KASAN: null-ptr-deref Write in simple_write_to_buffer

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    c1c07416cdd4 Merge tag 'kbuild-fixes-v4.17' of git://git.k..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17912237800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=5a1dc06635c10d27
dashboard link: https://syzkaller.appspot.com/bug?extid=ae590932da6e45d6564d
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=108c8e07800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=142f907b800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+ae590932da6e45d6564d@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)
Restarting kernel threads ... done.
==================================================================
BUG: KASAN: null-ptr-deref in _copy_from_user+0x100/0x150 lib/usercopy.c:12
Write of size 32 at addr 0000000000000020 by task syz-executor101/4498

CPU: 1 PID: 4498 Comm: syz-executor101 Not tainted 4.17.0-rc3+ #34
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+0x1b9/0x294 lib/dump_stack.c:113
 kasan_report_error mm/kasan/report.c:352 [inline]
 kasan_report.cold.7+0x6d/0x2fe mm/kasan/report.c:412
 check_memory_region_inline mm/kasan/kasan.c:260 [inline]
 check_memory_region+0x13e/0x1b0 mm/kasan/kasan.c:267
 kasan_check_write+0x14/0x20 mm/kasan/kasan.c:278
 _copy_from_user+0x100/0x150 lib/usercopy.c:12
 copy_from_user include/linux/uaccess.h:147 [inline]
 simple_write_to_buffer+0xaf/0x130 fs/libfs.c:669
 snapshot_write+0x16d/0x270 kernel/power/user.c:189
 __vfs_write+0x10b/0x960 fs/read_write.c:485
 vfs_write+0x1f8/0x560 fs/read_write.c:549
 ksys_write+0xf9/0x250 fs/read_write.c:598
 __do_sys_write fs/read_write.c:610 [inline]
 __se_sys_write fs/read_write.c:607 [inline]
 __x64_sys_write+0x73/0xb0 fs/read_write.c:607
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4455a9
RSP: 002b:00007f973a88ada8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000006dac24 RCX: 00000000004455a9
RDX: 0000000000000020 RSI: 0000000020000040 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dac20
R13: 616e732f7665642f R14: 00007f973a88b9c0 R15: 0000000000000003
==================================================================


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