BUG: unable to handle kernel NULL pointer dereference in ext4_file_read_iter

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: e195ca6cb6f2 Merge branch 'for-linus' of git://git.kernel...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=145a3093400000
kernel config: https://syzkaller.appspot.com/x/.config?x=73e2bc0cb6463446
dashboard link: https://syzkaller.appspot.com/bug?extid=471c3a979e0eb9d53aec
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
CC: [adilger...@dilger.ca linux...@vger.kernel.org
linux-...@vger.kernel.org ty...@mit.edu]

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+471c3a...@syzkaller.appspotmail.com

EXT4-fs (sda1): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
EXT4-fs (sda1): warning: refusing change of dax flag with busy inodes while
remounting
audit: type=1800 audit(1543120282.406:110): pid=11494 uid=0 auid=4294967295
ses=4294967295 subj=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
op=collect_data cause=failed(directio) comm="syz-executor1" name="file0"
dev="sda1" ino=16541 res=0
EXT4-fs (sda1): re-mounted. Opts: dax,
BUG: unable to handle kernel NULL pointer dereference at 0000000000000000
PGD 1c066d067 P4D 1c066d067 PUD 1a5fe2067 PMD 0
Oops: 0010 [#1] PREEMPT SMP KASAN
audit: type=1804 audit(1543120282.446:111): pid=11491 uid=0 auid=4294967295
ses=4294967295 subj=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
op=invalid_pcr cause=open_writers comm="syz-executor2"
name="/root/syzkaller-testdir007062170/syzkaller.pNOrGl/148/file0"
dev="sda1" ino=16542 res=1
CPU: 0 PID: 11494 Comm: syz-executor1 Not tainted 4.20.0-rc3+ #128
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010: (null)
Code: Bad RIP value.
RSP: 0018:ffff8881a310f098 EFLAGS: 00010246
RAX: 0000000000000000 RBX: dffffc0000000000 RCX: ffffc90007e8a000
RDX: 0000000000000b98 RSI: ffffea0006ef8f40 RDI: ffff8881c6923d40
RBP: ffff8881a310f890 R08: ffff88817b81e640 R09: fffff94000ddf1ee
R10: fffff94000ddf1ee R11: ffffea0006ef8f77 R12: ffffea0006ef8f48
R13: ffffffff883a75c0 R14: 0000000000000000 R15: ffffea0006ef8f40
FS: 00007f4ebf9d4700(0000) GS:ffff8881dae00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: ffffffffffffffd6 CR3: 00000001b9c3d000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
ext4_file_read_iter+0x18b/0x3c0 fs/ext4/file.c:77
call_read_iter include/linux/fs.h:1851 [inline]
generic_file_splice_read+0x5a2/0x9a0 fs/splice.c:308
do_splice_to+0x12e/0x190 fs/splice.c:880
splice_direct_to_actor+0x270/0x8f0 fs/splice.c:952
do_splice_direct+0x2d4/0x420 fs/splice.c:1061
do_sendfile+0x62a/0xe20 fs/read_write.c:1439
__do_sys_sendfile64 fs/read_write.c:1500 [inline]
__se_sys_sendfile64 fs/read_write.c:1486 [inline]
__x64_sys_sendfile64+0x1fd/0x250 fs/read_write.c:1486
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457569
Code: fd b3 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 cb b3 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f4ebf9d3c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000000457569
RDX: 0000000000000000 RSI: 0000000000000005 RDI: 0000000000000004
RBP: 000000000072bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000010000 R11: 0000000000000246 R12: 00007f4ebf9d46d4
R13: 00000000004c3c2a R14: 00000000004d63d0 R15: 00000000ffffffff
Modules linked in:
CR2: 0000000000000000
kobject: 'loop3' (00000000762fc872): kobject_uevent_env
kobject: 'loop3' (00000000762fc872): fill_kobj_path: path
= '/devices/virtual/block/loop3'
---[ end trace 0137b516ef583247 ]---
RIP: 0010: (null)
Code: Bad RIP value.
RSP: 0018:ffff8881a310f098 EFLAGS: 00010246
RAX: 0000000000000000 RBX: dffffc0000000000 RCX: ffffc90007e8a000
RDX: 0000000000000b98 RSI: ffffea0006ef8f40 RDI: ffff8881c6923d40
RBP: ffff8881a310f890 R08: ffff88817b81e640 R09: fffff94000ddf1ee
kobject: 'loop4' (00000000aea6cd3d): kobject_uevent_env
kobject: 'loop4' (00000000aea6cd3d): fill_kobj_path: path
= '/devices/virtual/block/loop4'
R10: fffff94000ddf1ee R11: ffffea0006ef8f77 R12: ffffea0006ef8f48
R13: ffffffff883a75c0 R14: 0000000000000000 R15: ffffea0006ef8f40
FS: 00007f4ebf9d4700(0000) GS:ffff8881daf00000(0000) knlGS:0000000000000000
kauditd_printk_skb: 2 callbacks suppressed
audit: type=1800 audit(1543120283.306:114): pid=11510 uid=0 auid=4294967295
ses=4294967295 subj=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
op=collect_data cause=failed(directio) comm="syz-executor2" name="file0"
dev="sda1" ino=16543 res=0
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
kobject: 'loop2' (000000004bb50341): kobject_uevent_env
audit: type=1804 audit(1543120283.356:115): pid=11510 uid=0 auid=4294967295
ses=4294967295 subj=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
op=invalid_pcr cause=open_writers comm="syz-executor2"
name="/root/syzkaller-testdir007062170/syzkaller.pNOrGl/149/file0"
dev="sda1" ino=16543 res=1
CR2: 0000001b30735000 CR3: 00000001b9c3d000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
kobject: 'loop2' (000000004bb50341): fill_kobj_path: path
= '/devices/virtual/block/loop2'
kobject: 'loop3' (00000000762fc872): kobject_uevent_env
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
audit: type=1804 audit(1543120283.466:116): pid=11519 uid=0 auid=4294967295
ses=4294967295 subj=unconfined_u:system_r:insmod_t:s0-s0:c0.c1023
op=invalid_pcr cause=open_writers comm="syz-executor2"
name="/root/syzkaller-testdir007062170/syzkaller.pNOrGl/149/file0"
dev="sda1" ino=16543 res=1


---
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[Dmitry Vyukov]

Willem, you said this looks like a dup of "WARNING in csum_and_copy_to_iter":
https://syzkaller.appspot.com/bug?extid=2053e00f81c4174b4206
We can mark it as dup now. But are you sure? That bug splices into dup
socket and causes a warning and this one splices into ext4 and causes
a NULL deref.

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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.