linux-kernel - Re: [PATCH v11 00/11] x86: PIE support to extend KASLR randomization

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Message-ID: <CAJcbSZHBB1u2Vq0jZKsmd0UcRj=aichxTtbGvbWgf8-g8WPa7w@mail.gmail.com>
Date:   Wed, 4 Mar 2020 11:19:44 -0800
From:   Thomas Garnier <thgarnie@...omium.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Kristen Carlson Accardi <kristen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Juergen Gross <jgross@...e.com>,
        Thomas Hellstrom <thellstrom@...are.com>,
        "VMware, Inc." <pv-drivers@...are.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, Pavel Machek <pavel@....cz>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jiri Slaby <jslaby@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Cao jin <caoj.fnst@...fujitsu.com>,
        Allison Randal <allison@...utok.net>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        virtualization@...ts.linux-foundation.org,
        Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH v11 00/11] x86: PIE support to extend KASLR randomization

On Wed, Mar 4, 2020 at 10:45 AM H. Peter Anvin <hpa@...or.com> wrote:
>
> On 2020-03-04 10:21, Kees Cook wrote:
> > On Wed, Mar 04, 2020 at 10:21:36AM +0100, Peter Zijlstra wrote:
> >> But at what cost; it does unspeakable ugly to the asm. And didn't a
> >> kernel compiled with the extended PIE range produce a measurably slower
> >> kernel due to all the ugly?
> >
> > Was that true? I thought the final results were a wash and that earlier
> > benchmarks weren't accurate for some reason? I can't find the thread
> > now. Thomas, do you have numbers on that?

I have never seen a significant performance impact. Performance and
size is better on more recent versions of gcc as it has better
generation of PIE code (for example generation of switches).

> >
> > BTW, I totally agree that fgkaslr is the way to go in the future. I
> > am mostly arguing for this under the assumption that it doesn't
> > have meaningful performance impact and that it gains the kernel some
> > flexibility in the kinds of things it can do in the future. If the former
> > is not true, then I'd agree, the benefit needs to be more clear.
> >
>
> "Making the assembly really ugly" by itself is a reason not to do it, in my
> Not So Humble Opinion[TM]; but the reason the kernel and small memory models
> exist in the first place is because there is a nonzero performance impact of
> the small-PIC memory model. Having modules in separate regions would further
> add the cost of a GOT references all over the place (PLT is optional, useless
> and deprecated for eager binding) *plus* might introduce at least one new
> vector of attack: overwrite a random GOT slot, and just wait until it gets hit
> by whatever code path it happens to be in; the exact code path doesn't matter.
> From an kASLR perspective this is *very* bad, since you only need to guess the
> general region of a GOT rather than an exact address.

I agree that it would add GOT references and I can explore that more
in terms of performance impact and size. This patchset makes the GOT
readonly too so I don't think the attack vector applies.

>
> The huge memory model, required for arbitrary placement, has a very
> significant performance impact.

I assume you mean mcmodel=large, it doesn't use it. It uses -fPIE and
removes -mcmodel=kernel. It favors relative references whenever
possible.

>
> The assembly code is *very* different across memory models.
>
>         -hpa