[syzbot] [squashfs?] general protection fault in lookup_open

[syzbot <syzbot+0b7a8fb8279a59e8a188@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    84abb955b101 Add linux-next specific files for 20261113
git tree:       linux-next
console+strace: https://syzkaller.appspot.com/x/log.txt?x=1082844d580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d9d5eeb0ba036071
dashboard link: https://syzkaller.appspot.com/bug?extid=0b7a8fb8279a59e8a188
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=148b45be580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=160f55a1580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/ffae1d4a717e/disk-84abb955.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/fd2a35a63abf/vmlinux-84abb955.xz
kernel image: https://storage.googleapis.com/syzbot-assets/039ccf647778/bzImage-84abb955.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/90d273be2ea1/mount_0.gz

AI-generated root cause summary:
In fs/squashfs/namei.c:squashfs_lookup(), sanity checks using
squashfs_dir_space() jump directly to label read_failure instead of data_error
upon detecting that a directory entry extends beyond inode size. Jumping to
read_failure bypasses the assignment of err = -EIO, leaving err set to either an
uninitialized value or a positive value returned by a prior successful call to
squashfs_read_metadata() (such as sizeof(struct squashfs_dir_entry) == 8).
Consequently, squashfs_lookup() returns ERR_PTR(8), which IS_ERR() does not
recognize as an error pointer; VFS lookup_open() subsequently dereferences
dentry->d_inode at address 0x8 + 0x68 = 0x70, triggering a near-NULL pointer
dereference crash.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=0b7a8fb8279a59e8a188

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+0b7a8fb8279a59e8a188@syzkaller.appspotmail.com

loop0: detected capacity change from 0 to 8
SQUASHFS error: Unable to read directory block [0:26]
Oops: general protection fault, probably for non-canonical address 0xdffffc000000000d: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000068-0x000000000000006f]
CPU: 1 UID: 0 PID: 5645 Comm: syz-executor126 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
RIP: 0010:lookup_open+0x890/0x16e0 fs/namei.c:4648
Code: 09 00 00 e8 a2 60 68 ff 49 81 ff 01 f0 ff ff 0f 83 2a ff ff ff 4d 8d 77 68 4c 89 f0 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 4c 89 f7 e8 92 54 e8 ff 49 83 3e 00 74 07 e8 67
RSP: 0018:ffffc900039579c0 EFLAGS: 00010203
RAX: 000000000000000d RBX: 0000000000000040 RCX: dffffc0000000000
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 00000000ffffffff
RBP: 00000000ffffffe2 R08: ffff8880134403fb R09: 1ffff1100268807f
R10: dffffc0000000000 R11: ffffed1002688080 R12: 1ffff110026f064c
R13: ffffc90003957e00 R14: 000000000000006e R15: 0000000000000006
FS:  0000555559c4d400(0000) GS:ffff8881234cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000020000000b400 CR3: 0000000071a58000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 open_last_lookups fs/namei.c:4880 [inline]
 path_openat+0x7a5/0x20e0 fs/namei.c:5119
 do_file_open+0x23e/0x4a0 fs/namei.c:5151
 do_sys_openat2+0x115/0x200 fs/open.c:1492
 do_sys_open fs/open.c:1498 [inline]
 __do_sys_openat fs/open.c:1838 [inline]
 __se_sys_openat fs/open.c:1833 [inline]
 __x64_sys_openat+0x138/0x170 fs/open.c:1833
 do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
 do_syscall_64+0x171/0x540 arch/x86/entry/syscall_64.c:89
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x00007fc862573909
Code: c0 79 93 eb d5 48 8d 7c 1d 00 eb 99 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 d8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fff58fa7d68 EFLAGS: 00000246 ORIG_RAX: 0000000000000101
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007fc862573909
RDX: 000000000000275a RSI: 000020000000b400 RDI: 00000000ffffff9c
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000555559c4e580
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff58fa7d90
R13: 0000000000000002 R14: 431bde82d7b634db R15: 00007fc8625d304b
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:lookup_open+0x890/0x16e0 fs/namei.c:4648
Code: 09 00 00 e8 a2 60 68 ff 49 81 ff 01 f0 ff ff 0f 83 2a ff ff ff 4d 8d 77 68 4c 89 f0 48 c1 e8 03 48 b9 00 00 00 00 00 fc ff df <80> 3c 08 00 74 08 4c 89 f7 e8 92 54 e8 ff 49 83 3e 00 74 07 e8 67
RSP: 0018:ffffc900039579c0 EFLAGS: 00010203
RAX: 000000000000000d RBX: 0000000000000040 RCX: dffffc0000000000
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 00000000ffffffff
RBP: 00000000ffffffe2 R08: ffff8880134403fb R09: 1ffff1100268807f
R10: dffffc0000000000 R11: ffffed1002688080 R12: 1ffff110026f064c
R13: ffffc90003957e00 R14: 000000000000006e R15: 0000000000000006
FS:  0000555559c4d400(0000) GS:ffff8881233cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000557377da1760 CR3: 0000000071a58000 CR4: 00000000003526f0
----------------
Code disassembly (best guess), 1 bytes skipped:
   0:	00 00                	add    %al,(%rax)
   2:	e8 a2 60 68 ff       	call   0xff6860a9
   7:	49 81 ff 01 f0 ff ff 	cmp    $0xfffffffffffff001,%r15
   e:	0f 83 2a ff ff ff    	jae    0xffffff3e
  14:	4d 8d 77 68          	lea    0x68(%r15),%r14
  18:	4c 89 f0             	mov    %r14,%rax
  1b:	48 c1 e8 03          	shr    $0x3,%rax
  1f:	48 b9 00 00 00 00 00 	movabs $0xdffffc0000000000,%rcx
  26:	fc ff df
* 29:	80 3c 08 00          	cmpb   $0x0,(%rax,%rcx,1) <-- trapping instruction
  2d:	74 08                	je     0x37
  2f:	4c 89 f7             	mov    %r14,%rdi
  32:	e8 92 54 e8 ff       	call   0xffe854c9
  37:	49 83 3e 00          	cmpq   $0x0,(%r14)
  3b:	74 07                	je     0x44
  3d:	e8                   	.byte 0xe8
  3e:	67                   	addr32


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