linux-kernel - Re: VLAs and security

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Date:   Wed, 5 Sep 2018 09:35:09 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     "Uecker, Martin" <Martin.Uecker@....uni-goettingen.de>
Cc:     "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "keescook@...omium.org" <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: VLAs and security

On Tue, Sep 4, 2018 at 8:22 PM, Uecker, Martin
<Martin.Uecker@....uni-goettingen.de> wrote:
> Am Dienstag, den 04.09.2018, 10:00 +0200 schrieb Dmitry Vyukov:
>> On Tue, Sep 4, 2018 at 8:27 AM, Uecker, Martin
>> <Martin.Uecker@....uni-goettingen.de> wrote:
>> > Am Montag, den 03.09.2018, 14:28 -0700 schrieb Linus Torvalds:
>
>
> Hi Dmitry,
>
>> Compiler and KASAN should still be able to do checking against the
>> static array size.
>
> ...and it is probably true that this is currently more useful
> than the limited amount of checking compilers can do for VLAs.
>
>> If you mean that there is some smaller dynamic logical bound n (<N)
>> and we are not supposed to use memory beyond that,
>
> Yes, this is what I mean.
>
> My concern is that this dynamic bound is valuable information
> which was put there by programmers by hand and I believe that
> this information can not always be recovered automatically
> by static analysis. So by removing VLAs from the source tree,
> this information ist lost.
>
>> then KMSAN [1] can
>> detect uses of the uninitialized part of the array. So we have some
>> coverage on the checking side too.
>>
>> [1] https://github.com/google/kmsan#kmsan-kernelmemorysanitizer
>
> But detecting reads of uninitialized parts can detect only some
> of the errors which could be detected with precise bounds.
> It can not detect out-of-bounds writes (which still fall into
> the larger fixed-size array) and it does not detect out-of-bounds
> reads (which still fall into the larger fixed-size array) if
> the larger fixed-size array was completely initialized
> before for some reason.

Well, I agree maybe it harms checking ability to some degree. But it's
always tradeoffs all way down. And in the end nothing can safe from
logical bugs (well, except for unit tests). With VLA one give a right
bound but the use wrong bytes in the buffer, or simply give a wrong
bound.