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

[syzbot <syzbot+e809069bc15f26300526@syzkaller.appspotmail.com>]

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+e809069bc15f26300526@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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