[syzbot] [fuse?] KASAN: null-ptr-deref Read in fuse_copy_do

[syzbot <syzbot+87b8e6ed25dbc41759f7@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    445d9f05fa14 Merge tag 'nfsd-6.13' of git://git.kernel.org..
git tree:       upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=12733530580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=3c44a32edb32752c
dashboard link: https://syzkaller.appspot.com/bug?extid=87b8e6ed25dbc41759f7
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11fd43c0580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15cf2f5f980000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/9fd8dd2a6550/disk-445d9f05.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/af034d90afcb/vmlinux-445d9f05.xz
kernel image: https://storage.googleapis.com/syzbot-assets/07a713832258/bzImage-445d9f05.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+87b8e6ed25dbc41759f7@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in fuse_copy_do+0x183/0x320 fs/fuse/dev.c:809
Write of size 5 at addr 0000000000000000 by task syz-executor159/5840

CPU: 0 UID: 0 PID: 5840 Comm: syz-executor159 Not tainted 6.12.0-syzkaller-09734-g445d9f05fa14 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/13/2024
Call Trace:
 <TASK>
 __dump_stack lib/dump_stack.c:94 [inline]
 dump_stack_lvl+0x116/0x1f0 lib/dump_stack.c:120
 kasan_report+0xd9/0x110 mm/kasan/report.c:602
 check_region_inline mm/kasan/generic.c:183 [inline]
 kasan_check_range+0xef/0x1a0 mm/kasan/generic.c:189
 __asan_memcpy+0x3c/0x60 mm/kasan/shadow.c:106
 fuse_copy_do+0x183/0x320 fs/fuse/dev.c:809
 fuse_copy_one fs/fuse/dev.c:1065 [inline]
 fuse_copy_args+0x1e6/0x770 fs/fuse/dev.c:1083
 copy_out_args fs/fuse/dev.c:1966 [inline]
 fuse_dev_do_write+0x1cc1/0x3720 fs/fuse/dev.c:2052
 fuse_dev_write+0x14f/0x1e0 fs/fuse/dev.c:2087
 new_sync_write fs/read_write.c:586 [inline]
 vfs_write+0x5ae/0x1150 fs/read_write.c:679
 ksys_write+0x12b/0x250 fs/read_write.c:731
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x250 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7fa4c9df3c0f
Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 89 5e 02 00 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 dc 5e 02 00 48
RSP: 002b:00007fa4c9da71e0 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007fa4c9e7f3e8 RCX: 00007fa4c9df3c0f
RDX: 0000000000000015 RSI: 0000000020000540 RDI: 0000000000000003
RBP: 00007fa4c9e7f3e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000293 R12: 00007fa4c9e4c33c
R13: 00007fa4c9e44027 R14: 00007fff3bcf2380 R15: 00007fa4c9e4a338
 </TASK>
==================================================================


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