Re: [syzbot] [fs?] general protection fault in __umount_mnt

[syzbot <syzbot+e4470cc28308f2081ec8@syzkaller.appspotmail.com>]

syzbot has found a reproducer for the following issue on:

HEAD commit:    3b058d1aeeef Add linux-next specific files for 20260327
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=11bd16f6580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4acce231a75c746f
dashboard link: https://syzkaller.appspot.com/bug?extid=e4470cc28308f2081ec8
compiler:       Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11dc6606580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15baca26580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/5a5b73363d45/disk-3b058d1a.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/ecba5c3ac106/vmlinux-3b058d1a.xz
kernel image: https://storage.googleapis.com/syzbot-assets/348124eb71de/bzImage-3b058d1a.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+e4470cc28308f2081ec8@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xfbd59c0000000024: 0000 [#1] SMP KASAN PTI
KASAN: maybe wild-memory-access in range [0xdead000000000120-0xdead000000000127]
CPU: 0 UID: 0 PID: 6113 Comm: syz.0.28 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
RIP: 0010:__hlist_del include/linux/list.h:992 [inline]
RIP: 0010:hlist_del_init include/linux/list.h:1020 [inline]
RIP: 0010:__umount_mnt+0x24e/0x490 fs/namespace.c:997
Code: 85 e4 74 61 4d 8d be f8 00 00 00 4c 89 f8 48 c1 e8 03 80 3c 28 00 74 08 4c 89 ff e8 ac 3f e3 ff 4d 8b 2f 4c 89 e0 48 c1 e8 03 <80> 3c 28 00 74 08 4c 89 e7 e8 84 40 e3 ff 4d 89 2c 24 4d 85 ed 74
RSP: 0018:ffffc900033a7a20 EFLAGS: 00010a06
RAX: 1bd5a00000000024 RBX: ffff88807dcb5188 RCX: ffff88803244bd00
RDX: 0000000000000000 RSI: ffffffff8e8d83a0 RDI: ffff88807dcb51f8
RBP: dffffc0000000000 R08: ffffffff90128af7 R09: 1ffffffff202515e
R10: dffffc0000000000 R11: fffffbfff202515f R12: dead000000000122
R13: dead000000000100 R14: ffff88807dcb5180 R15: ffff88807dcb5278
FS:  000055558d53f500(0000) GS:ffff88812543f000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f50a85ea078 CR3: 0000000076400000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 umount_mnt fs/namespace.c:1008 [inline]
 umount_tree+0x925/0xd90 fs/namespace.c:1819
 put_mnt_ns+0x1d6/0x2f0 fs/namespace.c:6264
 evict+0x61e/0xb10 fs/inode.c:846
 __dentry_kill+0x1a2/0x5e0 fs/dcache.c:670
 finish_dput+0xc9/0x480 fs/dcache.c:879
 __fput+0x691/0xa70 fs/file_table.c:477
 task_work_run+0x1d9/0x270 kernel/task_work.c:233
 resume_user_mode_work include/linux/resume_user_mode.h:50 [inline]
 __exit_to_user_mode_loop kernel/entry/common.c:67 [inline]
 exit_to_user_mode_loop+0xed/0x480 kernel/entry/common.c:98
 __exit_to_user_mode_prepare include/linux/irq-entry-common.h:238 [inline]
 syscall_exit_to_user_mode_prepare include/linux/irq-entry-common.h:269 [inline]
 syscall_exit_to_user_mode include/linux/entry-common.h:328 [inline]
 do_syscall_64+0x33e/0xf80 arch/x86/entry/syscall_64.c:100
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f265d99c799
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:00007ffe16be4ce8 EFLAGS: 00000246 ORIG_RAX: 00000000000001b4
RAX: 0000000000000000 RBX: 00007f265dc17da0 RCX: 00007f265d99c799
RDX: 0000000000000000 RSI: 000000000000001e RDI: 0000000000000003
RBP: 00007f265dc17da0 R08: 00007f265dc16038 R09: 0000000000000000
R10: 000000000003fda8 R11: 0000000000000246 R12: 00000000000161c9
R13: 00007f265dc1609c R14: 0000000000015fb1 R15: 00007f265dc16090
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__hlist_del include/linux/list.h:992 [inline]
RIP: 0010:hlist_del_init include/linux/list.h:1020 [inline]
RIP: 0010:__umount_mnt+0x24e/0x490 fs/namespace.c:997
Code: 85 e4 74 61 4d 8d be f8 00 00 00 4c 89 f8 48 c1 e8 03 80 3c 28 00 74 08 4c 89 ff e8 ac 3f e3 ff 4d 8b 2f 4c 89 e0 48 c1 e8 03 <80> 3c 28 00 74 08 4c 89 e7 e8 84 40 e3 ff 4d 89 2c 24 4d 85 ed 74
RSP: 0018:ffffc900033a7a20 EFLAGS: 00010a06
RAX: 1bd5a00000000024 RBX: ffff88807dcb5188 RCX: ffff88803244bd00
RDX: 0000000000000000 RSI: ffffffff8e8d83a0 RDI: ffff88807dcb51f8
RBP: dffffc0000000000 R08: ffffffff90128af7 R09: 1ffffffff202515e
R10: dffffc0000000000 R11: fffffbfff202515f R12: dead000000000122
R13: dead000000000100 R14: ffff88807dcb5180 R15: ffff88807dcb5278
FS:  000055558d53f500(0000) GS:ffff88812543f000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f50a85ea078 CR3: 0000000076400000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	85 e4                	test   %esp,%esp
   2:	74 61                	je     0x65
   4:	4d 8d be f8 00 00 00 	lea    0xf8(%r14),%r15
   b:	4c 89 f8             	mov    %r15,%rax
   e:	48 c1 e8 03          	shr    $0x3,%rax
  12:	80 3c 28 00          	cmpb   $0x0,(%rax,%rbp,1)
  16:	74 08                	je     0x20
  18:	4c 89 ff             	mov    %r15,%rdi
  1b:	e8 ac 3f e3 ff       	call   0xffe33fcc
  20:	4d 8b 2f             	mov    (%r15),%r13
  23:	4c 89 e0             	mov    %r12,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	80 3c 28 00          	cmpb   $0x0,(%rax,%rbp,1) <-- trapping instruction
  2e:	74 08                	je     0x38
  30:	4c 89 e7             	mov    %r12,%rdi
  33:	e8 84 40 e3 ff       	call   0xffe340bc
  38:	4d 89 2c 24          	mov    %r13,(%r12)
  3c:	4d 85 ed             	test   %r13,%r13
  3f:	74                   	.byte 0x74


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