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

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Date:	Tue, 1 Dec 2015 10:38:07 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Daniel Borkmann <daniel@...earbox.net>,
	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 Mon, Nov 30, 2015 at 7:29 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Nov 30, 2015 at 01:02:55PM +0100, Daniel Borkmann wrote:
>> During own review but also reported by Dmitry's syzkaller [1] it has been
>> noticed that we trigger a heap out-of-bounds access on eBPF array maps
>> when updating elements. This happens with each map whose map->value_size
>> (specified during map creation time) is not multiple of 8 bytes.
> ...
>> In case of array_map_lookup_elem(), the verifier prevents eBPF programs
>> from accessing beyond map->value_size through check_map_access(). Also
>> from syscall side map_lookup_elem() only copies map->value_size back to
>> user, so nothing could leak.
>>
>>   [1] http://github.com/google/syzkaller
>>
>> Fixes: 28fbcfa08d8e ("bpf: add array type of eBPF maps")
>> Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
>> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
>
> 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.
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