linux-cve-announce - CVE-2025-38491: mptcp: make fallback action and fallback decision atomic

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Message-ID: <2025072817-CVE-2025-38491-859c@gregkh>
Date: Mon, 28 Jul 2025 13:22:33 +0200
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-38491: mptcp: make fallback action and fallback decision atomic

From: Greg Kroah-Hartman <gregkh@...nel.org>

Description
===========

In the Linux kernel, the following vulnerability has been resolved:

mptcp: make fallback action and fallback decision atomic

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 net/mptcp/protocol.h:1244 [inline]
  WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 check_fully_established net/mptcp/options.c:982 [inline]
  WARNING: CPU: 1 PID: 7704 at net/mptcp/protocol.h:1223 mptcp_incoming_options+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-gbd5ce2324dba #20 PREEMPT(voluntary)
  Hardware name: QEMU Ubuntu 24.04 PC (i440FX + PIIX, 1996), BIOS 1.16.3-debian-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 f4 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:
   <IRQ>
   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
   </IRQ>
   <TASK>
   __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 ff 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
   </TASK>
  irq event stamp: 4068
  hardirqs last  enabled at (4076): [<ffffffff81544816>] __up_console_sem+0x76/0x80 kernel/printk/printk.c:344
  hardirqs last disabled at (4085): [<ffffffff815447fb>] __up_console_sem+0x5b/0x80 kernel/printk/printk.c:342
  softirqs last  enabled at (3096): [<ffffffff840e1be0>] local_bh_enable include/linux/bottom_half.h:33 [inline]
  softirqs last  enabled at (3096): [<ffffffff840e1be0>] inet_csk_listen_stop+0x2c0/0x1070 net/ipv4/inet_connection_sock.c:1524
  softirqs last disabled at (3097): [<ffffffff813b6b9f>] 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.

Since the fallback operation can now fail and return value should be
checked, rename the helper accordingly.

The Linux kernel CVE team has assigned CVE-2025-38491 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.19 with commit 0530020a7c8f2204e784f0dbdc882bbd961fdbde and fixed in 6.12.40 with commit 54999dea879fecb761225e28f274b40662918c30
	Issue introduced in 5.19 with commit 0530020a7c8f2204e784f0dbdc882bbd961fdbde and fixed in 6.15.8 with commit 1d82a8fe6ee4afdc92f4e8808c9dad2a6095bbc5
	Issue introduced in 5.19 with commit 0530020a7c8f2204e784f0dbdc882bbd961fdbde and fixed in 6.16 with commit f8a1d9b18c5efc76784f5a326e905f641f839894
	Issue introduced in 5.10.228 with commit 609937aa962a62e93acfc04dd370b665e6152dfb
	Issue introduced in 5.15.169 with commit 6654efe264b014d8ea9fc38f79efb568b1b79069

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2025-38491
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/mptcp/options.c
	net/mptcp/protocol.c
	net/mptcp/protocol.h
	net/mptcp/subflow.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/54999dea879fecb761225e28f274b40662918c30
	https://git.kernel.org/stable/c/1d82a8fe6ee4afdc92f4e8808c9dad2a6095bbc5
	https://git.kernel.org/stable/c/f8a1d9b18c5efc76784f5a326e905f641f839894