Subject: [syzbot] [netfilter?] possible deadlock in nf_tables_dumpreset_obj

From: syzbot <[email-masked]>
Date: Fri, 19 Dec 2025 16:58:28 -0800

Hello,

syzbot found the following issue on:

HEAD commit:    8f0b4cce4481 Linux 6.19-rc1
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=104f2d92580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a11e0f726bfb6765
dashboard link: https://syzkaller.appspot.com/bug?extid=ff16b505ec9152e5f448
compiler:       gcc (Debian 12.2.0-14+deb12u1) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

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

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-8f0b4cce.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/64c9a36f3f29/vmlinux-8f0b4cce.xz
kernel image: https://storage.googleapis.com/syzbot-assets/27a5e8a8a4b8/bzImage-8f0b4cce.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email-masked]

======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
syz.3.970/9330 is trying to acquire lock:
ffff888012d4ccd8 (&nft_net->commit_mutex){+.+.}-{4:4}, at: nf_tables_dumpreset_obj+0x6f/0xa0 net/netfilter/nf_tables_api.c:8491

but task is already holding lock:
ffff88802bce36f0 (nlk_cb_mutex-NETFILTER){+.+.}-{4:4}, at: __netlink_dump_start+0x150/0x990 net/netlink/af_netlink.c:2404

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #2 (nlk_cb_mutex-NETFILTER){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:614 [inline]
       __mutex_lock+0x1aa/0x1ca0 kernel/locking/mutex.c:776
       __netlink_dump_start+0x150/0x990 net/netlink/af_netlink.c:2404
       netlink_dump_start include/linux/netlink.h:341 [inline]
       ip_set_dump+0x17f/0x210 net/netfilter/ipset/ip_set_core.c:1717
       nfnetlink_rcv_msg+0x9fc/0x1200 net/netfilter/nfnetlink.c:302
       netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2550
       nfnetlink_rcv+0x1b3/0x430 net/netfilter/nfnetlink.c:669
       netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
       netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
       netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1894
       sock_sendmsg_nosec net/socket.c:727 [inline]
       __sock_sendmsg net/socket.c:742 [inline]
       ____sys_sendmsg+0xa5d/0xc30 net/socket.c:2592
       ___sys_sendmsg+0x134/0x1d0 net/socket.c:2646
       __sys_sendmsg+0x16d/0x220 net/socket.c:2678
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (nfnl_subsys_ipset){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/mutex.c:614 [inline]
       __mutex_lock+0x1aa/0x1ca0 kernel/locking/mutex.c:776
       ip_set_nfnl_get_byindex+0x7c/0x290 net/netfilter/ipset/ip_set_core.c:909
       set_target_v1_checkentry+0x1ac/0x570 net/netfilter/xt_set.c:313
       xt_check_target+0x27c/0xa40 net/netfilter/x_tables.c:1038
       nft_target_init+0x459/0x7d0 net/netfilter/nft_compat.c:267
       nf_tables_newexpr net/netfilter/nf_tables_api.c:3527 [inline]
       nf_tables_newrule+0xedd/0x2910 net/netfilter/nf_tables_api.c:4358
       nfnetlink_rcv_batch+0x190d/0x2350 net/netfilter/nfnetlink.c:526
       nfnetlink_rcv_skb_batch net/netfilter/nfnetlink.c:649 [inline]
       nfnetlink_rcv+0x3c1/0x430 net/netfilter/nfnetlink.c:667
       netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
       netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
       netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1894
       sock_sendmsg_nosec net/socket.c:727 [inline]
       __sock_sendmsg net/socket.c:742 [inline]
       ____sys_sendmsg+0xa5d/0xc30 net/socket.c:2592
       ___sys_sendmsg+0x134/0x1d0 net/socket.c:2646
       __sys_sendmsg+0x16d/0x220 net/socket.c:2678
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&nft_net->commit_mutex){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3165 [inline]
       check_prevs_add kernel/locking/lockdep.c:3284 [inline]
       validate_chain kernel/locking/lockdep.c:3908 [inline]
       __lock_acquire+0x1669/0x2890 kernel/locking/lockdep.c:5237
       lock_acquire kernel/locking/lockdep.c:5868 [inline]
       lock_acquire+0x179/0x330 kernel/locking/lockdep.c:5825
       __mutex_lock_common kernel/locking/mutex.c:614 [inline]
       __mutex_lock+0x1aa/0x1ca0 kernel/locking/mutex.c:776
       nf_tables_dumpreset_obj+0x6f/0xa0 net/netfilter/nf_tables_api.c:8491
       netlink_dump+0x539/0xd30 net/netlink/af_netlink.c:2325
       __netlink_dump_start+0x6d6/0x990 net/netlink/af_netlink.c:2440
       netlink_dump_start include/linux/netlink.h:341 [inline]
       nft_netlink_dump_start_rcu+0x81/0x1f0 net/netfilter/nf_tables_api.c:1286
       nf_tables_getobj_reset+0x56b/0x6b0 net/netfilter/nf_tables_api.c:8626
       nfnetlink_rcv_msg+0x583/0x1200 net/netfilter/nfnetlink.c:290
       netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2550
       nfnetlink_rcv+0x1b3/0x430 net/netfilter/nfnetlink.c:669
       netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
       netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
       netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1894
       sock_sendmsg_nosec net/socket.c:727 [inline]
       __sock_sendmsg net/socket.c:742 [inline]
       ____sys_sendmsg+0xa5d/0xc30 net/socket.c:2592
       ___sys_sendmsg+0x134/0x1d0 net/socket.c:2646
       __sys_sendmsg+0x16d/0x220 net/socket.c:2678
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &nft_net->commit_mutex --> nfnl_subsys_ipset --> nlk_cb_mutex-NETFILTER

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(nlk_cb_mutex-NETFILTER);
                               lock(nfnl_subsys_ipset);
                               lock(nlk_cb_mutex-NETFILTER);
  lock(&nft_net->commit_mutex);

 *** DEADLOCK ***

1 lock held by syz.3.970/9330:
 #0: ffff88802bce36f0 (nlk_cb_mutex-NETFILTER){+.+.}-{4:4}, at: __netlink_dump_start+0x150/0x990 net/netlink/af_netlink.c:2404

stack backtrace:
CPU: 0 UID: 0 PID: 9330 Comm: syz.3.970 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 print_circular_bug+0x275/0x340 kernel/locking/lockdep.c:2043
 check_noncircular+0x146/0x160 kernel/locking/lockdep.c:2175
 check_prev_add kernel/locking/lockdep.c:3165 [inline]
 check_prevs_add kernel/locking/lockdep.c:3284 [inline]
 validate_chain kernel/locking/lockdep.c:3908 [inline]
 __lock_acquire+0x1669/0x2890 kernel/locking/lockdep.c:5237
 lock_acquire kernel/locking/lockdep.c:5868 [inline]
 lock_acquire+0x179/0x330 kernel/locking/lockdep.c:5825
 __mutex_lock_common kernel/locking/mutex.c:614 [inline]
 __mutex_lock+0x1aa/0x1ca0 kernel/locking/mutex.c:776
 nf_tables_dumpreset_obj+0x6f/0xa0 net/netfilter/nf_tables_api.c:8491
 netlink_dump+0x539/0xd30 net/netlink/af_netlink.c:2325
 __netlink_dump_start+0x6d6/0x990 net/netlink/af_netlink.c:2440
 netlink_dump_start include/linux/netlink.h:341 [inline]
 nft_netlink_dump_start_rcu+0x81/0x1f0 net/netfilter/nf_tables_api.c:1286
 nf_tables_getobj_reset+0x56b/0x6b0 net/netfilter/nf_tables_api.c:8626
 nfnetlink_rcv_msg+0x583/0x1200 net/netfilter/nfnetlink.c:290
 netlink_rcv_skb+0x158/0x420 net/netlink/af_netlink.c:2550
 nfnetlink_rcv+0x1b3/0x430 net/netfilter/nfnetlink.c:669
 netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
 netlink_unicast+0x5aa/0x870 net/netlink/af_netlink.c:1344
 netlink_sendmsg+0x8c8/0xdd0 net/netlink/af_netlink.c:1894
 sock_sendmsg_nosec net/socket.c:727 [inline]
 __sock_sendmsg net/socket.c:742 [inline]
 ____sys_sendmsg+0xa5d/0xc30 net/socket.c:2592
 ___sys_sendmsg+0x134/0x1d0 net/socket.c:2646
 __sys_sendmsg+0x16d/0x220 net/socket.c:2678
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xcd/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fb7e7b8f7c9
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:00007fb7e8a9c038 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
RAX: ffffffffffffffda RBX: 00007fb7e7de5fa0 RCX: 00007fb7e7b8f7c9
RDX: 0000000004004004 RSI: 0000200000000140 RDI: 0000000000000003
RBP: 00007fb7e7c13f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007fb7e7de6038 R14: 00007fb7e7de5fa0 R15: 00007fffe518fab8
 </TASK>

---
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