[v5.15] WARNING: refcount bug in subflow_ulp_release

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 91d48252ad4b Linux 5.15.202
git tree: linux-5.15.y
console output: https://syzkaller.appspot.com/x/log.txt?x=11523e02580000
kernel config: https://syzkaller.appspot.com/x/.config?x=353ae28c40b35af5
dashboard link: https://syzkaller.appspot.com/bug?extid=3f6ee9c01d69bbf9206b
compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8

Unfortunately, I don't have any reproducer for this issue yet.

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/74e75dfcb812/disk-91d48252.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bfa72aab00f2/vmlinux-91d48252.xz
kernel image: https://storage.googleapis.com/syzbot-assets/47ea72d1c7dc/bzImage-91d48252.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3f6ee9...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 4377 at lib/refcount.c:28 refcount_warn_saturate+0x11b/0x1a0 lib/refcount.c:28
Modules linked in:
CPU: 0 PID: 4377 Comm: syz.2.29 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
RIP: 0010:refcount_warn_saturate+0x11b/0x1a0 lib/refcount.c:28
Code: 09 01 48 c7 c7 a0 ba 79 8a e8 01 9e cd 05 0f 0b eb c4 e8 a8 45 95 fd c6 05 24 60 92 09 01 48 c7 c7 00 bb 79 8a e8 e5 9d cd 05 <0f> 0b eb a8 e8 8c 45 95 fd c6 05 05 60 92 09 01 48 c7 c7 40 ba 79
RSP: 0000:ffffc90002fff688 EFLAGS: 00010246
RAX: 24c1888db8503000 RBX: 0000000000000003 RCX: 0000000000080000
RDX: ffffc9000f61c000 RSI: 0000000000009aa8 RDI: 0000000000009aa9
RBP: 0000000000000000 R08: ffffc90002fff1a7 R09: 1ffff920005ffe34
R10: dffffc0000000000 R11: fffff520005ffe35 R12: ffff88807a308cc0
R13: dffffc0000000000 R14: 0000000000000003 R15: ffff88807a308c40
FS: 00007f6cfc3656c0(0000) GS:ffff8880b9000000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f6cfe179e80 CR3: 000000007db78000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__refcount_sub_and_test include/linux/refcount.h:283 [inline]
__refcount_dec_and_test include/linux/refcount.h:315 [inline]
refcount_dec_and_test include/linux/refcount.h:333 [inline]
sock_put include/net/sock.h:1988 [inline]
subflow_ulp_release+0x24d/0x290 net/mptcp/subflow.c:1746
tcp_cleanup_ulp+0x6e/0xe0 net/ipv4/tcp_ulp.c:124
tcp_v4_destroy_sock+0x177/0x6e0 net/ipv4/tcp_ipv4.c:2300
inet_csk_destroy_sock+0x15c/0x390 net/ipv4/inet_connection_sock.c:1051
inet_csk_listen_stop+0x41f/0xac0 net/ipv4/inet_connection_sock.c:1272
__tcp_close+0x93/0xfa0 net/ipv4/tcp.c:2763
__mptcp_close_ssk+0x1ea/0x460 net/mptcp/protocol.c:2375
__mptcp_destroy_sock+0x3b5/0x6f0 net/mptcp/protocol.c:2809
mptcp_close+0x5a7/0x9f0 net/mptcp/protocol.c:2866
inet_release+0x139/0x180 net/ipv4/af_inet.c:432
__sock_release net/socket.c:651 [inline]
sock_close+0xd5/0x240 net/socket.c:1345
__fput+0x234/0x930 fs/file_table.c:311
task_work_run+0x125/0x1a0 kernel/task_work.c:188
get_signal+0x1222/0x12c0 kernel/signal.c:2672
arch_do_signal_or_restart+0xe7/0x12c0 arch/x86/kernel/signal.c:867
handle_signal_work kernel/entry/common.c:154 [inline]
exit_to_user_mode_loop+0x9e/0x130 kernel/entry/common.c:178
exit_to_user_mode_prepare+0xee/0x180 kernel/entry/common.c:214
__syscall_exit_to_user_mode_work kernel/entry/common.c:296 [inline]
syscall_exit_to_user_mode+0x16/0x40 kernel/entry/common.c:307
do_syscall_64+0x58/0xa0 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x66/0xd0
RIP: 0033:0x7f6cfe12c799
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f6cfc365028 EFLAGS: 00000246 ORIG_RAX: 0000000000000120
RAX: fffffffffffffe00 RBX: 00007f6cfe3a6090 RCX: 00007f6cfe12c799
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000007
RBP: 00007f6cfe1c2c99 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000080000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f6cfe3a6128 R14: 00007f6cfe3a6090 R15: 00007ffeca7c3728
</TASK>


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