linux-kernel - Re: VLAs and security

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Date:   Tue, 4 Sep 2018 18:22:18 +0000
From:   "Uecker, Martin" <Martin.Uecker@....uni-goettingen.de>
To:     "dvyukov@...gle.com" <dvyukov@...gle.com>
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

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.

Martin