netdev - Re: bpf: undefined shift in __bpf_prog

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Message-ID: <20151204203522.GB64517@ast-mbp.thefacebook.com>
Date:	Fri, 4 Dec 2015 12:35:23 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	David Miller <davem@...emloft.net>,
	Alexei Starovoitov <ast@...nel.org>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Andrey Ryabinin <ryabinin.a.a@...il.com>
Subject: Re: bpf: undefined shift in __bpf_prog_run

On Fri, Dec 04, 2015 at 08:48:57PM +0100, Dmitry Vyukov wrote:
> 
> For example, a compiler can assume that result of left shift is larger
> or equal to first operand, which in turn can allow it to elide some
> bounds check in code, which in turn can lead to an exploit. I am not
> saying that this particular pattern is present in the code, what I
> want to say is that such undefined behaviors can lead to very
> unpredictable and unexpected consequences.

Within bpf it cannot.
shift is not used in any memory or bounds operations.
so reg <<= 1234 cannot be exploited.

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