linux-kernel - Re: bpf: undefined shift in __bpf_prog

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Message-Id: <20151204.163753.594151588498690658.davem@davemloft.net>
Date:	Fri, 04 Dec 2015 16:37:53 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	alexei.starovoitov@...il.com
Cc:	dvyukov@...gle.com, ast@...nel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, syzkaller@...glegroups.com,
	kcc@...gle.com, glider@...gle.com, sasha.levin@...cle.com,
	edumazet@...gle.com, ryabinin.a.a@...il.com
Subject: Re: bpf: undefined shift in __bpf_prog_run

From: Alexei Starovoitov <alexei.starovoitov@...il.com>
Date: Fri, 4 Dec 2015 12:35:23 -0800

> 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.

+1
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