CVE: CVE-2025-21705
Published: 2025-02-27
Description: In the Linux kernel, the following vulnerability has been resolved:

mptcp: handle fastopen disconnect correctly

Syzbot was able to trigger a data stream corruption:

  WARNING: CPU: 0 PID: 9846 at net/mptcp/protocol.c:1024 __mptcp_clean_una+0xddb/0xff0 net/mptcp/protocol.c:1024
  Modules linked in:
  CPU: 0 UID: 0 PID: 9846 Comm: syz-executor351 Not tainted 6.13.0-rc2-syzkaller-00059-g00a5acdbf398 #0
  Hardware name: Google Compute Engine/Google Compute Engine, BIOS Google 11/25/2024
  RIP: 0010:__mptcp_clean_una+0xddb/0xff0 net/mptcp/protocol.c:1024
  Code: fa ff ff 48 8b 4c 24 18 80 e1 07 fe c1 38 c1 0f 8c 8e fa ff ff 48 8b 7c 24 18 e8 e0 db 54 f6 e9 7f fa ff ff e8 e6 80 ee f5 90 <0f> 0b 90 4c 8b 6c 24 40 4d 89 f4 e9 04 f5 ff ff 44 89 f1 80 e1 07
  RSP: 0018:ffffc9000c0cf400 EFLAGS: 00010293
  RAX: ffffffff8bb0dd5a RBX: ffff888033f5d230 RCX: ffff888059ce8000
  RDX: [PII_REDACTED] RSI: [PII_REDACTED] RDI: [PII_REDACTED]
  RBP: ffffc9000c0cf518 R08: ffffffff8bb0d1dd R09: 1ffff110170c8928
  R10: dffffc[PII_REDACTED] R11: ffffed10170c8929 R12: [PII_REDACTED]
  R13: ffff888033f5d220 R14: dffffc[PII_REDACTED] R15: ffff8880592b8000
  FS:  00007f6e866496c0(0000) GS:ffff8880b8600000(0000) knlGS:[PII_REDACTED]
  CS:  0010 DS: 0000 ES: 0000 CR0: [PII_REDACTED]
  CR2: 00007f6e86f491a0 CR3:[PII_REDACTED]0e6000 CR4:[PII_REDACTED]3526f0
  DR0: [PII_REDACTED] DR1: [PII_REDACTED] DR2: [PII_REDACTED]
  DR3: [PII_REDACTED] DR6: 00000000fffe0ff0 DR7: [PII_REDACTED]
  Call Trace:
   <TASK>
   __mptcp_clean_una_wakeup+0x7f/0x2d0 net/mptcp/protocol.c:1074
   mptcp_release_cb+0x7cb/0xb30 net/mptcp/protocol.c:3493
   release_sock+0x1aa/0x1f0 net/core/sock.c:3640
   inet_wait_for_connect net/ipv4/af_inet.c:609 [inline]
   __inet_stream_connect+0x8bd/0xf30 net/ipv4/af_inet.c:703
   mptcp_sendmsg_fastopen+0x2a2/0x530 net/mptcp/protocol.c:1755
   mptcp_sendmsg+0x1884/0x1b10 net/mptcp/protocol.c:1830
   sock_sendmsg_nosec net/socket.c:711 [inline]
   __sock_sendmsg+0x1a6/0x270 net/socket.c:726
   ____sys_sendmsg+0x52a/0x7e0 net/socket.c:2583
   ___sys_sendmsg net/socket.c:2637 [inline]
   __sys_sendmsg+0x269/0x350 net/socket.c:2669
   do_syscall_x64 arch/x86/entry/common.c:52 [inline]
   do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83
   entry_SYSCALL_64_after_hwframe+0x77/0x7f
  RIP: 0033:0x7f6e86ebfe69
  Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 b1 1f 00 00 90 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:00007f6e86649168 EFLAGS: 00000246 ORIG_RAX:[PII_REDACTED]00002e
  RAX: ffffffffffffffda RBX: 00007f6e86f491b8 RCX: 00007f6e86ebfe69
  RDX: [PII_REDACTED] RSI: [PII_REDACTED] RDI: [PII_REDACTED]
  RBP: 00007f6e86f491b0 R08: 00007f6e866496c0 R09: [PII_REDACTED]
  R10: [PII_REDACTED] R11: [PII_REDACTED] R12: 00007f6e86f491bc
  R13:[PII_REDACTED]00006e R14: 00007ffe445d9420 R15: 00007ffe445d9508
   </TASK>

The root cause is the bad handling of disconnect() generated internally
by the MPTCP protocol in case of connect FASTOPEN errors.

Address the issue increasing the socket disconnect counter even on such
a case, to allow other threads waiting on the same socket lock to
properly error out.
CVSS Score: 5.5 (MEDIUM)
References: https://git.kernel.org/stable/c/0263fb2e7b7b88075a5d86e74c4384ee4400828d; https://git.kernel.org/stable/c/619af16b3b57a3a4ee50b9a30add9ff155541e71; https://git.kernel.org/stable/c/6ec806762318a4adde0ea63342d42d0feae95079