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

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Date:	Fri, 4 Dec 2015 20:48:57 +0100
From:	Dmitry Vyukov <dvyukov@...gle.com>
To:	David Miller <davem@...emloft.net>
Cc:	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	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 4, 2015 at 8:26 PM, David Miller <davem@...emloft.net> wrote:
> From: Alexei Starovoitov <alexei.starovoitov@...il.com>
> Date: Fri, 4 Dec 2015 11:10:15 -0800
>
>> just don't generate random bpf programs with such shifts.
>
> Agreed, it is exactly the same as if the compiler emitted real cpu
> shift instructions with undefined behavior.
>
> The creator of the BPF code in question is what should be fixed.


There is another magical gcc flag enabled in kernel that alleviates
this undefined behavior? Or we are just assuming that C compilers are
still dump translators to machine code without analysis and
optimizations?

C standard per-se is pretty clear on this code:

6.5.7/3
The integer promotions are performed on each of the operands. The type
of the result is that of the promoted left operand. If the value of
the right operand is negative or is greater than or equal to the width
of the promoted left operand, the behavior is undefined.

3.4.3
undefined behavior
1 behavior, upon use of a nonportable or erroneous program construct
or of erroneous data, for which this International Standard imposes no
requirements
2 NOTE Possible undefined behavior ranges from ignoring the situation
completely with unpredictable results, to behaving during translation
or program execution in a documented manner characteristic of the
environment (with or without the issuance of a diagnostic message), to
terminating a translation or execution


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.
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