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

[syzbot <syzbot+03fdf84c0915fd48f361@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=10a329ad580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d9d5eeb0ba036071
dashboard link: https://syzkaller.appspot.com/bug?extid=03fdf84c0915fd48f361
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=12db8d3d580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=140eebfd580000

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/d95c1baa8012/mount_0.gz

AI-generated root cause summary:
In squashfs_lookup() (fs/squashfs/namei.c), squashfs_read_metadata() returns the
positive number of bytes read on success, which is stored in err. When a
subsequent squashfs_dir_space() check fails, execution branches directly to
read_failure without setting err to a negative errno like -EIO, causing
squashfs_lookup() to return ERR_PTR(err) where err is a positive integer (e.g.,
6). The VFS caller lookup_one_qstr_excl() treats this positive pointer as a
valid struct dentry pointer because IS_ERR() evaluates to false, and immediately
dereferences dentry->d_flags (offset 0), causing a near-NULL pointer dereference
crash.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=03fdf84c0915fd48f361

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+03fdf84c0915fd48f361@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 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
CPU: 0 UID: 0 PID: 5646 Comm: syz-executor151 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
RIP: 0010:__d_entry_type include/linux/dcache.h:452 [inline]
RIP: 0010:d_is_miss include/linux/dcache.h:457 [inline]
RIP: 0010:d_is_negative include/linux/dcache.h:503 [inline]
RIP: 0010:lookup_one_qstr_excl+0x155/0x360 fs/namei.c:1847
Code: ff d3 48 85 c0 0f 85 8f 01 00 00 e8 a5 a6 66 ff 49 81 fe 01 f0 ff ff 72 0a e8 97 a6 66 ff e9 86 00 00 00 4c 89 f0 48 c1 e8 03 <0f> b6 04 28 84 c0 0f 85 b6 01 00 00 bd 00 00 38 00 41 23 2e 41 89
RSP: 0018:ffffc90003c57cb0 EFLAGS: 00010247
RAX: 0000000000000000 RBX: 0000000000060000 RCX: ffff8880269a1f40
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 00000000ffffffff
RBP: dffffc0000000000 R08: ffff88807705343b R09: 1ffff1100ee0a687
R10: dffffc0000000000 R11: ffffed100ee0a688 R12: ffff888077053380
R13: ffff888079bb7240 R14: 0000000000000006 R15: 0000000000000006
FS:  00005555603cd400(0000) GS:ffff8881233cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000000 CR3: 0000000026d50000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 __start_dirop fs/namei.c:2971 [inline]
 start_dirop fs/namei.c:2993 [inline]
 filename_create+0x20e/0x370 fs/namei.c:5214
 filename_symlinkat+0xe8/0x430 fs/namei.c:5913
 __do_sys_symlink fs/namei.c:5946 [inline]
 __se_sys_symlink+0x4d/0x2b0 fs/namei.c:5942
 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: 0033:0x00007fe4baeb7c49
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:00007ffe25a11318 EFLAGS: 00000246 ORIG_RAX: 0000000000000058
RAX: ffffffffffffffda RBX: 00007ffe25a11340 RCX: 00007fe4baeb7c49
RDX: 00007fe4baeb7c49 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 00007fe4baf170b0 R08: 00005555603ce060 R09: 00005555603ce060
R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe25a11340
R13: 0000000000000002 R14: 431bde82d7b634db R15: 00007fe4baf1704b
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__d_entry_type include/linux/dcache.h:452 [inline]
RIP: 0010:d_is_miss include/linux/dcache.h:457 [inline]
RIP: 0010:d_is_negative include/linux/dcache.h:503 [inline]
RIP: 0010:lookup_one_qstr_excl+0x155/0x360 fs/namei.c:1847
Code: ff d3 48 85 c0 0f 85 8f 01 00 00 e8 a5 a6 66 ff 49 81 fe 01 f0 ff ff 72 0a e8 97 a6 66 ff e9 86 00 00 00 4c 89 f0 48 c1 e8 03 <0f> b6 04 28 84 c0 0f 85 b6 01 00 00 bd 00 00 38 00 41 23 2e 41 89
RSP: 0018:ffffc90003c57cb0 EFLAGS: 00010247
RAX: 0000000000000000 RBX: 0000000000060000 RCX: ffff8880269a1f40
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 00000000ffffffff
RBP: dffffc0000000000 R08: ffff88807705343b R09: 1ffff1100ee0a687
R10: dffffc0000000000 R11: ffffed100ee0a688 R12: ffff888077053380
R13: ffff888079bb7240 R14: 0000000000000006 R15: 0000000000000006
FS:  00005555603cd400(0000) GS:ffff8881234cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffe25a12000 CR3: 0000000026d50000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	ff d3                	call   *%rbx
   2:	48 85 c0             	test   %rax,%rax
   5:	0f 85 8f 01 00 00    	jne    0x19a
   b:	e8 a5 a6 66 ff       	call   0xff66a6b5
  10:	49 81 fe 01 f0 ff ff 	cmp    $0xfffffffffffff001,%r14
  17:	72 0a                	jb     0x23
  19:	e8 97 a6 66 ff       	call   0xff66a6b5
  1e:	e9 86 00 00 00       	jmp    0xa9
  23:	4c 89 f0             	mov    %r14,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	0f b6 04 28          	movzbl (%rax,%rbp,1),%eax <-- trapping instruction
  2e:	84 c0                	test   %al,%al
  30:	0f 85 b6 01 00 00    	jne    0x1ec
  36:	bd 00 00 38 00       	mov    $0x380000,%ebp
  3b:	41 23 2e             	and    (%r14),%ebp
  3e:	41                   	rex.B
  3f:	89                   	.byte 0x89


---
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