CVE-2025-21705

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: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000

RBP: ffffc9000c0cf518 R08: ffffffff8bb0d1dd R09: 1ffff110170c8928

R10: dffffc0000000000 R11: ffffed10170c8929 R12: 0000000000000000

R13: ffff888033f5d220 R14: dffffc0000000000 R15: ffff8880592b8000

FS: 00007f6e866496c0(0000) GS:ffff8880b8600000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 00007f6e86f491a0 CR3: 00000000310e6000 CR4: 00000000003526f0

DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000

DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400

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: 000000000000002e

RAX: ffffffffffffffda RBX: 00007f6e86f491b8 RCX: 00007f6e86ebfe69

RDX: 0000000030004001 RSI: 0000000020000080 RDI: 0000000000000003

RBP: 00007f6e86f491b0 R08: 00007f6e866496c0 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000246 R12: 00007f6e86f491bc

R13: 000000000000006e 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.