[syzbot] [fs?] general protection fault in fd_install

[syzbot <syzbot+40f42779048f7476e2e0@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    4a26e7032d7d Merge tag 'core-bugs-2025-12-01' of git://git..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15321512580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=553f3db6410d5a82
dashboard link: https://syzkaller.appspot.com/bug?extid=40f42779048f7476e2e0
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=16d76192580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16bfb8c2580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-4a26e703.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/bf3025099b65/vmlinux-4a26e703.xz
kernel image: https://storage.googleapis.com/syzbot-assets/9e022e7e7365/bzImage-4a26e703.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+40f42779048f7476e2e0@syzkaller.appspotmail.com

Oops: general protection fault, probably for non-canonical address 0xdffffc0000000008: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x0000000000000040-0x0000000000000047]
CPU: 0 UID: 0 PID: 5517 Comm: syz.0.17 Not tainted syzkaller #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
RIP: 0010:fd_install+0x57/0x3d0 fs/file.c:685
Code: 48 81 c3 48 09 00 00 48 89 d8 48 c1 e8 03 80 3c 28 00 74 08 48 89 df e8 c7 4c e6 ff 4c 8b 3b 49 8d 5e 40 48 89 d8 48 c1 e8 03 <0f> b6 04 28 84 c0 0f 85 29 03 00 00 8b 1b 89 de 81 e6 00 00 00 01
RSP: 0018:ffffc9000cb27ca0 EFLAGS: 00010202
RAX: 0000000000000008 RBX: 0000000000000041 RCX: ffff888035b14980
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000006
RBP: dffffc0000000000 R08: ffff88801c0af0e3 R09: 1ffff11003815e1c
R10: dffffc0000000000 R11: ffffed1003815e1d R12: 0000000000000006
R13: 0000000000000006 R14: 0000000000000001 R15: ffff88801f408f00
FS:  000055557b0dc500(0000) GS:ffff88808d6ba000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007fe4967bb43c CR3: 000000001c158000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 do_mq_open+0x5a0/0x770 ipc/mqueue.c:932
 __do_sys_mq_open ipc/mqueue.c:945 [inline]
 __se_sys_mq_open ipc/mqueue.c:938 [inline]
 __x64_sys_mq_open+0x16a/0x1c0 ipc/mqueue.c:938
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f7cfa38f7c9
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 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 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007ffd8f2c3568 EFLAGS: 00000246 ORIG_RAX: 00000000000000f0
RAX: ffffffffffffffda RBX: 00007f7cfa5e5fa0 RCX: 00007f7cfa38f7c9
RDX: 0000000000000110 RSI: 0000000000000040 RDI: 00002000000004c0
RBP: 00007f7cfa413f91 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f7cfa5e5fa0 R14: 00007f7cfa5e5fa0 R15: 0000000000000004
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:fd_install+0x57/0x3d0 fs/file.c:685
Code: 48 81 c3 48 09 00 00 48 89 d8 48 c1 e8 03 80 3c 28 00 74 08 48 89 df e8 c7 4c e6 ff 4c 8b 3b 49 8d 5e 40 48 89 d8 48 c1 e8 03 <0f> b6 04 28 84 c0 0f 85 29 03 00 00 8b 1b 89 de 81 e6 00 00 00 01
RSP: 0018:ffffc9000cb27ca0 EFLAGS: 00010202
RAX: 0000000000000008 RBX: 0000000000000041 RCX: ffff888035b14980
RDX: 0000000000000000 RSI: 0000000000000001 RDI: 0000000000000006
RBP: dffffc0000000000 R08: ffff88801c0af0e3 R09: 1ffff11003815e1c
R10: dffffc0000000000 R11: ffffed1003815e1d R12: 0000000000000006
R13: 0000000000000006 R14: 0000000000000001 R15: ffff88801f408f00
FS:  000055557b0dc500(0000) GS:ffff88808d6ba000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f0a4abe2fe8 CR3: 000000001c158000 CR4: 0000000000352ef0
----------------
Code disassembly (best guess):
   0:	48 81 c3 48 09 00 00 	add    $0x948,%rbx
   7:	48 89 d8             	mov    %rbx,%rax
   a:	48 c1 e8 03          	shr    $0x3,%rax
   e:	80 3c 28 00          	cmpb   $0x0,(%rax,%rbp,1)
  12:	74 08                	je     0x1c
  14:	48 89 df             	mov    %rbx,%rdi
  17:	e8 c7 4c e6 ff       	call   0xffe64ce3
  1c:	4c 8b 3b             	mov    (%rbx),%r15
  1f:	49 8d 5e 40          	lea    0x40(%r14),%rbx
  23:	48 89 d8             	mov    %rbx,%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 29 03 00 00    	jne    0x35f
  36:	8b 1b                	mov    (%rbx),%ebx
  38:	89 de                	mov    %ebx,%esi
  3a:	81 e6 00 00 00 01    	and    $0x1000000,%esi


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