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key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-02.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 316B51956086 for ; Sat, 5 Jul 2025 07:24:58 +0000 (UTC) Received: from gerbillo.redhat.com (unknown [10.45.224.15]) by mx-prod-int-05.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 2864D195608F for ; Sat, 5 Jul 2025 07:24:56 +0000 (UTC) From: Paolo Abeni To: mptcp@lists.linux.dev Subject: [PATCH mptcp-net v2 1/5] mptcp: make fallback action and fallback decision atomic Date: Sat, 5 Jul 2025 09:24:45 +0200 Message-ID: <7220b3c204d75d50e480f6045295a825ae80fd38.1751699999.git.pabeni@redhat.com> In-Reply-To: References: Precedence: bulk X-Mailing-List: mptcp@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.0 on 10.30.177.17 X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: 6ZnAY9I4H7QQoq9wVJLRRbMjjPEqBA4nhG7WuUTVqFc_1751700298 X-Mimecast-Originator: redhat.com Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8"; x-default="true" Syzkaller reported the following splat: WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 __mptcp_do_fallback = net/mptcp/protocol.h:1223 [inline] WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 mptcp_do_fallback ne= t/mptcp/protocol.h:1244 [inline] WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 check_fully_establis= hed net/mptcp/options.c:982 [inline] WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 mptcp_incoming_optio= ns+0x21a8/0x2510 net/mptcp/options.c:1153 Modules linked in: CPU: 1 UID: 0 PID: 7704 Comm: syz.3.1419 Not tainted 6.16.0-rc3-gbd5ce2324d= ba #20 PREEMPT(voluntary) Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debi= an-1.16.3-2 04/01/2014 RIP: 0010:__mptcp_do_fallback net/mptcp/protocol.h:1223 [inline] RIP: 0010:mptcp_do_fallback net/mptcp/protocol.h:1244 [inline] RIP: 0010:check_fully_established net/mptcp/options.c:982 [inline] RIP: 0010:mptcp_incoming_options+0x21a8/0x2510 net/mptcp/options.c:1153 Code: 24 18 e8 bb 2a 00 fd e9 1b df ff ff e8 b1 21 0f 00 e8 ec 5f c4 fc 44 = 0f b7 ac 24 b0 00 00 00 e9 54 f1 ff ff e8 d9 5f c4 fc 90 <0f> 0b 90 e9 b8 f= 4 ff ff e8 8b 2a 00 fd e9 8d e6 ff ff e8 81 2a 00 RSP: 0018:ffff8880a3f08448 EFLAGS: 00010246 RAX: 0000000000000000 RBX: ffff8880180a8000 RCX: ffffffff84afcf45 RDX: ffff888090223700 RSI: ffffffff84afdaa7 RDI: 0000000000000001 RBP: ffff888017955780 R08: 0000000000000001 R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000 R13: ffff8880180a8910 R14: ffff8880a3e9d058 R15: 0000000000000000 FS: 00005555791b8500(0000) GS:ffff88811c495000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 000000110c2800b7 CR3: 0000000058e44000 CR4: 0000000000350ef0 Call Trace: tcp_reset+0x26f/0x2b0 net/ipv4/tcp_input.c:4432 tcp_validate_incoming+0x1057/0x1b60 net/ipv4/tcp_input.c:5975 tcp_rcv_established+0x5b5/0x21f0 net/ipv4/tcp_input.c:6166 tcp_v4_do_rcv+0x5dc/0xa70 net/ipv4/tcp_ipv4.c:1925 tcp_v4_rcv+0x3473/0x44a0 net/ipv4/tcp_ipv4.c:2363 ip_protocol_deliver_rcu+0xba/0x480 net/ipv4/ip_input.c:205 ip_local_deliver_finish+0x2f1/0x500 net/ipv4/ip_input.c:233 NF_HOOK include/linux/netfilter.h:317 [inline] NF_HOOK include/linux/netfilter.h:311 [inline] ip_local_deliver+0x1be/0x560 net/ipv4/ip_input.c:254 dst_input include/net/dst.h:469 [inline] ip_rcv_finish net/ipv4/ip_input.c:447 [inline] NF_HOOK include/linux/netfilter.h:317 [inline] NF_HOOK include/linux/netfilter.h:311 [inline] ip_rcv+0x514/0x810 net/ipv4/ip_input.c:567 __netif_receive_skb_one_core+0x197/0x1e0 net/core/dev.c:5975 __netif_receive_skb+0x1f/0x120 net/core/dev.c:6088 process_backlog+0x301/0x1360 net/core/dev.c:6440 __napi_poll.constprop.0+0xba/0x550 net/core/dev.c:7453 napi_poll net/core/dev.c:7517 [inline] net_rx_action+0xb44/0x1010 net/core/dev.c:7644 handle_softirqs+0x1d0/0x770 kernel/softirq.c:579 do_softirq+0x3f/0x90 kernel/softirq.c:480 __local_bh_enable_ip+0xed/0x110 kernel/softirq.c:407 local_bh_enable include/linux/bottom_half.h:33 [inline] inet_csk_listen_stop+0x2c5/0x1070 net/ipv4/inet_connection_sock.c:1524 mptcp_check_listen_stop.part.0+0x1cc/0x220 net/mptcp/protocol.c:2985 mptcp_check_listen_stop net/mptcp/mib.h:118 [inline] __mptcp_close+0x9b9/0xbd0 net/mptcp/protocol.c:3000 mptcp_close+0x2f/0x140 net/mptcp/protocol.c:3066 inet_release+0xed/0x200 net/ipv4/af_inet.c:435 inet6_release+0x4f/0x70 net/ipv6/af_inet6.c:487 __sock_release+0xb3/0x270 net/socket.c:649 sock_close+0x1c/0x30 net/socket.c:1439 __fput+0x402/0xb70 fs/file_table.c:465 task_work_run+0x150/0x240 kernel/task_work.c:227 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline] exit_to_user_mode_loop+0xd4/0xe0 kernel/entry/common.c:114 exit_to_user_mode_prepare include/linux/entry-common.h:330 [inline] syscall_exit_to_user_mode_work include/linux/entry-common.h:414 [inline] syscall_exit_to_user_mode include/linux/entry-common.h:449 [inline] do_syscall_64+0x245/0x360 arch/x86/entry/syscall_64.c:100 entry_SYSCALL_64_after_hwframe+0x77/0x7f RIP: 0033:0x7fc92f8a36ad Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 90 f3 0f 1e fa 48 89 f8 48 89 f7 = 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff f= f 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48 RSP: 002b:00007ffcf52802d8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4 RAX: 0000000000000000 RBX: 00007ffcf52803a8 RCX: 00007fc92f8a36ad RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003 RBP: 00007fc92fae7ba0 R08: 0000000000000001 R09: 0000002800000000 R10: 00007fc92f700000 R11: 0000000000000246 R12: 00007fc92fae5fac R13: 00007fc92fae5fa0 R14: 0000000000026d00 R15: 0000000000026c51 irq event stamp: 4068 hardirqs last enabled at (4076): [] __up_console_sem+0x7= 6/0x80 kernel/printk/printk.c:344 hardirqs last disabled at (4085): [] __up_console_sem+0x5= b/0x80 kernel/printk/printk.c:342 softirqs last enabled at (3096): [] local_bh_enable incl= ude/linux/bottom_half.h:33 [inline] softirqs last enabled at (3096): [] inet_csk_listen_stop= +0x2c0/0x1070 net/ipv4/inet_connection_sock.c:1524 softirqs last disabled at (3097): [] do_softirq+0x3f/0x90= kernel/softirq.c:480 Since we need to track the 'fallback is possible' condition and the fallback status separately, there are a few possible races open between the check and the actual fallback action. Add a spinlock to protect the fallback related information and use it close all the possible related races. While at it also remove the too-early clearing of allow_infinite_fallback in __mptcp_subflow_connect(): the field will be correctly cleared by subflow_finish_connect() if/ when the connection will complete successfully. If fallback is not possible, as per RFC, reset the current subflow. Reported by: Matthieu Baerts Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/570 Reported-by: syzbot+5cf807c20386d699b524@syzkaller.appspotmail.com Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/555 Fixes: 0530020a7c8f ("mptcp: track and update contiguous data status") Signed-off-by: Paolo Abeni --- v1 -> v2: - remove unneeded and racing allow_infinite_fallback in __mptcp_subflow_connect() - really close race in subflow_check_data_avail() Sharing earlier to feed syzkaller. Hopefully a follow-up on mptcp-net-next will cleanup a bit the fallback handling, I tried to minimize the delta for backport's sake --- net/mptcp/options.c | 3 ++- net/mptcp/protocol.c | 40 +++++++++++++++++++++++++++++++++++----- net/mptcp/protocol.h | 21 +++++++++++++++------ net/mptcp/subflow.c | 8 ++++---- 4 files changed, 56 insertions(+), 16 deletions(-) diff --git a/net/mptcp/options.c b/net/mptcp/options.c index 56fd7bcb786c..4680d980e605 100644 --- a/net/mptcp/options.c +++ b/net/mptcp/options.c @@ -978,8 +978,9 @@ static bool check_fully_established(struct mptcp_sock *= msk, struct sock *ssk, if (subflow->mp_join) goto reset; subflow->mp_capable =3D 0; + if (mptcp_do_fallback(ssk)) + goto reset; pr_fallback(msk); - mptcp_do_fallback(ssk); return false; } =20 diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c index 1de42dc4e8ea..38bb11865b50 100644 --- a/net/mptcp/protocol.c +++ b/net/mptcp/protocol.c @@ -560,10 +560,9 @@ static bool mptcp_check_data_fin(struct sock *sk) =20 static void mptcp_dss_corruption(struct mptcp_sock *msk, struct sock *ssk) { - if (READ_ONCE(msk->allow_infinite_fallback)) { + if (mptcp_do_fallback(ssk)) { MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_DSSCORRUPTIONFALLBACK); - mptcp_do_fallback(ssk); } else { MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_DSSCORRUPTIONRESET); mptcp_subflow_reset(ssk); @@ -803,6 +802,14 @@ static bool __mptcp_finish_join(struct mptcp_sock *msk= , struct sock *ssk) if (sk->sk_state !=3D TCP_ESTABLISHED) return false; =20 + spin_lock_bh(&msk->fallback_lock); + if (__mptcp_check_fallback(msk)) { + spin_unlock_bh(&msk->fallback_lock); + return false; + } + mptcp_subflow_joined(msk, ssk); + spin_unlock_bh(&msk->fallback_lock); + /* attach to msk socket only after we are sure we will deal with it * at close time */ @@ -811,7 +818,6 @@ static bool __mptcp_finish_join(struct mptcp_sock *msk,= struct sock *ssk) =20 mptcp_subflow_ctx(ssk)->subflow_id =3D msk->subflow_id++; mptcp_sockopt_sync_locked(msk, ssk); - mptcp_subflow_joined(msk, ssk); mptcp_stop_tout_timer(sk); __mptcp_propagate_sndbuf(sk, ssk); return true; @@ -1136,10 +1142,14 @@ static void mptcp_update_infinite_map(struct mptcp_= sock *msk, mpext->infinite_map =3D 1; mpext->data_len =3D 0; =20 + if (!mptcp_do_fallback(ssk)) { + mptcp_subflow_reset(ssk); + return; + } + MPTCP_INC_STATS(sock_net(ssk), MPTCP_MIB_INFINITEMAPTX); mptcp_subflow_ctx(ssk)->send_infinite_map =3D 0; pr_fallback(msk); - mptcp_do_fallback(ssk); } =20 #define MPTCP_MAX_GSO_SIZE (GSO_LEGACY_MAX_SIZE - (MAX_TCP_HEADER + 1)) @@ -2543,9 +2553,9 @@ static void mptcp_check_fastclose(struct mptcp_sock *= msk) =20 static void __mptcp_retrans(struct sock *sk) { + struct mptcp_sendmsg_info info =3D { .data_lock_held =3D true, }; struct mptcp_sock *msk =3D mptcp_sk(sk); struct mptcp_subflow_context *subflow; - struct mptcp_sendmsg_info info =3D {}; struct mptcp_data_frag *dfrag; struct sock *ssk; int ret, err; @@ -2590,6 +2600,18 @@ static void __mptcp_retrans(struct sock *sk) info.sent =3D 0; info.limit =3D READ_ONCE(msk->csum_enabled) ? dfrag->data_len : dfrag->already_sent; + + /* + * make the whole retrans decision, xmit, disallow + * fallback atomic + */ + spin_lock_bh(&msk->fallback_lock); + if (__mptcp_check_fallback(msk)) { + spin_unlock_bh(&msk->fallback_lock); + release_sock(ssk); + return; + } + while (info.sent < info.limit) { ret =3D mptcp_sendmsg_frag(sk, ssk, dfrag, &info); if (ret <=3D 0) @@ -2605,6 +2627,7 @@ static void __mptcp_retrans(struct sock *sk) info.size_goal); WRITE_ONCE(msk->allow_infinite_fallback, false); } + spin_unlock_bh(&msk->fallback_lock); =20 release_sock(ssk); } @@ -2738,6 +2761,7 @@ static void __mptcp_init_sock(struct sock *sk) msk->last_ack_recv =3D tcp_jiffies32; =20 mptcp_pm_data_init(msk); + spin_lock_init(&msk->fallback_lock); =20 /* re-use the csk retrans timer for MPTCP-level retrans */ timer_setup(&msk->sk.icsk_retransmit_timer, mptcp_retransmit_timer, 0); @@ -3524,7 +3548,13 @@ bool mptcp_finish_join(struct sock *ssk) =20 /* active subflow, already present inside the conn_list */ if (!list_empty(&subflow->node)) { + spin_lock_bh(&msk->fallback_lock); + if (__mptcp_check_fallback(msk)) { + spin_unlock_bh(&msk->fallback_lock); + return false; + } mptcp_subflow_joined(msk, ssk); + spin_unlock_bh(&msk->fallback_lock); mptcp_propagate_sndbuf(parent, ssk); return true; } diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h index e36ae88b10ad..52c9175c7e49 100644 --- a/net/mptcp/protocol.h +++ b/net/mptcp/protocol.h @@ -352,6 +352,7 @@ struct mptcp_sock { u32 subflow_id; u32 setsockopt_seq; char ca_name[TCP_CA_NAME_MAX]; + spinlock_t fallback_lock; /* protects fallback && allow_infinite_fallback= */ }; =20 #define mptcp_data_lock(sk) spin_lock_bh(&(sk)->sk_lock.slock) @@ -1214,15 +1215,21 @@ static inline bool mptcp_check_fallback(const struc= t sock *sk) return __mptcp_check_fallback(msk); } =20 -static inline void __mptcp_do_fallback(struct mptcp_sock *msk) +static inline bool __mptcp_do_fallback(struct mptcp_sock *msk) { if (__mptcp_check_fallback(msk)) { pr_debug("TCP fallback already done (msk=3D%p)\n", msk); - return; + return true; } - if (WARN_ON_ONCE(!READ_ONCE(msk->allow_infinite_fallback))) - return; + spin_lock_bh(&msk->fallback_lock); + if (!msk->allow_infinite_fallback) { + spin_unlock_bh(&msk->fallback_lock); + return false; + } + set_bit(MPTCP_FALLBACK_DONE, &msk->flags); + spin_unlock_bh(&msk->fallback_lock); + return true; } =20 static inline bool __mptcp_has_initial_subflow(const struct mptcp_sock *ms= k) @@ -1234,14 +1241,15 @@ static inline bool __mptcp_has_initial_subflow(cons= t struct mptcp_sock *msk) TCPF_SYN_RECV | TCPF_LISTEN)); } =20 -static inline void mptcp_do_fallback(struct sock *ssk) +static inline bool mptcp_do_fallback(struct sock *ssk) { struct mptcp_subflow_context *subflow =3D mptcp_subflow_ctx(ssk); struct sock *sk =3D subflow->conn; struct mptcp_sock *msk; =20 msk =3D mptcp_sk(sk); - __mptcp_do_fallback(msk); + if (!__mptcp_do_fallback(msk)) + return false; if (READ_ONCE(msk->snd_data_fin_enable) && !(ssk->sk_shutdown & SEND_SHUT= DOWN)) { gfp_t saved_allocation =3D ssk->sk_allocation; =20 @@ -1253,6 +1261,7 @@ static inline void mptcp_do_fallback(struct sock *ssk) tcp_shutdown(ssk, SEND_SHUTDOWN); ssk->sk_allocation =3D saved_allocation; } + return true; } =20 #define pr_fallback(a) pr_debug("%s:fallback to TCP (msk=3D%p)\n", __func_= _, a) diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c index 15613d691bfe..433c0b25dbc1 100644 --- a/net/mptcp/subflow.c +++ b/net/mptcp/subflow.c @@ -544,9 +544,11 @@ static void subflow_finish_connect(struct sock *sk, co= nst struct sk_buff *skb) mptcp_get_options(skb, &mp_opt); if (subflow->request_mptcp) { if (!(mp_opt.suboptions & OPTION_MPTCP_MPC_SYNACK)) { + if (!mptcp_do_fallback(sk)) + goto do_reset; + MPTCP_INC_STATS(sock_net(sk), MPTCP_MIB_MPCAPABLEACTIVEFALLBACK); - mptcp_do_fallback(sk); pr_fallback(msk); goto fallback; } @@ -1395,7 +1397,7 @@ static bool subflow_check_data_avail(struct sock *ssk) return true; } =20 - if (!READ_ONCE(msk->allow_infinite_fallback)) { + if (!mptcp_do_fallback(ssk)) { /* fatal protocol error, close the socket. * subflow_error_report() will introduce the appropriate barriers */ @@ -1414,7 +1416,6 @@ static bool subflow_check_data_avail(struct sock *ssk) return false; } =20 - mptcp_do_fallback(ssk); } =20 skb =3D skb_peek(&ssk->sk_receive_queue); @@ -1679,7 +1680,6 @@ int __mptcp_subflow_connect(struct sock *sk, const st= ruct mptcp_pm_local *local, /* discard the subflow socket */ mptcp_sock_graft(ssk, sk->sk_socket); iput(SOCK_INODE(sf)); - WRITE_ONCE(msk->allow_infinite_fallback, false); mptcp_stop_tout_timer(sk); return 0; =20 --=20 2.49.0