Date: Fri, 4 Dec 2015 12:35:23 -0800
From: Alexei Starovoitov <alexei.starovoitov@gmail.com>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: David Miller <davem@davemloft.net>, Alexei Starovoitov <ast@kernel.org>,
        netdev <netdev@vger.kernel.org>, LKML <linux-kernel@vger.kernel.org>,
        syzkaller <syzkaller@googlegroups.com>,
        Kostya Serebryany <kcc@google.com>,
        Alexander Potapenko <glider@google.com>,
        Sasha Levin <sasha.levin@oracle.com>,
        Eric Dumazet <edumazet@google.com>,
        Andrey Ryabinin <ryabinin.a.a@gmail.com>
Subject: Re: bpf: undefined shift in __bpf_prog_run
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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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