[syzbot] [kernel?] stack segment fault in nsproxy_free

[syzbot <syzbot+504479f733b2b0cbf617@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    6d7e7251d03f Add linux-next specific files for 20251113
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=14b86658580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8a6b17620801a7cb
dashboard link: https://syzkaller.appspot.com/bug?extid=504479f733b2b0cbf617
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ab5308631192/disk-6d7e7251.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/53e266b1dfe2/vmlinux-6d7e7251.xz
kernel image: https://storage.googleapis.com/syzbot-assets/a24e6bca1ac0/bzImage-6d7e7251.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+504479f733b2b0cbf617@syzkaller.appspotmail.com

Oops: stack segment: 0000 [#1] SMP KASAN PTI
CPU: 0 UID: 0 PID: 16550 Comm: syz.2.4585 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/25/2025
RIP: 0010:is_ns_init_id include/linux/ns_common.h:25 [inline]
RIP: 0010:__ns_ref_put include/linux/ns_common.h:70 [inline]
RIP: 0010:put_net include/net/net_namespace.h:292 [inline]
RIP: 0010:nsproxy_free+0x40a/0x7c0 kernel/nsproxy.c:72
Code: 03 42 80 3c 28 00 74 08 4c 89 f7 e8 90 c2 99 00 4d 8b 36 4d 85 f6 0f 84 df 00 00 00 4d 8d be 78 01 00 00 4c 89 fd 48 c1 ed 03 <42> 80 7c 2d 00 00 74 08 4c 89 ff e8 66 c2 99 00 4d 8b 27 31 ff 4c
RSP: 0018:ffffc90004c67d08 EFLAGS: 00010207
RAX: 1ffff1100c3dc94a RBX: ffff888061ee4a28 RCX: 0000000000080000
RDX: ffffc9001b253000 RSI: 000000000007ffff RDI: 0000000000080000
RBP: 000000000000002e R08: ffff88807d5b6e1f R09: 1ffff1100fab6dc3
R10: dffffc0000000000 R11: ffffed100fab6dc4 R12: 000000000000029c
R13: dffffc0000000000 R14: fffffffffffffffc R15: 0000000000000174
FS:  00007feaebb846c0(0000) GS:ffff888125a78000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000020000032a030 CR3: 0000000029c20000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __free_nsproxy_free kernel/nsproxy.c:76 [inline]
 create_new_namespaces+0x47a/0x540 kernel/nsproxy.c:132
 unshare_nsproxy_namespaces+0x11c/0x170 kernel/nsproxy.c:197
 ksys_unshare+0x4c8/0x8c0 kernel/fork.c:3129
 __do_sys_unshare kernel/fork.c:3200 [inline]
 __se_sys_unshare kernel/fork.c:3198 [inline]
 __x64_sys_unshare+0x38/0x50 kernel/fork.c:3198
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7feaead8f6c9
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:00007feaebb84038 EFLAGS: 00000246 ORIG_RAX: 0000000000000110
RAX: ffffffffffffffda RBX: 00007feaeafe5fa0 RCX: 00007feaead8f6c9
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000060000600
RBP: 00007feaeae11f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007feaeafe6038 R14: 00007feaeafe5fa0 R15: 00007fff573f8438
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:is_ns_init_id include/linux/ns_common.h:25 [inline]
RIP: 0010:__ns_ref_put include/linux/ns_common.h:70 [inline]
RIP: 0010:put_net include/net/net_namespace.h:292 [inline]
RIP: 0010:nsproxy_free+0x40a/0x7c0 kernel/nsproxy.c:72
Code: 03 42 80 3c 28 00 74 08 4c 89 f7 e8 90 c2 99 00 4d 8b 36 4d 85 f6 0f 84 df 00 00 00 4d 8d be 78 01 00 00 4c 89 fd 48 c1 ed 03 <42> 80 7c 2d 00 00 74 08 4c 89 ff e8 66 c2 99 00 4d 8b 27 31 ff 4c
RSP: 0018:ffffc90004c67d08 EFLAGS: 00010207
RAX: 1ffff1100c3dc94a RBX: ffff888061ee4a28 RCX: 0000000000080000
RDX: ffffc9001b253000 RSI: 000000000007ffff RDI: 0000000000080000
RBP: 000000000000002e R08: ffff88807d5b6e1f R09: 1ffff1100fab6dc3
R10: dffffc0000000000 R11: ffffed100fab6dc4 R12: 000000000000029c
R13: dffffc0000000000 R14: fffffffffffffffc R15: 0000000000000174
FS:  00007feaebb846c0(0000) GS:ffff888125a78000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000200000404030 CR3: 0000000029c20000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	03 42 80             	add    -0x80(%rdx),%eax
   3:	3c 28                	cmp    $0x28,%al
   5:	00 74 08 4c          	add    %dh,0x4c(%rax,%rcx,1)
   9:	89 f7                	mov    %esi,%edi
   b:	e8 90 c2 99 00       	call   0x99c2a0
  10:	4d 8b 36             	mov    (%r14),%r14
  13:	4d 85 f6             	test   %r14,%r14
  16:	0f 84 df 00 00 00    	je     0xfb
  1c:	4d 8d be 78 01 00 00 	lea    0x178(%r14),%r15
  23:	4c 89 fd             	mov    %r15,%rbp
  26:	48 c1 ed 03          	shr    $0x3,%rbp
* 2a:	42 80 7c 2d 00 00    	cmpb   $0x0,0x0(%rbp,%r13,1) <-- trapping instruction
  30:	74 08                	je     0x3a
  32:	4c 89 ff             	mov    %r15,%rdi
  35:	e8 66 c2 99 00       	call   0x99c2a0
  3a:	4d 8b 27             	mov    (%r15),%r12
  3d:	31 ff                	xor    %edi,%edi
  3f:	4c                   	rex.WR


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