[syzbot] [hams?] WARNING: refcount bug in ax25_setsockopt

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: faeefc173be4 sock: Correct error checking condition for (a..
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=157b9c04580000
kernel config: https://syzkaller.appspot.com/x/.config?x=eecd7902e39d7933
dashboard link: https://syzkaller.appspot.com/bug?extid=0ee4da32f91ae2a3f015
compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15875398580000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=137b9c04580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/25a813551e04/disk-faeefc17.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/06484bfac01b/vmlinux-faeefc17.xz
kernel image: https://storage.googleapis.com/syzbot-assets/df48cbb45ee8/bzImage-faeefc17.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0ee4da...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 1 PID: 6151 at lib/refcount.c:25 refcount_warn_saturate+0x13a/0x1d0 lib/refcount.c:25
Modules linked in:
CPU: 1 UID: 0 PID: 6151 Comm: syz-executor223 Not tainted 6.15.0-rc1-syzkaller-00209-gfaeefc173be4 #0 PREEMPT(full)
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2025
RIP: 0010:refcount_warn_saturate+0x13a/0x1d0 lib/refcount.c:25
Code: 00 6b a1 8c e8 17 87 7d fc 90 0f 0b 90 90 eb b9 e8 0b 65 be fc c6 05 94 dc 44 0b 01 90 48 c7 c7 60 6b a1 8c e8 f7 86 7d fc 90 <0f> 0b 90 90 eb 99 e8 eb 64 be fc c6 05 75 dc 44 0b 01 90 48 c7 c7
RSP: 0018:ffffc900046afc68 EFLAGS: 00010246
RAX: 975256fdc5619000 RBX: ffff888034835ac8 RCX: ffff888078041e00
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000000
RBP: 0000000000000002 R08: ffffffff81828a12 R09: fffffbfff1d7a978
R10: dffffc0000000000 R11: fffffbfff1d7a978 R12: 1ffff110069bac05
R13: ffff888034835a00 R14: ffff888013018000 R15: ffff888034dd6028
FS: 000055556b9c9380(0000) GS:ffff888125093000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffdd88feca8 CR3: 0000000029122000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
ax25_setsockopt+0xbb7/0xf00 net/ax25/af_ax25.c:705
do_sock_setsockopt+0x3b1/0x710 net/socket.c:2296
__sys_setsockopt net/socket.c:2321 [inline]
__do_sys_setsockopt net/socket.c:2327 [inline]
__se_sys_setsockopt net/socket.c:2324 [inline]
__x64_sys_setsockopt+0x1ee/0x280 net/socket.c:2324
do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xf3/0x230 arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f59be6f7969
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 31 1a 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 b8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffdd88feda8 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000200000000000 RCX: 00007f59be6f7969
RDX: 0000000000000019 RSI: 0000000000000101 RDI: 0000000000000004
RBP: 0000000000000000 R08: 0000000000000010 R09: 00007f59be745214
R10: 0000200000000000 R11: 0000000000000246 R12: 00007ffdd88fedcc
R13: 00007ffdd88fee00 R14: 00007ffdd88fede0 R15: 0000000000000036
</TASK>


---
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[Hillf Danton]

> Date: Fri, 11 Jul 2025 08:28:34 -0700 [thread overview]

> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: faeefc173be4 sock: Correct error checking condition for (a..
> git tree: net-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=157b9c04580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=eecd7902e39d7933
> dashboard link: https://syzkaller.appspot.com/bug?extid=0ee4da32f91ae2a3f015
> compiler: Debian clang version 15.0.6, GNU ld (GNU Binutils for Debian) 2.40
> syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15875398580000
> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=137b9c04580000

#syz test

--- x/net/ax25/af_ax25.c
+++ y/net/ax25/af_ax25.c
@@ -694,8 +694,10 @@ static int ax25_setsockopt(struct socket
ax25_dev_put(ax25->ax25_dev);
}

+ spin_lock_bh(&ax25_dev_lock);
ax25->ax25_dev = ax25_dev_ax25dev(dev);
if (!ax25->ax25_dev) {
+ spin_unlock_bh(&ax25_dev_lock);
rcu_read_unlock();
res = -ENODEV;
break;
@@ -703,6 +705,7 @@ static int ax25_setsockopt(struct socket
ax25_fillin_cb(ax25, ax25->ax25_dev);
netdev_hold(dev, &ax25->dev_tracker, GFP_ATOMIC);
ax25_dev_hold(ax25->ax25_dev);
+ spin_unlock_bh(&ax25_dev_lock);
rcu_read_unlock();
break;

--

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-by: syzbot+0ee4da...@syzkaller.appspotmail.com
Tested-by: syzbot+0ee4da...@syzkaller.appspotmail.com

Tested on:

commit: a52f9f0d Merge tag 'batadv-next-pullrequest-20250710' ..
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=10bc7bd4580000
kernel config: https://syzkaller.appspot.com/x/.config?x=491e511dfc95d0a5
dashboard link: https://syzkaller.appspot.com/bug?extid=0ee4da32f91ae2a3f015
compiler: Debian clang version 20.1.7 (++20250616065708+6146a88f6049-1~exp1~20250616065826.132), Debian LLD 20.1.7
patch: https://syzkaller.appspot.com/x/patch.diff?x=17cb2e8c580000

Note: testing is done by a robot and is best-effort only.