Re: [syzbot] [mptcp?] possible deadlock in mptcp_pm_mp_prio_send_ack

[Matthieu Baerts <matttbe@kernel.org>]

Hello,

On 09/04/2026 19:13, syzbot wrote:
> Hello,
> 
> syzbot found the following issue on:
> 
> HEAD commit:    1caa871bb061 Merge branch 'net-stmmac-fix-tegra234-mgbe-cl..
> git tree:       net
> console output: https://syzkaller.appspot.com/x/log.txt?x=11d74e06580000
> kernel config:  https://syzkaller.appspot.com/x/.config?x=6754c86e8d9e4c91
> dashboard link: https://syzkaller.appspot.com/bug?extid=2204dbe6a049b3218db9
> 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/014aae23b990/disk-1caa871b.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/c574a710638c/vmlinux-1caa871b.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/b29909f4efc4/bzImage-1caa871b.xz
> 
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+2204dbe6a049b3218db9@syzkaller.appspotmail.com
> 
> netlink: 8 bytes leftover after parsing attributes in process `syz.2.2034'.
> netlink: 8 bytes leftover after parsing attributes in process `syz.2.2034'.
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Not tainted
> ------------------------------------------------------
> syz.2.2034/13659 is trying to acquire lock:
> ffff888031173560 (k-sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_mp_prio_send_ack+0xaf8/0xba0 net/mptcp/pm.c:296
> 
> but task is already holding lock:
> ffff88807e300ea0 (sk_lock-AF_INET){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1709 [inline]
> ffff88807e300ea0 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_nl_set_flags_all net/mptcp/pm_kernel.c:1482 [inline]
> ffff88807e300ea0 (sk_lock-AF_INET){+.+.}-{0:0}, at: mptcp_pm_nl_set_flags+0x795/0xc90 net/mptcp/pm_kernel.c:1551
> 
> which lock already depends on the new lock.
> 
> 
> the existing dependency chain (in reverse order) is:
> 
> -> #7 (sk_lock-AF_INET){+.+.}-{0:0}:
>        lock_sock_nested+0x48/0x100 net/core/sock.c:3780
>        lock_sock include/net/sock.h:1709 [inline]
>        inet_shutdown+0x6a/0x390 net/ipv4/af_inet.c:919
>        nbd_mark_nsock_dead+0x2e9/0x560 drivers/block/nbd.c:318

If I'm not mistaken, it looks like this issue is also due to nbd
introducing a lockdep dependency between reclaim and af_socket, and this
is similar to a previous report:

#syz dup: [syzbot] [mptcp?] possible deadlock in mptcp_subflow_create_socket (2)

If that's not correct, please unduplicate it.

Cheers,
Matt
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