[syzbot] [netfs?] BUG: unable to handle kernel NULL pointer dereference in netfs_unbuffered_write

[syzbot <syzbot+7227db0fbac9f348dba0@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    c107785c7e8d Merge tag 'modules-7.0-rc3.fixes' of git://gi..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15db7b5a580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=976ba5a93c4add9e
dashboard link: https://syzkaller.appspot.com/bug?extid=7227db0fbac9f348dba0
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1628ab5a580000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16a5414a580000

Downloadable assets:
disk image (non-bootable): https://storage.googleapis.com/syzbot-assets/d900f083ada3/non_bootable_disk-c107785c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/3a4a4abcd973/vmlinux-c107785c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/f60667f16840/bzImage-c107785c.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+7227db0fbac9f348dba0@syzkaller.appspotmail.com

netfs: Couldn't get user pages (rc=-14)
BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 31867067 P4D 31867067 PUD 0 
Oops: Oops: 0010 [#1] SMP KASAN NOPTI
CPU: 3 UID: 0 PID: 6079 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 04/01/2014
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
RSP: 0018:ffffc90003b7fb90 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88803bd3a5b0 RCX: ffffffff82c49d0a
RDX: ffff88802b9ca4c0 RSI: ffffffff82c49b9c RDI: ffff88803bd3a500
RBP: 0000000000140000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff88803bd3a598
R13: dffffc0000000000 R14: ffff88803bd3a500 R15: ffff888023066580
FS:  00007f9e9a09f6c0(0000) GS:ffff8880d6644000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000002c65b000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 netfs_unbuffered_write+0xae5/0x2080 fs/netfs/direct_write.c:189
 netfs_unbuffered_write_iter_locked+0x801/0xab0 fs/netfs/direct_write.c:287
 netfs_unbuffered_write_iter+0x40c/0x710 fs/netfs/direct_write.c:377
 v9fs_file_write_iter+0xbf/0x100 fs/9p/vfs_file.c:409
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1070 fs/read_write.c:688
 ksys_write+0x12a/0x250 fs/read_write.c:740
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x106/0xf80 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f9e9919c799
Code: ff c3 66 2e 0f 1f 84 00 00 00 00 00 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 e8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f9e9a09f028 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f9e99415fa0 RCX: 00007f9e9919c799
RDX: 000000000208e24b RSI: 0000200000000000 RDI: 0000000000000003
RBP: 00007f9e99232bd9 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 00007f9e99416038 R14: 00007f9e99415fa0 R15: 00007fff05034208
 </TASK>
Modules linked in:
CR2: 0000000000000000
---[ end trace 0000000000000000 ]---
RIP: 0010:0x0
Code: Unable to access opcode bytes at 0xffffffffffffffd6.
RSP: 0018:ffffc90003b7fb90 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff88803bd3a5b0 RCX: ffffffff82c49d0a
RDX: ffff88802b9ca4c0 RSI: ffffffff82c49b9c RDI: ffff88803bd3a500
RBP: 0000000000140000 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000001 R11: 0000000000000000 R12: ffff88803bd3a598
R13: dffffc0000000000 R14: ffff88803bd3a500 R15: ffff888023066580
FS:  00007f9e9a09f6c0(0000) GS:ffff8880d6644000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 000000002c65b000 CR4: 0000000000352ef0


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