KASAN: null-ptr-deref Write in xdp_umem_unaccount_pages

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    1c8c5a9d38f6 Merge git://git.kernel.org/pub/scm/linux/kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=13a72bdf800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=4f1acdf888c9d4e9
dashboard link: https://syzkaller.appspot.com/bug?extid=979217770b09ebf5c407
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=12aca2af800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=161d4ddf800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+979217770b09ebf5c407@xxxxxxxxxxxxxxxxxxxxxxxxx

RDX: 0000000000000004 RSI: 000000000000011b RDI: 0000000000000004
RBP: 00000000006cb018 R08: 0000000000000018 R09: 00007fffc4750032
R10: 0000000020000040 R11: 0000000000000246 R12: 0000000000000005
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
==================================================================
BUG: KASAN: null-ptr-deref in atomic64_sub  
include/asm-generic/atomic-instrumented.h:144 [inline]
BUG: KASAN: null-ptr-deref in atomic_long_sub  
include/asm-generic/atomic-long.h:199 [inline]
BUG: KASAN: null-ptr-deref in xdp_umem_unaccount_pages.isra.4+0x3d/0x80  
net/xdp/xdp_umem.c:135
Write of size 8 at addr 0000000000000060 by task syz-executor246/4527

CPU: 1 PID: 4527 Comm: syz-executor246 Not tainted 4.17.0+ #89
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
 atomic64_sub include/asm-generic/atomic-instrumented.h:144 [inline]
 atomic_long_sub include/asm-generic/atomic-long.h:199 [inline]
 xdp_umem_unaccount_pages.isra.4+0x3d/0x80 net/xdp/xdp_umem.c:135
 xdp_umem_reg net/xdp/xdp_umem.c:334 [inline]
 xdp_umem_create+0xd6c/0x10f0 net/xdp/xdp_umem.c:349
 xsk_setsockopt+0x443/0x550 net/xdp/xsk.c:531
 __sys_setsockopt+0x1bd/0x390 net/socket.c:1935
 __do_sys_setsockopt net/socket.c:1946 [inline]
 __se_sys_setsockopt net/socket.c:1943 [inline]
 __x64_sys_setsockopt+0xbe/0x150 net/socket.c:1943
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440549
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 5b 14 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fffc475d008 EFLAGS: 00000246 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 0000000000440549
RDX: 0000000000000004 RSI: 000000000000011b RDI: 0000000000000004
RBP: 00000000006cb018 R08: 0000000000000018 R09: 00007fffc4750032
R10: 0000000020000040 R11: 0000000000000246 R12: 0000000000000005
R13: ffffffffffffffff R14: 0000000000000000 R15: 0000000000000000
==================================================================


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