Date: Mon, 12 Mar 2018 22:53:44 -0600 From: Tycho Andersen <tycho@...ho.ws> To: Salvatore Mesoraca <s.mesoraca16@...il.com> Cc: "Tobin C. Harding" <tobin@...orbit.com>, Kees Cook <keescook@...omium.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com> Subject: Re: VLA commit log 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 . > > > > ... > > > > : 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. > > > > 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. Cheers, Tycho
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