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

[syzbot <syzbot+5d7ba5f6127ed887f459@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=122fbda1580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d9d5eeb0ba036071
dashboard link: https://syzkaller.appspot.com/bug?extid=5d7ba5f6127ed887f459
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=174cb369580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1136b011580000

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/3d55211a7134/mount_0.gz

AI-generated root cause summary:
In fs/squashfs/inode.c:squashfs_read_inode(), squashfs_read_metadata() returns
the number of bytes read on success (e.g. sizeof(struct squashfs_dir_inode) ==
32) into err. When checking directory inodes (SQUASHFS_DIR_TYPE and
SQUASHFS_LDIR_TYPE) where inode->i_size < 3, the code jumps to failed_read via
goto without updating err to a negative error code (such as -EINVAL). As a
result, squashfs_read_inode() returns a positive integer (32 or 40), which
squashfs_iget() wraps with ERR_PTR(err) into (struct inode *)0x20. Callers like
d_obtain_alias() check IS_ERR(inode), which evaluates to false for positive
values, and subsequently dereference inode->i_sb at offset 0x28 (address 0x48),
triggering a near-NULL pointer dereference crash.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=5d7ba5f6127ed887f459

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+5d7ba5f6127ed887f459@syzkaller.appspotmail.com

loop0: detected capacity change from 0 to 8
SQUASHFS error: Unable to read inode 0x87
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000009: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000048-0x000000000000004f]
CPU: 0 UID: 0 PID: 5647 Comm: syz-executor372 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
RIP: 0010:__d_obtain_alias+0x50/0x880 fs/dcache.c:2359
Code: e8 25 e6 64 ff eb 53 e8 1e e6 64 ff 49 c7 c5 8c ff ff ff eb 45 49 bf 00 00 00 00 00 fc ff df 49 8d 7d 28 48 89 f8 48 c1 e8 03 <42> 80 3c 38 00 74 05 e8 14 da e4 ff 4d 8b 75 28 4c 89 ef e8 a8 a8
RSP: 0018:ffffc90003b2f9d0 EFLAGS: 00010206
RAX: 0000000000000009 RBX: 0000000000000002 RCX: ffff888079600000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000048
RBP: 0000000000000001 R08: ffffffff91bec8bf R09: 1ffffffff237d917
R10: dffffc0000000000 R11: fffffbfff237d918 R12: ffff88807deca000
R13: 0000000000000020 R14: 1ffff92000765f4c R15: dffffc0000000000
FS:  000055556efe9400(0000) GS:ffff8881233cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffc93144b5c CR3: 000000007b0fe000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 squashfs_export_iget+0x1d8/0x370 fs/squashfs/export.c:117
 exportfs_decode_fh_raw+0x1c8/0x770 fs/exportfs/expfs.c:460
 do_handle_to_path+0xa4/0x1a0 fs/fhandle.c:274
 handle_to_path fs/fhandle.c:423 [inline]
 do_handle_open+0x6dd/0xb10 fs/fhandle.c:447
 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: 0033:0x00007fab5525dc49
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:00007fff9e969ae8 EFLAGS: 00000246 ORIG_RAX: 0000000000000130
RAX: ffffffffffffffda RBX: 00007fab552bd0b0 RCX: 00007fab5525dc49
RDX: 0000000000000000 RSI: 0000200000000340 RDI: 0000000000000004
RBP: 0000000000000001 R08: 000055556efea580 R09: 000055556efea580
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fff9e969b10
R13: 0000000000000002 R14: 431bde82d7b634db R15: 00007fab552bd04b
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:__d_obtain_alias+0x50/0x880 fs/dcache.c:2359
Code: e8 25 e6 64 ff eb 53 e8 1e e6 64 ff 49 c7 c5 8c ff ff ff eb 45 49 bf 00 00 00 00 00 fc ff df 49 8d 7d 28 48 89 f8 48 c1 e8 03 <42> 80 3c 38 00 74 05 e8 14 da e4 ff 4d 8b 75 28 4c 89 ef e8 a8 a8
RSP: 0018:ffffc90003b2f9d0 EFLAGS: 00010206
RAX: 0000000000000009 RBX: 0000000000000002 RCX: ffff888079600000
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000048
RBP: 0000000000000001 R08: ffffffff91bec8bf R09: 1ffffffff237d917
R10: dffffc0000000000 R11: fffffbfff237d918 R12: ffff88807deca000
R13: 0000000000000020 R14: 1ffff92000765f4c R15: dffffc0000000000
FS:  000055556efe9400(0000) GS:ffff8881233cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ffc93144b5c CR3: 000000007b0fe000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	e8 25 e6 64 ff       	call   0xff64e62a
   5:	eb 53                	jmp    0x5a
   7:	e8 1e e6 64 ff       	call   0xff64e62a
   c:	49 c7 c5 8c ff ff ff 	mov    $0xffffffffffffff8c,%r13
  13:	eb 45                	jmp    0x5a
  15:	49 bf 00 00 00 00 00 	movabs $0xdffffc0000000000,%r15
  1c:	fc ff df
  1f:	49 8d 7d 28          	lea    0x28(%r13),%rdi
  23:	48 89 f8             	mov    %rdi,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	42 80 3c 38 00       	cmpb   $0x0,(%rax,%r15,1) <-- trapping instruction
  2f:	74 05                	je     0x36
  31:	e8 14 da e4 ff       	call   0xffe4da4a
  36:	4d 8b 75 28          	mov    0x28(%r13),%r14
  3a:	4c 89 ef             	mov    %r13,%rdi
  3d:	e8                   	.byte 0xe8
  3e:	a8 a8                	test   $0xa8,%al


---
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