netdev - Re: [PATCH net] bpf, array: fix heap out-of-bounds access when updating elements

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Date:	Tue, 01 Dec 2015 11:30:01 +0100
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Dmitry Vyukov <dvyukov@...gle.com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:	David Miller <davem@...emloft.net>,
	Alexei Starovoitov <ast@...nel.org>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	syzkaller <syzkaller@...glegroups.com>,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH net] bpf, array: fix heap out-of-bounds access when updating
 elements

On 12/01/2015 10:38 AM, Dmitry Vyukov wrote:
> On Mon, Nov 30, 2015 at 7:29 PM, Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
...
>> Dmitry, thanks a lot for applying syzkaller to bpf. The issues
>> got cought much sooner than they would have been discovered otherwise.
>> Looks like the fuzzing has limited dependency chains described
>> in sys/sys.txt. Can they be improved into doing something like:
>> single call to map_create followed by many calls to update to
>> stress oom ? I did it manually so far without kasan.
>
> Hi Alexei,
>
> Please elaborate.
> sys.txt describes signatures of syscalls. Based on that syzkaller
> generates programs that can contain a map_create call followed by
> multiple map update calls. Though, it won't generate millions of
> update calls on a single map, because it directly conflicts with the
> idea of coverage guided fuzzing. For OOMs I guess you want to try
> kmalloc fault injection.

Wrt dependency chains, I believe what is meant is that there are some
options that could only be covered by the fuzzer after having succeeded
a couple of dependencies first.

Perhaps not directly related to BPF, but f.e. some things can only be
reached after having a session established, like the transfer of fds via
SCM_RIGHTS, options that are being fuzzed while having a tcp/udp/sctp/
netlink/etc session established and such.

Perhaps in BPF case, f.e. updating of a program array, which itself would
require some semi-autogenerated program to get loaded first. (The latter
is already a different beast by itself wrt testing the verifier, though.)

Cheers,
Daniel
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