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

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Message-ID: <20151130182946.GB30762@ast-mbp.thefacebook.com>
Date:	Mon, 30 Nov 2015 10:29:46 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Daniel Borkmann <daniel@...earbox.net>
Cc:	davem@...emloft.net, ast@...nel.org, dvyukov@...gle.com,
	kcc@...gle.com, glider@...gle.com, edumazet@...gle.com,
	sasha.levin@...cle.com, syzkaller@...glegroups.com,
	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 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.

Daniel, thanks for the fix. Commit log is all good.
Acked-by: Alexei Starovoitov <ast@...nel.org>

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