------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: lib/refcount.c:25 at refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25, CPU#1: syz.3.1968/14685
Modules linked in:
CPU: 1 UID: 0 PID: 14685 Comm: syz.3.1968 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 04/18/2026
RIP: 0010:refcount_warn_saturate+0x9f/0x110 lib/refcount.c:25
Code: eb 66 85 db 74 3e 83 fb 01 75 4c e8 0b a3 13 fd 48 8d 3d 24 7b 89 0b 67 48 0f b9 3a eb 4a e8 f8 a2 13 fd 48 8d 3d 21 7b 89 0b <67> 48 0f b9 3a eb 37 e8 e5 a2 13 fd 48 8d 3d 1e 7b 89 0b 67 48 0f
RSP: 0018:ffffc90000a08c28 EFLAGS: 00010246
RAX: ffffffff84b21ed8 RBX: 0000000000000002 RCX: ffff888034793d80
RDX: 0000000000000100 RSI: ffffffff8f10fa20 RDI: ffffffff903b9a00
RBP: 0000000000000000 R08: ffff888034793d80 R09: 0000000000000005
R10: 0000000000000004 R11: 0000000000000100 R12: dffffc0000000000
R13: 0000000000000001 R14: ffff888056832004 R15: ffff8880568323c0
FS:  00007f62aae076c0(0000) GS:ffff88812538c000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f44c6e06717 CR3: 0000000057198000 CR4: 00000000003526f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000600
Call Trace:
 <IRQ>
 sctp_generate_timeout_event+0x1c8/0x390 net/sctp/sm_sideeffect.c:284
 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+0x652/0x8b0 kernel/time/timer.c:2386
 run_timer_base kernel/time/timer.c:2395 [inline]
 run_timer_softirq+0xb7/0x170 kernel/time/timer.c:2405
 handle_softirqs+0x22a/0x840 kernel/softirq.c:622
 __do_softirq kernel/softirq.c:656 [inline]
 invoke_softirq kernel/softirq.c:496 [inline]
 __irq_exit_rcu+0xca/0x220 kernel/softirq.c:735
 irq_exit_rcu+0x9/0x30 kernel/softirq.c:752
 instr_sysvec_apic_timer_interrupt arch/x86/kernel/apic/apic.c:1061 [inline]
 sysvec_apic_timer_interrupt+0xa6/0xc0 arch/x86/kernel/apic/apic.c:1061
 </IRQ>
 <TASK>
 asm_sysvec_apic_timer_interrupt+0x1a/0x20 arch/x86/include/asm/idtentry.h:697
RIP: 0010:task_work_add+0xa9/0x440 kernel/task_work.c:70
Code: 89 e6 e8 4a 09 36 00 41 83 fc 04 44 89 64 24 14 75 18 65 48 8b 05 2f 83 a9 11 49 39 c5 0f 85 5b 03 00 00 e8 e9 04 36 00 eb 0d <e8> e2 04 36 00 4c 89 f7 e8 7a 3b a1 00 4c 89 7c 24 08 4c 89 6c 24
RSP: 0018:ffffc9000d57f740 EFLAGS: 00000293
RAX: ffffffff818fbcc6 RBX: dffffc0000000000 RCX: 0000000000000002
RDX: ffff888034793d80 RSI: 0000000000000001 RDI: 0000000000000004
RBP: ffffc9000d57f7f0 R08: ffffffff8e6149bb R09: 1ffffffff1cc2937
R10: dffffc0000000000 R11: fffffbfff1cc2938 R12: 0000000000000001
R13: ffff888034793d80 R14: ffff88801c687a80 R15: 1ffff92001aafeec
 mntput_no_expire_slowpath+0x70c/0xbd0 fs/namespace.c:1379
 mntput fs/namespace.c:1416 [inline]
 namespace_unlock+0x704/0x7a0 fs/namespace.c:1723
 nsproxy_free kernel/nsproxy.c:66 [inline]
 deactivate_nsproxy+0x41/0x140 kernel/nsproxy.c:80
 copy_process+0x2fcb/0x4440 kernel/fork.c:2556
 kernel_clone+0x284/0x8f0 kernel/fork.c:2721
 __do_sys_clone kernel/fork.c:2862 [inline]
 __se_sys_clone kernel/fork.c:2846 [inline]
 __x64_sys_clone+0x1b6/0x230 kernel/fork.c:2846
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x15f/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f62a9f9ce59
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f62aae06fd8 EFLAGS: 00000202 ORIG_RAX: 0000000000000038
RAX: ffffffffffffffda RBX: 00007f62aa215fa0 RCX: 00007f62a9f9ce59
RDX: 0000000000000000 RSI: 0000000000000700 RDI: 00000000a49a4080
RBP: 00007f62aa032d6f R08: ffffffffff600000 R09: ffffffffff600000
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000000000
R13: 00007f62aa216038 R14: 00007f62aa215fa0 R15: 00007ffca2dd20d8
 </TASK>
----------------
Code disassembly (best guess):
   0:	eb 66                	jmp    0x68
   2:	85 db                	test   %ebx,%ebx
   4:	74 3e                	je     0x44
   6:	83 fb 01             	cmp    $0x1,%ebx
   9:	75 4c                	jne    0x57
   b:	e8 0b a3 13 fd       	call   0xfd13a31b
  10:	48 8d 3d 24 7b 89 0b 	lea    0xb897b24(%rip),%rdi        # 0xb897b3b
  17:	67 48 0f b9 3a       	ud1    (%edx),%rdi
  1c:	eb 4a                	jmp    0x68
  1e:	e8 f8 a2 13 fd       	call   0xfd13a31b
  23:	48 8d 3d 21 7b 89 0b 	lea    0xb897b21(%rip),%rdi        # 0xb897b4b
* 2a:	67 48 0f b9 3a       	ud1    (%edx),%rdi <-- trapping instruction
  2f:	eb 37                	jmp    0x68
  31:	e8 e5 a2 13 fd       	call   0xfd13a31b
  36:	48 8d 3d 1e 7b 89 0b 	lea    0xb897b1e(%rip),%rdi        # 0xb897b5b
  3d:	67                   	addr32
  3e:	48                   	rex.W
  3f:	0f                   	.byte 0xf