Subject: [syzbot] [net?] possible deadlock in ipv6_sock_mc_close (4)

From: syzbot <[email-masked]>
Date: Mon, 08 Dec 2025 05:22:26 -0800

Hello,

syzbot found the following issue on:

HEAD commit:    8f7aa3d3c732 Merge tag 'net-next-6.19' of git://git.kernel..
git tree:       net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1564201a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9e5198eaf003f1d1
dashboard link: https://syzkaller.appspot.com/bug?extid=c59e6bb54e7620495725
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=17a53192580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13f3301a580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5fe312c4cf90/disk-8f7aa3d3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/c7a1f54ef730/vmlinux-8f7aa3d3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/64a3779458bb/bzImage-8f7aa3d3.xz

The issue was bisected to:

commit b7ddb55f31279f4e59acde3395fc03c3d94b6e5f
Author: Kuniyuki Iwashima <[email-masked]>
Date:   Thu Oct 23 23:16:56 2025 +0000

    sctp: Use sctp_clone_sock() in sctp_do_peeloff().

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=105db01a580000
final oops:     https://syzkaller.appspot.com/x/report.txt?x=125db01a580000
console output: https://syzkaller.appspot.com/x/log.txt?x=145db01a580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]
Fixes: b7ddb55f3127 ("sctp: Use sctp_clone_sock() in sctp_do_peeloff().")

============================================
WARNING: possible recursive locking detected
syzkaller #0 Not tainted
--------------------------------------------
syz.0.17/5997 is trying to acquire lock:
ffff88803021dee0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1700 [inline]
ffff88803021dee0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: ipv6_sock_mc_close+0xd3/0x140 net/ipv6/mcast.c:348

but task is already holding lock:
ffff88803021d5a0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1700 [inline]
ffff88803021d5a0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: sctp_getsockopt+0x135/0xb60 net/sctp/socket.c:8131

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

       CPU0
       ----
  lock(sk_lock-AF_INET6);
  lock(sk_lock-AF_INET6);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

1 lock held by syz.0.17/5997:
 #0: ffff88803021d5a0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1700 [inline]
 #0: ffff88803021d5a0 (sk_lock-AF_INET6){+.+.}-{0:0}, at: sctp_getsockopt+0x135/0xb60 net/sctp/socket.c:8131

stack backtrace:
CPU: 1 UID: 0 PID: 5997 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_deadlock_bug+0x279/0x290 kernel/locking/lockdep.c:3041
 check_deadlock kernel/locking/lockdep.c:3093 [inline]
 validate_chain kernel/locking/lockdep.c:3895 [inline]
 __lock_acquire+0x2540/0x2cf0 kernel/locking/lockdep.c:5237
 lock_acquire+0x117/0x340 kernel/locking/lockdep.c:5868
 lock_sock_nested+0x48/0x100 net/core/sock.c:3780
 lock_sock include/net/sock.h:1700 [inline]
 ipv6_sock_mc_close+0xd3/0x140 net/ipv6/mcast.c:348
 inet6_release+0x47/0x70 net/ipv6/af_inet6.c:482
 __sock_release net/socket.c:653 [inline]
 sock_release+0x85/0x150 net/socket.c:681
 sctp_getsockopt_peeloff_common+0x56b/0x770 net/sctp/socket.c:5732
 sctp_getsockopt_peeloff_flags+0x13b/0x230 net/sctp/socket.c:5801
 sctp_getsockopt+0x3ab/0xb60 net/sctp/socket.c:8151
 do_sock_getsockopt+0x2b4/0x3d0 net/socket.c:2399
 __sys_getsockopt net/socket.c:2428 [inline]
 __do_sys_getsockopt net/socket.c:2435 [inline]
 __se_sys_getsockopt net/socket.c:2432 [inline]
 __x64_sys_getsockopt+0x1a5/0x250 net/socket.c:2432
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f8a4018f749
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffc8290fbe8 EFLAGS: 00000246 ORIG_RAX: 0000000000000037
RAX: ffffffffffffffda RBX: 00007f8a403e5fa0 RCX: 00007f8a4018f749
RDX: 000000000000007a RSI: 0000000000000084 RDI: 0000000000000003
RBP: 00007f8a40213f91 R08: 0000200000000040 R09: 0000000000000000
R10: 0000200000000340 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f8a403e5fa0 R14: 00007f8a403e5fa0 R15: 0000000000000005
 </TASK>

---
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