BUG: unable to handle kernel NULL pointer dereference in generic_perform_write (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    5b8b9d0c Merge branch 'akpm' (patches from Andrew)
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13507b43e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=23c5a352e32a1944
dashboard link: https://syzkaller.appspot.com/bug?extid=bca9799bf129256190da
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1620c007e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=117f975de00000

The bug was bisected to:

commit 5f0663bb4a64f588f0a2dd6d1be68d40f9af0086
Author: Dan Williams <dan.j.williams@xxxxxxxxx>
Date:   Thu Dec 21 20:25:11 2017 +0000

    ext4, dax: introduce ext4_dax_aops

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=11046b5de00000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=13046b5de00000
console output: https://syzkaller.appspot.com/x/log.txt?x=15046b5de00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+bca9799bf129256190da@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 5f0663bb4a64 ("ext4, dax: introduce ext4_dax_aops")

BUG: kernel NULL pointer dereference, address: 0000000000000000
#PF: supervisor instruction fetch in kernel mode
#PF: error_code(0x0010) - not-present page
PGD 86de7067 P4D 86de7067 PUD 8eb7e067 PMD 0 
Oops: 0010 [#1] PREEMPT SMP KASAN
CPU: 0 PID: 8522 Comm: syz-executor124 Not tainted 5.6.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
RIP: 0010:0x0
Code: Bad RIP value.
RSP: 0018:ffffc90004707a38 EFLAGS: 00010246
RAX: ffffffff883cb0a0 RBX: 0000000000000000 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffff8880880c68e0 RDI: ffff888098a0c300
RBP: ffff8880880c68e0 R08: 0000000000000000 R09: ffffc90004707ac0
R10: ffff88808ddee607 R11: ffffed1011bbdcc0 R12: 0000000000000001
R13: 0000000000000000 R14: 0000000000000000 R15: ffffc90004707d18
FS:  00007ff4c6fcc700(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 0000000086de6000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 generic_perform_write+0x20a/0x4e0 mm/filemap.c:3302
 ext4_buffered_write_iter+0x1f7/0x450 fs/ext4/file.c:270
 ext4_file_write_iter+0x1ec/0x13f0 fs/ext4/file.c:642
 call_write_iter include/linux/fs.h:1907 [inline]
 new_sync_write+0x4a2/0x700 fs/read_write.c:484
 __vfs_write+0xc9/0x100 fs/read_write.c:497
 vfs_write+0x268/0x5d0 fs/read_write.c:559
 ksys_write+0x12d/0x250 fs/read_write.c:612
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:295
 entry_SYSCALL_64_after_hwframe+0x49/0xb3
RIP: 0033:0x44ecf9
Code: bd ca fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 0f 83 8b ca fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ff4c6fcbce8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000006e79e8 RCX: 000000000044ecf9
RDX: 0000000000000001 RSI: 0000000020000080 RDI: 0000000000000003
RBP: 00000000006e79e0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006e79ec
R13: 00007ffce52c26cf R14: 00007ff4c6fcc9c0 R15: 0000000000000000
Modules linked in:
CR2: 0000000000000000
---[ end trace a03cde52885aaa2a ]---
RIP: 0010:0x0
Code: Bad RIP value.
RSP: 0018:ffffc90004707a38 EFLAGS: 00010246
RAX: ffffffff883cb0a0 RBX: 0000000000000000 RCX: 0000000000000001
RDX: 0000000000000000 RSI: ffff8880880c68e0 RDI: ffff888098a0c300
RBP: ffff8880880c68e0 R08: 0000000000000000 R09: ffffc90004707ac0
R10: ffff88808ddee607 R11: ffffed1011bbdcc0 R12: 0000000000000001
R13: 0000000000000000 R14: 0000000000000000 R15: ffffc90004707d18
FS:  00007ff4c6fcc700(0000) GS:ffff8880ae600000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007ff4c6f89db8 CR3: 0000000086de6000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
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