KASAN: null-ptr-deref Read in xattr_getsecurity

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
fd40ffc72e2f74c7db61e400903e7d50a88bc0b0 (Mon Apr 9 18:36:05 2018 +0000)
selinux: fix missing dput() before selinuxfs unmount
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=9369930ca44f29e60e2d

So far this crash happened 41 times on upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5421128315043840
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=-771321277174894814
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+9369930ca44f29e60e2d@xxxxxxxxxxxxxxxxxxxxxxxxx
It will help syzbot understand when the bug is fixed. See footer for  
details.
If you forward the report, please keep this part and the footer.

R13: 0000000000000098 R14: 00000000006f3ee0 R15: 0000000000000002
==================================================================
BUG: KASAN: null-ptr-deref in memcpy include/linux/string.h:345 [inline]
BUG: KASAN: null-ptr-deref in xattr_getsecurity+0x18a/0x1f0 fs/xattr.c:251
Read of size 20 at addr 0000000000000000 by task syz-executor5/12111

CPU: 0 PID: 12111 Comm: syz-executor5 Not tainted 4.16.0+ #16
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+0x13b/0x2f5 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
 memcpy+0x23/0x50 mm/kasan/kasan.c:302
 memcpy include/linux/string.h:345 [inline]
 xattr_getsecurity+0x18a/0x1f0 fs/xattr.c:251
 vfs_getxattr+0xf2/0x160 fs/xattr.c:333
 getxattr+0x139/0x2c0 fs/xattr.c:540
 SYSC_fgetxattr fs/xattr.c:598 [inline]
 SyS_fgetxattr+0x109/0x190 fs/xattr.c:589
 do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x455259
RSP: 002b:00007f20bc46fc68 EFLAGS: 00000246 ORIG_RAX: 00000000000000c1
RAX: ffffffffffffffda RBX: 00007f20bc4706d4 RCX: 0000000000455259
RDX: 0000000020000280 RSI: 00000000200002c0 RDI: 0000000000000014
RBP: 000000000072bea0 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffffffffff27 R11: 0000000000000246 R12: 0000000000000015
R13: 0000000000000098 R14: 00000000006f3ee0 R15: 0000000000000002
==================================================================


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzkaller@xxxxxxxxxxxxxxxxx

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is  
merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug  
report.
Note: all commands must start from beginning of the line in the email body.