linux-kernel - Re: randomize_kstack: To init or not to init?

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Date:   Thu, 9 Dec 2021 13:58:35 +0100
From:   Marco Elver <elver@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Potapenko <glider@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, linux-toolchains@...r.kernel.org
Subject: Re: randomize_kstack: To init or not to init?

On Thu, 9 Dec 2021 at 11:27, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote:
[...]
> > There are several options:
> >
> >       A. Make memset (and probably all other mem-transfer functions)
> >          noinstr compatible, if that is even possible. This only solves
> >          problem #2.
>
> While we can shut up objtool real easy, the bigger problem is that
> noinstr also excludes things like kprobes and breakpoints and other such
> goodness from being placed in the text.
>
> >       B. A workaround could be using a VLA with
> >          __attribute__((uninitialized)), but requires some restructuring
> >          to make sure the VLA remains in scope and other trickery to
> >          convince the compiler to not give up that stack space.
> >
> >       C. Introduce a new __builtin_alloca_uninitialized().
> >
> > I think #C would be the most robust solution, but means this would
> > remain as-is for a while.
> >
> > Preferences?
>
> I'm with you on C.

Seems simple enough, so I've sent https://reviews.llvm.org/D115440 --
either way, there needs to be support to not initialize alloca'd
memory. Let's see where we end up.