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From: "Uecker, Martin" <Martin.Uecker@med.uni-goettingen.de>
To: "dvyukov@google.com" <dvyukov@google.com>
Cc: "torvalds@linux-foundation.org" <torvalds@linux-foundation.org>,
	"keescook@chromium.org" <keescook@chromium.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: VLAs and security
Date: Tue, 4 Sep 2018 18:22:18 +0000
Message-ID: <1536085338.17991.1.camel@med.uni-goettingen.de> (raw)
In-Reply-To: <CACT4Y+bBPRfjpWFXoZmruuot37_KTemZ0PHUNrFfOPW5+6JD7w@mail.gmail.com>

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@med.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


  reply index

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-02  8:08 Uecker, Martin
2018-09-02 17:40 ` Kees Cook
2018-09-03  7:39   ` Uecker, Martin
2018-09-03 21:28     ` Linus Torvalds
2018-09-04  6:27       ` Uecker, Martin
2018-09-04  8:00         ` Dmitry Vyukov
2018-09-04 18:22           ` Uecker, Martin [this message]
2018-09-05  7:35             ` Dmitry Vyukov

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