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Message-ID: <861efe7a-e51f-e004-5979-aef0ed9ff1fe@embeddedor.com>
Date: Mon, 12 Mar 2018 00:38:04 -0500
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: "Tobin C. Harding" <tobin@...orbit.com>, Kees Cook
<keescook@...omium.org>, Tycho Andersen <tycho@...ho.ws>
Cc: kernel-hardening@...ts.openwall.com
Subject: Re: VLA commit log
On 03/12/2018 12:26 AM, Tobin C. Harding wrote:
> Hi,
>
> I got some push back on the commit log we have all started to use
> (copying Kees' initial commit log). If we are going to do hundreds of
> these patches should we write a perfectly correct commit log that can be
> included as the start of the 'why' of each VLA removal patch? Here is
> my attempt, I am quite bad at writing commit logs so would love someone
> to fix it up.
>
The same thing happened to me once and then I wrote this:
In preparation to enabling -Wvla, remove VLA and replace it
with a fixed-length array instead.
From a security viewpoint, the use of Variable Length Arrays can be
a vector for stack overflow attacks. Also, in general, as the code
evolves it is easy to lose track of how big a VLA can get. Thus, we
can end up having segfaults that are hard to debug.
Also, fixed as part of the directive to remove all VLAs from
the kernel: https://lkml.org/lkml/2018/3/7/621
The maintainer lived happily ever after. :)
--
Gustavo
> Kernel stack size is limited. Variable Length Arrays (VLA) open the
> kernel up to stack abuse in a couple of ways;
>
> 1. If the variable can be controlled by an attacker.
> 2. Not having the size of the stack right there in plain site makes it
> harder to maintain the code base because changes in one place can effect
> the stack in another place (i.e in another function).
>
> It would be nice to be able to build the kernel with -Wvla. There has
> been some consensus on this already [1].
>
> ...
>
> [1]: https://lkml.org/lkml/2018/3/7/621
>
> The '...' would of course be different for each patch. In case you
> missed it here is the catalyst for this email
>
> On Mon, Mar 12, 2018 at 03:49:40PM +1100, Tobin C. Harding wrote:
> > The kernel would like to have all stack VLA usage removed[1].
>
> Can you please stop writing this? The Linux kernel isn't
> sentient; it doesn't "like" anything. You need to explain why
> *you* (and other people) believe these changes should be made.
>
>
> Perhaps we should add a summary of all the gcc discussion i.e why const
> variables still cause gcc to emit a VLA warning.
>
>
> thanks,
> Tobin.
>
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