[syzbot] [net?] WARNING: refcount bug in __inet_csk_reqsk_queue_drop

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: eb3f4b7426cf Merge tag 'nfsd-7.1-2' of git://git.kernel.or..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=11e5b673980000
kernel config: https://syzkaller.appspot.com/x/.config?x=bd38685893011045
dashboard link: https://syzkaller.appspot.com/bug?extid=e809069bc15f26300526
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/3e8630fd2e70/disk-eb3f4b74.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3a23f2a3eb2a/vmlinux-eb3f4b74.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e102c22c34c3/bzImage-eb3f4b74.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e80906...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: lib/refcount.c:28 at refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28, CPU#0: ktimers/0/16
Modules linked in:
CPU: 0 UID: 0 PID: 16 Comm: ktimers/0 Tainted: G L syzkaller #0 PREEMPT_{RT,(full)}
Tainted: [L]=SOFTLOCKUP
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
RIP: 0010:refcount_warn_saturate+0xb2/0x110 lib/refcount.c:28
Code: e4 7d d1 0a 67 48 0f b9 3a eb 4a e8 38 3d 23 fd 48 8d 3d e1 7d d1 0a 67 48 0f b9 3a eb 37 e8 25 3d 23 fd 48 8d 3d de 7d d1 0a <67> 48 0f b9 3a eb 24 e8 12 3d 23 fd 48 8d 3d db 7d d1 0a 67 48 0f
RSP: 0000:ffffc90000157948 EFLAGS: 00010246
RAX: ffffffff84a1301b RBX: 0000000000000003 RCX: ffff88801ca98000
RDX: 0000000000000100 RSI: 0000000000000000 RDI: ffffffff8f72ae00
RBP: ffffffff99ae3b01 R08: ffff88801ca98000 R09: 0000000000000005
R10: 0000000000000100 R11: 0000000000000004 R12: ffff8880425ef568
R13: ffff8880425ef4f8 R14: ffff8880425ef578 R15: 0000000000000000
FS: 0000000000000000(0000) GS:ffff888126386000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f7b46710e9c CR3: 000000000dbb6000 CR4: 00000000003526f0
Call Trace:
<TASK>
__refcount_sub_and_test include/linux/refcount.h:400 [inline]
__refcount_dec_and_test include/linux/refcount.h:432 [inline]
refcount_dec_and_test include/linux/refcount.h:450 [inline]
reqsk_put include/net/request_sock.h:136 [inline]
__inet_csk_reqsk_queue_drop+0x3ce/0x440 net/ipv4/inet_connection_sock.c:1007
reqsk_timer_handler+0x651/0xdf0 net/ipv4/inet_connection_sock.c:1137
call_timer_fn+0x192/0x5e0 kernel/time/timer.c:1748
expire_timers kernel/time/timer.c:1799 [inline]
__run_timers kernel/time/timer.c:2374 [inline]
__run_timer_base+0x6a3/0x9f0 kernel/time/timer.c:2386
run_timer_base kernel/time/timer.c:2395 [inline]
run_timer_softirq+0x67/0x170 kernel/time/timer.c:2403
handle_softirqs+0x1de/0x6d0 kernel/softirq.c:622
__do_softirq kernel/softirq.c:656 [inline]
run_ktimerd+0x69/0x100 kernel/softirq.c:1151
smpboot_thread_fn+0x541/0xa50 kernel/smpboot.c:160
kthread+0x388/0x470 kernel/kthread.c:436
ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
</TASK>
----------------
Code disassembly (best guess):
0: e4 7d in $0x7d,%al
2: d1 0a rorl $1,(%rdx)
4: 67 48 0f b9 3a ud1 (%edx),%rdi
9: eb 4a jmp 0x55
b: e8 38 3d 23 fd call 0xfd233d48
10: 48 8d 3d e1 7d d1 0a lea 0xad17de1(%rip),%rdi # 0xad17df8
17: 67 48 0f b9 3a ud1 (%edx),%rdi
1c: eb 37 jmp 0x55
1e: e8 25 3d 23 fd call 0xfd233d48
23: 48 8d 3d de 7d d1 0a lea 0xad17dde(%rip),%rdi # 0xad17e08
* 2a: 67 48 0f b9 3a ud1 (%edx),%rdi <-- trapping instruction
2f: eb 24 jmp 0x55
31: e8 12 3d 23 fd call 0xfd233d48
36: 48 8d 3d db 7d d1 0a lea 0xad17ddb(%rip),%rdi # 0xad17e18
3d: 67 addr32
3e: 48 rex.W
3f: 0f .byte 0xf


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