KASAN: null-ptr-deref Write in copy_namespaces

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 2882b06ddaa5 Merge branch 'ravb-Duplex-handling-update-V3'
git tree: net-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1568a783400000
kernel config: https://syzkaller.appspot.com/x/.config?x=c36a72af2123e78a
dashboard link: https://syzkaller.appspot.com/bug?extid=3bfb7eaa7e81ecf1f603
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
CC: [ak...@osdl.org c...@fr.ibm.com ebie...@xmission.com
linux-...@vger.kernel.org serge....@canonical.com se...@us.ibm.com]

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+3bfb7e...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: null-ptr-deref in atomic_inc
include/asm-generic/atomic-instrumented.h:109 [inline]
BUG: KASAN: null-ptr-deref in get_nsproxy include/linux/nsproxy.h:85
[inline]
BUG: KASAN: null-ptr-deref in copy_namespaces+0x250/0x4d0
kernel/nsproxy.c:147
Write of size 4 at addr 0000000000000000 by task syz-executor3/6111

CPU: 1 PID: 6111 Comm: syz-executor3 Not tainted 4.20.0-rc3+ #311
list_add corruption. next->prev should be prev (ffff8881dae2d798), but was
ffffffff89f9e340. (next=ffff88819001a6b0).
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+0x244/0x39d lib/dump_stack.c:113
kasan_report_error mm/kasan/report.c:352 [inline]
kasan_report.cold.8+0x6d/0x309 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
atomic_inc include/asm-generic/atomic-instrumented.h:109 [inline]
get_nsproxy include/linux/nsproxy.h:85 [inline]
copy_namespaces+0x250/0x4d0 kernel/nsproxy.c:147
copy_process+0x4cd5/0x87a0 kernel/fork.c:1920
_do_fork+0x1cb/0x11d0 kernel/fork.c:2216
__do_sys_clone kernel/fork.c:2323 [inline]
__se_sys_clone kernel/fork.c:2317 [inline]
__x64_sys_clone+0xbf/0x150 kernel/fork.c:2317
do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x455b1a
Code: f7 d8 64 89 04 25 d4 02 00 00 64 4c 8b 0c 25 10 00 00 00 31 d2 4d 8d
91 d0 02 00 00 31 f6 bf 11 00 20 01 b8 38 00 00 00 0f 05 <48> 3d 00 f0 ff
ff 0f 87 f5 00 00 00 85 c0 41 89 c5 0f 85 fc 00 00
RSP: 002b:00007fff520d55c0 EFLAGS: 00000246 ORIG_RAX: 0000000000000038
RAX: ffffffffffffffda RBX: 00007fff520d55c0 RCX: 0000000000455b1a
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
RBP: 00007fff520d5600 R08: 0000000000000001 R09: 0000000002453940
R10: 0000000002453c10 R11: 0000000000000246 R12: 0000000000000001
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000003
==================================================================
------------[ cut here ]------------
kernel BUG at lib/list_debug.c:25!


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[syzbot]

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