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Date: Tue, 13 Mar 2018 16:46:29 +1100
From: "Tobin C. Harding" <tobin@...orbit.com>
To: Tycho Andersen <tycho@...ho.ws>
Cc: Salvatore Mesoraca <s.mesoraca16@...il.com>,
	Kees Cook <keescook@...omium.org>,
	Kernel Hardening <kernel-hardening@...ts.openwall.com>
Subject: Re: VLA commit log

On Mon, Mar 12, 2018 at 10:53:44PM -0600, Tycho Andersen wrote:
> On Mon, Mar 12, 2018 at 11:28:19AM +0100, Salvatore Mesoraca wrote:
> > 2018-03-12 6:26 GMT+01:00 Tobin C. Harding <tobin@...orbit.com>:
> > > 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.
> > >
> > >     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.
> > 
> > Maybe it will be useful to update the doc (e.g.
> > Documentation/process/coding-style.rst or a new
> > Documentation/process/vla-considered-harmful.rst) with an extensive
> > explanation of why VLAs shouldn't be used.
> > And then we can just refer to that.
> 
> This seems like a great idea. Perhaps we can combine Kees' recent
> reply + a link to the original Linus mail into something? There's also
> a similar thread from about four months ago when I originally started
> looking at this that we could grab stuff from.

We should include an explanation of why the warning is misleading and
all the stuff about the front end and back end of the compiler and
which end knows about vla's


	Tobin

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