netdev - Re: [PATCH bpf] bpf: fix a rcu usage warning in bpf_prog_array_copy

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Date:   Thu, 16 Aug 2018 02:17:53 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Roman Gushchin <guro@...com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Yonghong Song <yhs@...com>, ast@...com, netdev@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH bpf] bpf: fix a rcu usage warning in
 bpf_prog_array_copy_core()

On 08/15/2018 02:08 AM, Roman Gushchin wrote:
> On Tue, Aug 14, 2018 at 04:59:45PM -0700, Alexei Starovoitov wrote:
>> On Tue, Aug 14, 2018 at 11:01:12AM -0700, Yonghong Song wrote:
>>> Commit 394e40a29788 ("bpf: extend bpf_prog_array to store pointers
>>> to the cgroup storage") refactored the bpf_prog_array_copy_core()
>>> to accommodate new structure bpf_prog_array_item which contains
>>> bpf_prog array itself.
>>>
>>> In the old code, we had
>>>    perf_event_query_prog_array():
>>>      mutex_lock(...)
>>>      bpf_prog_array_copy_call():
>>>        prog = rcu_dereference_check(array, 1)->progs
>>>        bpf_prog_array_copy_core(prog, ...)
>>>      mutex_unlock(...)
>>>
>>> With the above commit, we had
>>>    perf_event_query_prog_array():
>>>      mutex_lock(...)
>>>      bpf_prog_array_copy_call():
>>>        bpf_prog_array_copy_core(array, ...):
>>>          item = rcu_dereference(array)->items;
>>>          ...
>>>      mutex_unlock(...)
>>>
>>> The new code will trigger a lockdep rcu checking warning.
>>> The fix is to change rcu_dereference() to rcu_dereference_check()
>>> to prevent such a warning.
>>>
>>> Reported-by: syzbot+6e72317008eef84a216b@...kaller.appspotmail.com
>>> Fixes: 394e40a29788 ("bpf: extend bpf_prog_array to store pointers to the cgroup storage")
>>> Cc: Roman Gushchin <guro@...com>
>>> Signed-off-by: Yonghong Song <yhs@...com>

Applied to bpf, thanks Yonghong!