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

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

[syzbot]

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=16da293d580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d9d5eeb0ba036071
dashboard link: https://syzkaller.appspot.com/bug?extid=d393ed2b27449be4992b
compiler:       Debian clang version 22.1.8 (1~deb13u4), Debian LLD 22.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=132b7fa5580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12dfd4d9580000

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/39a0c2b38acc/mount_1.gz

AI-generated root cause summary:
In squashfs_lookup() (fs/squashfs/namei.c), the local variable int err is
declared uninitialized and receives a positive return value (number of bytes
read) from preceding successful calls to squashfs_read_metadata(). When
directory boundary checks via squashfs_dir_space() fail, execution jumps
directly to the read_failure label rather than data_error (which sets err =
-EIO). Consequently, squashfs_lookup() returns ERR_PTR(err) holding a small
positive integer (e.g., (struct dentry *)0x6) or uninitialized stack data, which
VFS path resolution treats as a valid dentry pointer and subsequently
dereferences at offset 0 (d_flags) in traverse_mounts(), causing a kernel crash.

full analysis:  https://syzkaller.appspot.com/x/ai_root_cause?x=d393ed2b27449be4992b

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d393ed2b27449be4992b@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: 1 UID: 0 PID: 5644 Comm: syz-executor134 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/06/2026
RIP: 0010:traverse_mounts fs/namei.c:1665 [inline]
RIP: 0010:handle_mounts fs/namei.c:1777 [inline]
RIP: 0010:step_into_slowpath+0x112/0x810 fs/namei.c:2146
Code: 24 1f 00 49 bf 00 00 00 00 00 fc ff df 43 0f b6 04 3c 84 c0 0f 85 75 05 00 00 45 8b 75 00 4c 8b 6c 24 48 4c 89 e8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 76 05 00 00 45 8b 6d 00 44 89 ee 81 e6
RSP: 0018:ffffc90003ba7a40 EFLAGS: 00010247
RAX: 0000000000000000 RBX: ffffc90003ba7c20 RCX: ffff888037d43e80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90003ba7af0 R08: ffff888078fd331f R09: 1ffff1100f1fa663
R10: dffffc0000000000 R11: ffffed100f1fa664 R12: 1ffff92000774f8b
R13: 0000000000000006 R14: 0000000000000001 R15: dffffc0000000000
FS:  0000555564c07400(0000) GS:ffff8881234cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff34166e74 CR3: 000000007b074000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 step_into fs/namei.c:2194 [inline]
 walk_component fs/namei.c:2331 [inline]
 lookup_last fs/namei.c:2839 [inline]
 path_lookupat+0x433/0x8c0 fs/namei.c:2864
 filename_lookup+0x265/0x5d0 fs/namei.c:2893
 user_path_at+0x40/0x160 fs/namei.c:3692
 do_mount fs/namespace.c:4467 [inline]
 __do_sys_mount fs/namespace.c:4683 [inline]
 __se_sys_mount+0x2dc/0x420 fs/namespace.c:4660
 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: 0033:0x00007fdbf37e4949
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:00007fff0f8baee8 EFLAGS: 00000246 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000200000000000 RCX: 00007fdbf37e4949
RDX: 0000000000000000 RSI: 00002000000000c0 RDI: 0000000000000000
RBP: 0000000000000001 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000004000 R11: 0000000000000246 R12: 00007fff0f8baf20
R13: 0000000000000002 R14: 431bde82d7b634db R15: 00007fdbf384404b
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:traverse_mounts fs/namei.c:1665 [inline]
RIP: 0010:handle_mounts fs/namei.c:1777 [inline]
RIP: 0010:step_into_slowpath+0x112/0x810 fs/namei.c:2146
Code: 24 1f 00 49 bf 00 00 00 00 00 fc ff df 43 0f b6 04 3c 84 c0 0f 85 75 05 00 00 45 8b 75 00 4c 8b 6c 24 48 4c 89 e8 48 c1 e8 03 <42> 0f b6 04 38 84 c0 0f 85 76 05 00 00 45 8b 6d 00 44 89 ee 81 e6
RSP: 0018:ffffc90003ba7a40 EFLAGS: 00010247
RAX: 0000000000000000 RBX: ffffc90003ba7c20 RCX: ffff888037d43e80
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
RBP: ffffc90003ba7af0 R08: ffff888078fd331f R09: 1ffff1100f1fa663
R10: dffffc0000000000 R11: ffffed100f1fa664 R12: 1ffff92000774f8b
R13: 0000000000000006 R14: 0000000000000001 R15: dffffc0000000000
FS:  0000555564c07400(0000) GS:ffff8881234cd000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fff34166e74 CR3: 000000007b074000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	24 1f                	and    $0x1f,%al
   2:	00 49 bf             	add    %cl,-0x41(%rcx)
   5:	00 00                	add    %al,(%rax)
   7:	00 00                	add    %al,(%rax)
   9:	00 fc                	add    %bh,%ah
   b:	ff                   	lcall  (bad)
   c:	df 43 0f             	filds  0xf(%rbx)
   f:	b6 04                	mov    $0x4,%dh
  11:	3c 84                	cmp    $0x84,%al
  13:	c0 0f 85             	rorb   $0x85,(%rdi)
  16:	75 05                	jne    0x1d
  18:	00 00                	add    %al,(%rax)
  1a:	45 8b 75 00          	mov    0x0(%r13),%r14d
  1e:	4c 8b 6c 24 48       	mov    0x48(%rsp),%r13
  23:	4c 89 e8             	mov    %r13,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	42 0f b6 04 38       	movzbl (%rax,%r15,1),%eax <-- trapping instruction
  2f:	84 c0                	test   %al,%al
  31:	0f 85 76 05 00 00    	jne    0x5ad
  37:	45 8b 6d 00          	mov    0x0(%r13),%r13d
  3b:	44 89 ee             	mov    %r13d,%esi
  3e:	81                   	.byte 0x81
  3f:	e6                   	.byte 0xe6


---
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