KASAN: null-ptr-deref Read in refcount_inc_not_zero

[syzbot <syzbot+6a35cd2d9559c909d570@syzkaller.appspotmail.com>]

Hello,

syzbot hit the following crash on upstream commit
285848b0f4074f04ab606f1e5dca296482033d54 (Sun Apr 22 04:20:48 2018 +0000)
Merge tag 'random_for_linus_stable' of  
git://git.kernel.org/pub/scm/linux/kernel/git/tytso/random
syzbot dashboard link:  
https://syzkaller.appspot.com/bug?extid=6a35cd2d9559c909d570

So far this crash happened 1772 times on upstream.
C reproducer: https://syzkaller.appspot.com/x/repro.c?id=5975533900791808
syzkaller reproducer:  
https://syzkaller.appspot.com/x/repro.syz?id=4813418829709312
Raw console output:  
https://syzkaller.appspot.com/x/log.txt?id=5008564225572864
Kernel config:  
https://syzkaller.appspot.com/x/.config?id=1808800213120130118
compiler: gcc (GCC) 8.0.1 20180413 (experimental)

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+6a35cd2d9559c909d570@syzkaller.appspotmail.com
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random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
random: sshd: uninitialized urandom read (32 bytes read)
==================================================================
BUG: KASAN: null-ptr-deref in atomic_read  
include/asm-generic/atomic-instrumented.h:21 [inline]
BUG: KASAN: null-ptr-deref in refcount_inc_not_zero+0x8f/0x2d0  
lib/refcount.c:120
Read of size 4 at addr 0000000000000004 by task syzkaller633288/4488

CPU: 0 PID: 4488 Comm: syzkaller633288 Not tainted 4.17.0-rc1+ #12
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_read+0x11/0x20 mm/kasan/kasan.c:272
  atomic_read include/asm-generic/atomic-instrumented.h:21 [inline]
  refcount_inc_not_zero+0x8f/0x2d0 lib/refcount.c:120
  refcount_inc+0x15/0x70 lib/refcount.c:153
  llc_sap_hold include/net/llc.h:116 [inline]
  llc_ui_release+0xba/0x2b0 net/llc/af_llc.c:207
  sock_release+0x96/0x1b0 net/socket.c:594
  sock_close+0x16/0x20 net/socket.c:1149
  __fput+0x34d/0x890 fs/file_table.c:209
  ____fput+0x15/0x20 fs/file_table.c:243
  task_work_run+0x1e4/0x290 kernel/task_work.c:113
  exit_task_work include/linux/task_work.h:22 [inline]
  do_exit+0x1aee/0x2730 kernel/exit.c:865
  do_group_exit+0x16f/0x430 kernel/exit.c:968
  __do_sys_exit_group kernel/exit.c:979 [inline]
  __se_sys_exit_group kernel/exit.c:977 [inline]
  __x64_sys_exit_group+0x3e/0x50 kernel/exit.c:977
  do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x43e878
RSP: 002b:00007ffd854075f8 EFLAGS: 00000246 ORIG_RAX: 00000000000000e7
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 000000000043e878
RDX: 0000000000000000 RSI: 000000000000003c RDI: 0000000000000000
RBP: 00000000004be220 R08: 00000000000000e7 R09: ffffffffffffffd0
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000001
R13: 00000000006cc160 R14: 0000000000000000 R15: 0000000000000000
==================================================================


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