk, struct sk_buff *skb, bool ret =3D false; u64 ack_seq; =20 + memset(&opts->ext_copy, 0, sizeof(opts->ext_copy)); opts->csum_reqd =3D READ_ONCE(msk->csum_enabled); mpext =3D skb ? mptcp_get_ext(skb) : NULL; =20 --=20 2.43.0