[syzbot] [netfs?] general protection fault in netfs_prepare_write

Hello,

syzbot found the following issue on:

HEAD commit:    3d6d817622b0 Merge tag 'scsi-fixes' of git://git.kernel.or..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14832c79580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=c44651ea7dd2f307
dashboard link: https://syzkaller.appspot.com/bug?extid=6a13fc77eb6f0802be2d
compiler:       gcc (Debian 14.2.0-19) 14.2.0, GNU ld (GNU Binutils for Debian) 2.44
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=12832c79580000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: [email protected]

RBP: 0000000000000006 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2bf1f9f288
R13: 0000000000000005 R14: 0000000000000000 R15: 00007f2bf1fa3252
 </TASK>
Oops: general protection fault, probably for non-canonical address 0xdffffc0000000015: 0000 [#1] SMP KASAN NOPTI
KASAN: null-ptr-deref in range [0x00000000000000a8-0x00000000000000af]
CPU: 3 UID: 0 PID: 6023 Comm: syz-executor218 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:netfs_prepare_write+0xbc/0x7b0 fs/netfs/write_issue.c:173
Code: e1 07 38 ca 7f 08 84 d2 0f 85 04 06 00 00 48 8d bb ad 00 00 00 0f b6 45 52 48 ba 00 00 00 00 00 fc ff df 48 89 f9 48 c1 e9 03 <0f> b6 14 11 48 89 f9 83 e1 07 38 ca 7f 08 84 d2 0f 85 c1 05 00 00
RSP: 0018:ffffc90003ea7b40 EFLAGS: 00010203
RAX: 0000000000000005 RBX: 0000000000000000 RCX: 0000000000000015
RDX: dffffc0000000000 RSI: ffffffff82cb9219 RDI: 00000000000000ad
RBP: ffff8880383da660 R08: 00000000ffffffff R09: fffffbfff1dbdcfe
R10: ffffffff8edee7f3 R11: 0000000000000000 R12: ffff8880383da580
R13: ffff8880383da738 R14: 000000000000001c R15: ffff8880383da6b2
FS:  000055558535c400(0000) GS:ffff8880d60ec000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000557b8da30300 CR3: 0000000028cd1000 CR4: 0000000000352ef0
Call Trace:
 <TASK>
 netfs_unbuffered_write+0x75c/0x2020 fs/netfs/direct_write.c:111
 netfs_unbuffered_write_iter_locked+0x7d6/0xa80 fs/netfs/direct_write.c:290
 netfs_unbuffered_write_iter+0x442/0x720 fs/netfs/direct_write.c:382
 v9fs_file_write_iter+0xbf/0x100 fs/9p/vfs_file.c:409
 new_sync_write fs/read_write.c:595 [inline]
 vfs_write+0x6ac/0x1050 fs/read_write.c:687
 ksys_write+0x12a/0x250 fs/read_write.c:739
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0x115/0x870 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f2bf1f5355e
Code: 08 0f 85 f5 e2 ff ff 49 89 fb 48 89 f0 48 89 d7 48 89 ce 4c 89 c2 4d 89 ca 4c 8b 44 24 08 4c 8b 4c 24 10 4c 89 5c 24 08 0f 05 <c3> 90 41 57 41 56 4d 89 c6 41 55 4d 89 cd 41 54 55 53 48 83 ec 08
RSP: 002b:00007fffd127ea28 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 000055558535c400 RCX: 00007f2bf1f5355e
RDX: 0000000000000004 RSI: 00007f2bf1fa31f6 RDI: 0000000000000005
RBP: 0000000000000006 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f2bf1f9f288
R13: 0000000000000005 R14: 0000000000000000 R15: 00007f2bf1fa3252
 </TASK>
Modules linked in:
---[ end trace 0000000000000000 ]---
RIP: 0010:netfs_prepare_write+0xbc/0x7b0 fs/netfs/write_issue.c:173
Code: e1 07 38 ca 7f 08 84 d2 0f 85 04 06 00 00 48 8d bb ad 00 00 00 0f b6 45 52 48 ba 00 00 00 00 00 fc ff df 48 89 f9 48 c1 e9 03 <0f> b6 14 11 48 89 f9 83 e1 07 38 ca 7f 08 84 d2 0f 85 c1 05 00 00
RSP: 0018:ffffc90003ea7b40 EFLAGS: 00010203
RAX: 0000000000000005 RBX: 0000000000000000 RCX: 0000000000000015
RDX: dffffc0000000000 RSI: ffffffff82cb9219 RDI: 00000000000000ad
RBP: ffff8880383da660 R08: 00000000ffffffff R09: fffffbfff1dbdcfe
R10: ffffffff8edee7f3 R11: 0000000000000000 R12: ffff8880383da580
R13: ffff8880383da738 R14: 000000000000001c R15: ffff8880383da6b2
FS:  000055558535c400(0000) GS:ffff8880d60ec000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000557b8da30300 CR3: 0000000028cd1000 CR4: 0000000000352ef0
----------------
Code disassembly (best guess):
   0:	e1 07                	loope  0x9
   2:	38 ca                	cmp    %cl,%dl
   4:	7f 08                	jg     0xe
   6:	84 d2                	test   %dl,%dl
   8:	0f 85 04 06 00 00    	jne    0x612
   e:	48 8d bb ad 00 00 00 	lea    0xad(%rbx),%rdi
  15:	0f b6 45 52          	movzbl 0x52(%rbp),%eax
  19:	48 ba 00 00 00 00 00 	movabs $0xdffffc0000000000,%rdx
  20:	fc ff df
  23:	48 89 f9             	mov    %rdi,%rcx
  26:	48 c1 e9 03          	shr    $0x3,%rcx
* 2a:	0f b6 14 11          	movzbl (%rcx,%rdx,1),%edx <-- trapping instruction
  2e:	48 89 f9             	mov    %rdi,%rcx
  31:	83 e1 07             	and    $0x7,%ecx
  34:	38 ca                	cmp    %cl,%dl
  36:	7f 08                	jg     0x40
  38:	84 d2                	test   %dl,%dl
  3a:	0f 85 c1 05 00 00    	jne    0x601


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