WARNING in generic_make_request_checks

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    60f5a2173632 Merge tag 'usercopy-fix-v4.18-rc8' of git://g..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=15e200e2400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2dc0cd7c2eefb46f
dashboard link: https://syzkaller.appspot.com/bug?extid=21cfe1f803e0e158acf1
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=17b87bfc400000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=117ccc8c400000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+21cfe1f803e0e158acf1@xxxxxxxxxxxxxxxxxxxxxxxxx

random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
------------[ cut here ]------------
generic_make_request: Trying to write to read-only block-device loop0  
(partno 0)
WARNING: CPU: 1 PID: 4396 at block/blk-core.c:2161 bio_check_ro  
block/blk-core.c:2158 [inline]
WARNING: CPU: 1 PID: 4396 at block/blk-core.c:2161  
generic_make_request_checks+0x1618/0x24a0 block/blk-core.c:2263
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 4396 Comm: syz-executor533 Not tainted 4.18.0-rc7+ #177
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
 panic+0x238/0x4e7 kernel/panic.c:184
 __warn.cold.8+0x163/0x1ba kernel/panic.c:536
 report_bug+0x252/0x2d0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 do_error_trap+0x1fc/0x4d0 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:bio_check_ro block/blk-core.c:2158 [inline]
RIP: 0010:generic_make_request_checks+0x1618/0x24a0 block/blk-core.c:2263
Code: 8d bd 78 ff ff ff 45 8b b6 d4 04 00 00 48 89 df 4c 89 fe e8 ea 54 07  
00 44 89 f2 48 c7 c7 60 16 39 87 48 89 c6 e8 68 30 ff fd <0f> 0b e9 56 ed  
ff ff 48 8b bd 50 fd ff ff e8 75 f3 71 fe e9 ca ea
RSP: 0018:ffff8801acc8f640 EFLAGS: 00010282
RAX: 0000000000000000 RBX: ffff8801ae5e0400 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff81632481 RDI: 0000000000000001
RBP: ffff8801acc8f940 R08: ffff8801ace94500 R09: ffffed003b624fc0
R10: ffffed003b624fc0 R11: ffff8801db127e07 R12: ffff8801d3083140
R13: ffff8801ae5e0408 R14: 0000000000000000 R15: ffff8801acc8f8b8
 generic_make_request+0x2aa/0x1800 block/blk-core.c:2387
 submit_bio+0xba/0x460 block/blk-core.c:2552
 submit_bio_wait+0x134/0x1e0 block/bio.c:1004
 blkdev_issue_flush+0x204/0x300 block/blk-flush.c:553
 blkdev_fsync+0x93/0xd0 fs/block_dev.c:633
 vfs_fsync_range+0x140/0x220 fs/sync.c:197
 vfs_fsync fs/sync.c:211 [inline]
 do_fsync+0x52/0xa0 fs/sync.c:221
 __do_sys_fsync fs/sync.c:229 [inline]
 __se_sys_fsync fs/sync.c:227 [inline]
 __x64_sys_fsync+0x33/0x40 fs/sync.c:227
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x444999
Code: e8 ec b2 02 00 48 83 c4 18 c3 0f 1f 80 00 00 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 0f 83 bb d4 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffdc1fbd458 EFLAGS: 00000213 ORIG_RAX: 000000000000004a
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000444999
RDX: 0000000000401430 RSI: 0000000000000000 RDI: 0000000000000003
RBP: 0000000000000000 R08: 00000000021eb880 R09: 00000000000000c2
R10: 000000000000000f R11: 0000000000000213 R12: 00000000000074ed
R13: 0000000000402370 R14: 0000000000000000 R15: 0000000000000000
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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