linux-cve-announce - CVE-2025-71126: mptcp: avoid deadlock on fallback while reinjecting

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Message-ID: <2026011418-CVE-2025-71126-8538@gregkh>
Date: Wed, 14 Jan 2026 16:06:31 +0100
From: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To: linux-cve-announce@...r.kernel.org
Cc: Greg Kroah-Hartman <gregkh@...nel.org>
Subject: CVE-2025-71126: mptcp: avoid deadlock on fallback while reinjecting

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

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

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

mptcp: avoid deadlock on fallback while reinjecting

Jakub reported an MPTCP deadlock at fallback time:

 WARNING: possible recursive locking detected
 6.18.0-rc7-virtme #1 Not tainted
 --------------------------------------------
 mptcp_connect/20858 is trying to acquire lock:
 ff1100001da18b60 (&msk->fallback_lock){+.-.}-{3:3}, at: __mptcp_try_fallback+0xd8/0x280

 but task is already holding lock:
 ff1100001da18b60 (&msk->fallback_lock){+.-.}-{3:3}, at: __mptcp_retrans+0x352/0xaa0

 other info that might help us debug this:
  Possible unsafe locking scenario:

        CPU0
        ----
   lock(&msk->fallback_lock);
   lock(&msk->fallback_lock);

  *** DEADLOCK ***

  May be due to missing lock nesting notation

 3 locks held by mptcp_connect/20858:
  #0: ff1100001da18290 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_sendmsg+0x114/0x1bc0
  #1: ff1100001db40fd0 (k-sk_lock-AF_INET#2){+.+.}-{0:0}, at: __mptcp_retrans+0x2cb/0xaa0
  #2: ff1100001da18b60 (&msk->fallback_lock){+.-.}-{3:3}, at: __mptcp_retrans+0x352/0xaa0

 stack backtrace:
 CPU: 0 UID: 0 PID: 20858 Comm: mptcp_connect Not tainted 6.18.0-rc7-virtme #1 PREEMPT(full)
 Hardware name: Bochs, BIOS Bochs 01/01/2011
 Call Trace:
  <TASK>
  dump_stack_lvl+0x6f/0xa0
  print_deadlock_bug.cold+0xc0/0xcd
  validate_chain+0x2ff/0x5f0
  __lock_acquire+0x34c/0x740
  lock_acquire.part.0+0xbc/0x260
  _raw_spin_lock_bh+0x38/0x50
  __mptcp_try_fallback+0xd8/0x280
  mptcp_sendmsg_frag+0x16c2/0x3050
  __mptcp_retrans+0x421/0xaa0
  mptcp_release_cb+0x5aa/0xa70
  release_sock+0xab/0x1d0
  mptcp_sendmsg+0xd5b/0x1bc0
  sock_write_iter+0x281/0x4d0
  new_sync_write+0x3c5/0x6f0
  vfs_write+0x65e/0xbb0
  ksys_write+0x17e/0x200
  do_syscall_64+0xbb/0xfd0
  entry_SYSCALL_64_after_hwframe+0x4b/0x53
 RIP: 0033:0x7fa5627cbc5e
 Code: 4d 89 d8 e8 14 bd 00 00 4c 8b 5d f8 41 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 11 c9 c3 0f 1f 80 00 00 00 00 48 8b 45 10 0f 05 <c9> c3 83 e2 39 83 fa 08 75 e7 e8 13 ff ff ff 0f 1f 00 f3 0f 1e fa
 RSP: 002b:00007fff1fe14700 EFLAGS: 00000202 ORIG_RAX: 0000000000000001
 RAX: ffffffffffffffda RBX: 0000000000000005 RCX: 00007fa5627cbc5e
 RDX: 0000000000001f9c RSI: 00007fff1fe16984 RDI: 0000000000000005
 RBP: 00007fff1fe14710 R08: 0000000000000000 R09: 0000000000000000
 R10: 0000000000000000 R11: 0000000000000202 R12: 00007fff1fe16920
 R13: 0000000000002000 R14: 0000000000001f9c R15: 0000000000001f9c

The packet scheduler could attempt a reinjection after receiving an
MP_FAIL and before the infinite map has been transmitted, causing a
deadlock since MPTCP needs to do the reinjection atomically from WRT
fallback.

Address the issue explicitly avoiding the reinjection in the critical
scenario. Note that this is the only fallback critical section that
could potentially send packets and hit the double-lock.

The Linux kernel CVE team has assigned CVE-2025-71126 to this issue.


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

	Issue introduced in 6.1.149 with commit 5586518bec27666c747cd52aabb62d485686d0bf and fixed in 6.1.160 with commit 0107442e82c0f8d6010e07e6030741c59c520d6e
	Issue introduced in 6.6.101 with commit 75a4c9ab8a7af0d76b31ccd1188ed178c38b35d2 and fixed in 6.6.120 with commit 252892d5a6a2f163ce18f32716e46fa4da7d4e79
	Issue introduced in 6.12.40 with commit 54999dea879fecb761225e28f274b40662918c30 and fixed in 6.12.64 with commit 0ca9fb4335e726dab4f23b3bfe87271d8f005f41
	Issue introduced in 6.16 with commit f8a1d9b18c5efc76784f5a326e905f641f839894 and fixed in 6.18.3 with commit 50f47c02be419bf0a3ae94c118addf67beef359f
	Issue introduced in 6.16 with commit f8a1d9b18c5efc76784f5a326e905f641f839894 and fixed in 6.19-rc2 with commit ffb8c27b0539dd90262d1021488e7817fae57c42
	Issue introduced in 6.15.8 with commit 1d82a8fe6ee4afdc92f4e8808c9dad2a6095bbc5

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-2025-71126
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/protocol.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/0107442e82c0f8d6010e07e6030741c59c520d6e
	https://git.kernel.org/stable/c/252892d5a6a2f163ce18f32716e46fa4da7d4e79
	https://git.kernel.org/stable/c/0ca9fb4335e726dab4f23b3bfe87271d8f005f41
	https://git.kernel.org/stable/c/50f47c02be419bf0a3ae94c118addf67beef359f
	https://git.kernel.org/stable/c/ffb8c27b0539dd90262d1021488e7817fae57c42