netdev - Re: [PATCH net] net: filter: initialize A and X registers

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Message-Id: <20140423.125235.1748012189632512309.davem@davemloft.net>
Date:	Wed, 23 Apr 2014 12:52:35 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	dborkman@...hat.com
Cc:	ast@...mgrid.com, netdev@...r.kernel.org
Subject: Re: [PATCH net] net: filter: initialize A and X registers

From: Daniel Borkmann <dborkman@...hat.com>
Date: Wed, 23 Apr 2014 09:02:54 +0200

> On 04/23/2014 06:59 AM, Alexei Starovoitov wrote:
>> On Tue, Apr 22, 2014 at 8:57 PM, David Miller <davem@...emloft.net>
>> wrote:
>>> From: Alexei Starovoitov <ast@...mgrid.com>
>>> Date: Tue, 22 Apr 2014 20:18:57 -0700
>>>
>>>> exisiting BPF verifier allows uninitialized access to registers,
>>>> 'ret A' is considered to be a valid filter.
>>>> So initialize A and X to zero to prevent leaking kernel memory
>>>> In the future BPF verifier will be rejecting such filters
>>>>
>>>> Signed-off-by: Alexei Starovoitov <ast@...mgrid.com>
>>>
>>> Has the code always been like this?
> 
> I think it would be much cleaner to just prevent such filters that
> only contain a 'ret A', or 'ret X' w/o a load into X from attaching.

That choice had to be done two decades ago, not now.

We have to accept such filters, we always have.
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