[syzbot] [exfat?] general protection fault in exfat_utf16_to_nls

[syzbot <syzbot+3e9cb93e3c5f90d28e19@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    b5a4da2c459f Add linux-next specific files for 20250924
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=15ffad34580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=fc64d939cce41d2
dashboard link: https://syzkaller.appspot.com/bug?extid=3e9cb93e3c5f90d28e19
compiler:       Debian clang version 20.1.8 (++20250708063551+0c9f909b7976-1~exp1~20250708183702.136), Debian LLD 20.1.8
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1704cf12580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15c8d4e2580000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/127c931e6696/disk-b5a4da2c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/cf4957abd39e/vmlinux-b5a4da2c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/860d3ac61bac/bzImage-b5a4da2c.xz
mounted in repro: https://storage.googleapis.com/syzbot-assets/9006b1673f9f/mount_0.gz
  fsck result: failed (log: https://syzkaller.appspot.com/x/fsck.log?x=11c8d4e2580000)

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+3e9cb93e3c5f90d28e19@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000002: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
CPU: 1 UID: 0 PID: 5979 Comm: syz-executor Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/18/2025
RIP: 0010:exfat_convert_ucs2_to_char fs/exfat/nls.c:441 [inline]
RIP: 0010:__exfat_utf16_to_nls fs/exfat/nls.c:554 [inline]
RIP: 0010:exfat_utf16_to_nls+0x21c/0x840 fs/exfat/nls.c:638
Code: 2e 29 ff 66 41 83 fc 7f 77 14 e8 7f 2a 29 ff e9 b6 00 00 00 e8 75 2a 29 ff e9 a9 00 00 00 48 8b 4c 24 40 48 89 c8 48 c1 e8 03 <42> 80 3c 38 00 74 0f 48 8b 7c 24 40 e8 d3 6b 8e ff 48 8b 4c 24 40
RSP: 0018:ffffc90003fef760 EFLAGS: 00010202
RAX: 0000000000000002 RBX: 0000000000000004 RCX: 0000000000000010
RDX: ffff88802f18bc80 RSI: 00000000000000e1 RDI: 0000000000000080
RBP: ffffc90003fef850 R08: 0000000000000005 R09: 0000000000000000
R10: ffffc90003fef7e0 R11: fffff520007fdefc R12: 00000000000000e1
R13: ffffc90003fefa48 R14: 0000000000000000 R15: dffffc0000000000
FS:  000055557b9c2500(0000) GS:ffff888125b03000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055557b9e5608 CR3: 00000000734c8000 CR4: 00000000003526f0
Call Trace:
 <TASK>
 exfat_readdir fs/exfat/dir.c:143 [inline]
 exfat_iterate+0x19a7/0x2050 fs/exfat/dir.c:243
 wrap_directory_iterator+0x96/0xe0 fs/readdir.c:65
 iterate_dir+0x399/0x570 fs/readdir.c:108
 __do_sys_getdents64 fs/readdir.c:410 [inline]
 __se_sys_getdents64+0xe4/0x260 fs/readdir.c:396
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xfa0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2c837c1833
Code: c1 66 0f 1f 44 00 00 48 83 c4 08 48 89 ef 5b 5d e9 32 3d f8 ff 66 90 b8 ff ff ff 7f 48 39 c2 48 0f 47 d0 b8 d9 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 05 c3 0f 1f 40 00 48 c7 c2 a8 ff ff ff f7 d8
RSP: 002b:00007ffc8cf73ff8 EFLAGS: 00000293 ORIG_RAX: 00000000000000d9
RAX: ffffffffffffffda RBX: 000055557b9dd600 RCX: 00007f2c837c1833
RDX: 0000000000008000 RSI: 000055557b9dd600 RDI: 0000000000000005
RBP: 000055557b9dd5d4 R08: 0000000000028a41 R09: 0000000000000000
R10: 00007f2c839b7cc0 R11: 0000000000000293 R12: ffffffffffffffa8
R13: 0000000000000010 R14: 000055557b9dd5d0 R15: 00007ffc8cf762b0
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:exfat_convert_ucs2_to_char fs/exfat/nls.c:441 [inline]
RIP: 0010:__exfat_utf16_to_nls fs/exfat/nls.c:554 [inline]
RIP: 0010:exfat_utf16_to_nls+0x21c/0x840 fs/exfat/nls.c:638
Code: 2e 29 ff 66 41 83 fc 7f 77 14 e8 7f 2a 29 ff e9 b6 00 00 00 e8 75 2a 29 ff e9 a9 00 00 00 48 8b 4c 24 40 48 89 c8 48 c1 e8 03 <42> 80 3c 38 00 74 0f 48 8b 7c 24 40 e8 d3 6b 8e ff 48 8b 4c 24 40
RSP: 0018:ffffc90003fef760 EFLAGS: 00010202
RAX: 0000000000000002 RBX: 0000000000000004 RCX: 0000000000000010
RDX: ffff88802f18bc80 RSI: 00000000000000e1 RDI: 0000000000000080
RBP: ffffc90003fef850 R08: 0000000000000005 R09: 0000000000000000
R10: ffffc90003fef7e0 R11: fffff520007fdefc R12: 00000000000000e1
R13: ffffc90003fefa48 R14: 0000000000000000 R15: dffffc0000000000
FS:  000055557b9c2500(0000) GS:ffff888125a03000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000000c000692800 CR3: 00000000734c8000 CR4: 00000000003526f0
----------------
Code disassembly (best guess):
   0:	2e 29 ff             	cs sub %edi,%edi
   3:	66 41 83 fc 7f       	cmp    $0x7f,%r12w
   8:	77 14                	ja     0x1e
   a:	e8 7f 2a 29 ff       	call   0xff292a8e
   f:	e9 b6 00 00 00       	jmp    0xca
  14:	e8 75 2a 29 ff       	call   0xff292a8e
  19:	e9 a9 00 00 00       	jmp    0xc7
  1e:	48 8b 4c 24 40       	mov    0x40(%rsp),%rcx
  23:	48 89 c8             	mov    %rcx,%rax
  26:	48 c1 e8 03          	shr    $0x3,%rax
* 2a:	42 80 3c 38 00       	cmpb   $0x0,(%rax,%r15,1) <-- trapping instruction
  2f:	74 0f                	je     0x40
  31:	48 8b 7c 24 40       	mov    0x40(%rsp),%rdi
  36:	e8 d3 6b 8e ff       	call   0xff8e6c0e
  3b:	48 8b 4c 24 40       	mov    0x40(%rsp),%rcx


---
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