Date:   Tue, 9 Apr 2019 08:34:58 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     syzbot <syzbot+457d3e2ffbcf31aee5c0@...kaller.appspotmail.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        LKML <linux-kernel@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        John Fastabend <john.fastabend@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: KASAN: use-after-free Read in __dev_map_entry_free



On 04/09/2019 07:50 AM, Toke Høiland-Jørgensen wrote:
> Jesper Dangaard Brouer <brouer@...hat.com> writes:
> 
>> On Wed, 3 Apr 2019 20:59:24 -0700
>> Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
>>
>>> On Tue, Apr 2, 2019 at 1:03 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>>>>
>>>>
>>>>
>>>> On 06/20/2018 08:19 AM, syzbot wrote:  
>>>>> Hello,
>>>>>
>>>>> syzbot found the following crash on:
>>>>>
>>>>> HEAD commit:    f0dc7f9c6dd9 Merge git://git.kernel.org/pub/scm/linux/kern..
>>>>> git tree:       bpf-next
>>>>> console output: https://syzkaller.appspot.com/x/log.txt?x=15ad7d10400000
>>>>> kernel config:  https://syzkaller.appspot.com/x/.config?x=fa9c20c48788d1c1
>>>>> dashboard link: https://syzkaller.appspot.com/bug?extid=457d3e2ffbcf31aee5c0
>>>>> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
>>>>> syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1195225f800000
>>>>> C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=171a7ce4400000
>>>>>
>>>>> IMPORTANT: if you fix the bug, please add the following tag to the commit:
>>>>> Reported-by: syzbot+457d3e2ffbcf31aee5c0@...kaller.appspotmail.com
>>>>>
>>>>> ==================================================================
>>>>> BUG: KASAN: use-after-free in dev_map_flush_old kernel/bpf/devmap.c:365 [inline]
>>>>> BUG: KASAN: use-after-free in __dev_map_entry_free+0x2a8/0x300 kernel/bpf/devmap.c:379
>>>>> Read of size 8 at addr ffff8801b8da38c8 by task ksoftirqd/1/18
>>>>>
>>>>> CPU: 1 PID: 18 Comm: ksoftirqd/1 Not tainted 4.17.0+ #39
>>>>> 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
>>>>>  print_address_description+0x6c/0x20b mm/kasan/report.c:256
>>>>>  kasan_report_error mm/kasan/report.c:354 [inline]
>>>>>  kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412
>>>>>  __asan_report_load8_noabort+0x14/0x20 mm/kasan/report.c:433
>>>>>  dev_map_flush_old kernel/bpf/devmap.c:365 [inline]
>>>>>  __dev_map_entry_free+0x2a8/0x300 kernel/bpf/devmap.c:379
>>>>>  __rcu_reclaim kernel/rcu/rcu.h:178 [inline]
>>>>>  rcu_do_batch kernel/rcu/tree.c:2558 [inline]
>>>>>  invoke_rcu_callbacks kernel/rcu/tree.c:2818 [inline]
>>>>>  __rcu_process_callbacks kernel/rcu/tree.c:2785 [inline]
>>>>>  rcu_process_callbacks+0xe9d/0x1760 kernel/rcu/tree.c:2802
>>>>>  __do_softirq+0x2e0/0xaf5 kernel/softirq.c:284
>>>>>  run_ksoftirqd+0x86/0x100 kernel/softirq.c:645
>>>>>  smpboot_thread_fn+0x417/0x870 kernel/smpboot.c:164
>>>>>  kthread+0x345/0x410 kernel/kthread.c:240
>>>>>  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
>>>>>
>>>>> Allocated by task 6675:
>>>>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
>>>>>  set_track mm/kasan/kasan.c:460 [inline]
>>>>>  kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553
>>>>>  kmem_cache_alloc_trace+0x152/0x780 mm/slab.c:3620
>>>>>  kmalloc include/linux/slab.h:513 [inline]
>>>>>  kzalloc include/linux/slab.h:706 [inline]
>>>>>  dev_map_alloc+0x208/0x7f0 kernel/bpf/devmap.c:102
>>>>>  find_and_alloc_map kernel/bpf/syscall.c:129 [inline]
>>>>>  map_create+0x393/0x1010 kernel/bpf/syscall.c:453
>>>>>  __do_sys_bpf kernel/bpf/syscall.c:2351 [inline]
>>>>>  __se_sys_bpf kernel/bpf/syscall.c:2328 [inline]
>>>>>  __x64_sys_bpf+0x303/0x510 kernel/bpf/syscall.c:2328
>>>>>  do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:290
>>>>>  entry_SYSCALL_64_after_hwframe+0x49/0xbe
>>>>>
>>>>> Freed by task 26:
>>>>>  save_stack+0x43/0xd0 mm/kasan/kasan.c:448
>>>>>  set_track mm/kasan/kasan.c:460 [inline]
>>>>>  __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521
>>>>>  kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
>>>>>  __cache_free mm/slab.c:3498 [inline]
>>>>>  kfree+0xd9/0x260 mm/slab.c:3813
>>>>>  dev_map_free+0x4fa/0x670 kernel/bpf/devmap.c:191
>>>>>  bpf_map_free_deferred+0xba/0xf0 kernel/bpf/syscall.c:262
>>>>>  process_one_work+0xc64/0x1b70 kernel/workqueue.c:2153
>>>>>  worker_thread+0x181/0x13a0 kernel/workqueue.c:2296
>>>>>  kthread+0x345/0x410 kernel/kthread.c:240
>>>>>  ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412
>>>>>
>>>>> The buggy address belongs to the object at ffff8801b8da37c0
>>>>>  which belongs to the cache kmalloc-512 of size 512
>>>>> The buggy address is located 264 bytes inside of
>>>>>  512-byte region [ffff8801b8da37c0, ffff8801b8da39c0)
>>>>> The buggy address belongs to the page:
>>>>> page:ffffea0006e368c0 count:1 mapcount:0 mapping:ffff8801da800940 index:0xffff8801b8da3540
>>>>> flags: 0x2fffc0000000100(slab)
>>>>> raw: 02fffc0000000100 ffffea0007217b88 ffffea0006e30cc8 ffff8801da800940
>>>>> raw: ffff8801b8da3540 ffff8801b8da3040 0000000100000004 0000000000000000
>>>>> page dumped because: kasan: bad access detected
>>>>>
>>>>> Memory state around the buggy address:
>>>>>  ffff8801b8da3780: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
>>>>>  ffff8801b8da3800: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb  
>>>>>> ffff8801b8da3880: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb  
>>>>>                                               ^
>>>>>  ffff8801b8da3900: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
>>>>>  ffff8801b8da3980: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>>>>> ==================================================================
>>>>>
>>>>>
>>>>> ---
>>>>> This bug is generated by a bot. It may contain errors.
>>>>> See https://goo.gl/tpsmEJ for more information about syzbot.
>>>>> syzbot engineers can be reached at syzkaller@...glegroups.com.
>>>>>
>>>>> syzbot will keep track of this bug report. See:
>>>>> https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with syzbot.
>>>>> syzbot can test patches for this bug, for details see:
>>>>> https://goo.gl/tpsmEJ#testing-patches  
>>>>
>>>>
>>>> What about something like :
>>>>
>>>>
>>>>
>>>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>>>> index 191b79948424f4b21b7aa120abc03801264bf0a6..1e525d70f83354e451b738ffb8e42d83b5fa932f 100644
>>>> --- a/kernel/bpf/devmap.c
>>>> +++ b/kernel/bpf/devmap.c
>>>> @@ -164,6 +164,9 @@ static void dev_map_free(struct bpf_map *map)
>>>>         bpf_clear_redirect_map(map);
>>>>         synchronize_rcu();
>>>>
>>>> +       /* Make sure prior __dev_map_entry_free() have completed. */
>>>> +       rcu_barrier();
>>>> +  
>>>
>>> Eric, Thank you for looking at it. The fix makes sense to me.
>>>
>>> Jesper, thoughts?
>>
>> First I though it looked strange to have a synchronize_rcu() followed
>> by a rcu_barrier().  But I think the fix is actually correct.  We do
>> need the rcu_barrier() call as it states "Wait until all in-flight
>> call_rcu() callbacks complete".   I wonder if we also/still need the
>> synchronize_rcu(), but I think so as it functions as a memory-barrier
>> across all CPUs (as in bpf_clear_redirect_map we visit all CPUs and
>> clear any left-over per_cpu_ptr redirect_info).
>>
>> Acked-by: Jesper Dangaard Brouer <brouer@...hat.com>
> 
> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
> 
> Eric, are you going to submit a proper patch for this? :)

Yes I will, I am back from vacation.

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