syzbot hit the following crash on bpf-next commit
17dec0a949153d9ac00760ba2f5b78cb583e995f (Wed Apr 4 02:15:32 2018 +0000)
Merge branch 'userns-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/ebiederm/user-namespace syzbot dashboard link: https://syzkaller.appspot.com/bug?extid=7c449856228b63ac951e

So far this crash happened 2 times on bpf-next.
syzkaller reproducer: https://syzkaller.appspot.com/x/repro.syz?id=4514433808203776 Raw console output: https://syzkaller.appspot.com/x/log.txt?id=5570436679073792 Kernel config: https://syzkaller.appspot.com/x/.config?id=-2735707888269579554
compiler: gcc (GCC) 8.0.1 20180301 (experimental)

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Reported-by: syzbot+7c449856228b63ac9...@syzkaller.appspotmail.com
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IPVS: ftp: loaded support on port[0] = 21
BUG: KASAN: slab-out-of-bounds in perf_callchain_store include/linux/perf_event.h:1147 [inline] BUG: KASAN: slab-out-of-bounds in perf_callchain_user+0xe31/0xfe0 arch/x86/events/core.c:2485
Write of size 8 at addr ffff8801d87f2d40 by task udevd/2377

CPU: 0 PID: 2377 Comm: udevd Not tainted 4.16.0+ #3
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x1b9/0x29f lib/dump_stack.c:53
 print_address_description+0x6c/0x20b mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report.cold.7+0xac/0x2f5 mm/kasan/report.c:412
 __asan_report_store8_noabort+0x17/0x20 mm/kasan/report.c:438
 perf_callchain_store include/linux/perf_event.h:1147 [inline]
 perf_callchain_user+0xe31/0xfe0 arch/x86/events/core.c:2485
 get_perf_callchain+0x798/0xb20 kernel/events/callchain.c:227
 perf_callchain kernel/events/core.c:6354 [inline]
 perf_prepare_sample+0x123d/0x1900 kernel/events/core.c:6380
 __perf_event_output kernel/events/core.c:6494 [inline]
 perf_event_output_forward+0x10a/0x2b0 kernel/events/core.c:6512
 __perf_event_overflow+0x231/0x4b0 kernel/events/core.c:7748
 perf_swevent_overflow+0xad/0x150 kernel/events/core.c:7824
 perf_swevent_event+0x1f0/0x2e0 kernel/events/core.c:7857
 perf_tp_event+0x4da/0xc30 kernel/events/core.c:8280
 perf_trace_run_bpf_submit+0x23f/0x370 kernel/events/core.c:8254
 perf_trace_lock_acquire+0x4f1/0x980 include/trace/events/lock.h:13
 trace_lock_acquire include/trace/events/lock.h:13 [inline]
 lock_acquire+0x38e/0x520 kernel/locking/lockdep.c:3919
 finish_lock_switch kernel/sched/core.c:2602 [inline]
 finish_task_switch+0x1c2/0x820 kernel/sched/core.c:2701
 context_switch kernel/sched/core.c:2851 [inline]
 __schedule+0x80f/0x1e40 kernel/sched/core.c:3490
 schedule+0xef/0x430 kernel/sched/core.c:3549
 schedule_hrtimeout_range_clock+0x3c0/0x470 kernel/time/hrtimer.c:1883
 schedule_hrtimeout_range+0x2a/0x40 kernel/time/hrtimer.c:1940
 ep_poll+0xf2e/0x11d0 fs/eventpoll.c:1812
 do_epoll_wait+0x1b0/0x200 fs/eventpoll.c:2191
 SYSC_epoll_wait fs/eventpoll.c:2201 [inline]
 SyS_epoll_wait+0x2c/0x40 fs/eventpoll.c:2198
 do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287
RIP: 0033:0x7fe39adf3943
RSP: 002b:00007ffc13750708 EFLAGS: 00000246 ORIG_RAX: 00000000000000e8
RAX: ffffffffffffffda RBX: 00000000ffffffff RCX: 00007fe39adf3943
RDX: 0000000000000008 RSI: 00007ffc13750800 RDI: 000000000000000a
RBP: 0000000001b676c0 R08: 0000000000000000 R09: 00007fe39ae3ca60
R10: 00000000ffffffff R11: 0000000000000246 R12: 0000000000000a56
R13: 0000000000000000 R14: 0000000001b674a0 R15: 0000000001b4e250

Allocated by task 4544:
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:552
 __do_kmalloc_node mm/slab.c:3670 [inline]
 __kmalloc_node+0x47/0x70 mm/slab.c:3677
 kmalloc_node include/linux/slab.h:554 [inline]
 alloc_callchain_buffers kernel/events/callchain.c:91 [inline]
 get_callchain_buffers+0x31a/0x4b0 kernel/events/callchain.c:138
 perf_event_alloc.part.91+0x2274/0x30a0 kernel/events/core.c:10047
 perf_event_alloc kernel/events/core.c:10376 [inline]
 SYSC_perf_event_open+0xa8a/0x2fa0 kernel/events/core.c:10477
 SyS_perf_event_open+0x35/0x40 kernel/events/core.c:10366
 do_syscall_64+0x29e/0x9d0 arch/x86/entry/common.c:287

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8801d87f1c40
 which belongs to the cache kmalloc-8192 of size 8192
The buggy address is located 4352 bytes inside of
 8192-byte region [ffff8801d87f1c40, ffff8801d87f3c40)
The buggy address belongs to the page:
page:ffffea000761fc00 count:1 mapcount:0 mapping:ffff8801d87f1c40 index:0x0 compound_mapcount: 0
flags: 0x2fffc0000008100(slab|head)
raw: 02fffc0000008100 ffff8801d87f1c40 0000000000000000 0000000100000001
raw: ffffea0007612020 ffffea0006b0ff20 ffff8801dac02080 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8801d87f2c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8801d87f2c80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff8801d87f2d00: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
 ffff8801d87f2d80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff8801d87f2e00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc

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