linux-kernel - Re: [PATCH v5 07/17] arm64: add basic pointer authentication support

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Date:   Fri, 19 Oct 2018 16:49:48 +0100
From:   Will Deacon <will.deacon@....com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Kristina Martsenko <kristina.martsenko@....com>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        linux-arch <linux-arch@...r.kernel.org>,
        Andrew Jones <drjones@...hat.com>,
        Jacob Bramley <jacob.bramley@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Adam Wallis <awallis@...eaurora.org>,
        "Suzuki K . Poulose" <suzuki.poulose@....com>,
        Christoffer Dall <christoffer.dall@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Amit Kachhap <Amit.Kachhap@....com>,
        Dave P Martin <Dave.Martin@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: Re: [PATCH v5 07/17] arm64: add basic pointer authentication support

On Fri, Oct 19, 2018 at 08:36:45AM -0700, Kees Cook wrote:
> On Fri, Oct 19, 2018 at 4:24 AM, Will Deacon <will.deacon@....com> wrote:
> > FWIW: I think we should be entertaining a prctl() interface to use a new
> > key on a per-thread basis. Obviously, this would need to be used with care
> > (e.g. you'd fork(); use the prctl() and then you'd better not return from
> > the calling function!).
> >
> > Assuming we want this (Kees -- I was under the impression that everything in
> > Android would end up with the same key otherwise?), then the question is
> > do we want:
> >
> >   - prctl() get/set operations for the key, or
> >   - prctl() set_random_key operation, or
> >   - both of the above?
> >
> > Part of the answer to that may lie in the requirements of CRIU, where I
> > strongly suspect they need explicit get/set operations, although these
> > could be gated on CONFIG_CHECKPOINT_RESTORE=y.
> 
> Oh CRIU. Yikes. I'd like the get/set to be gated by the CONFIG, yes.
> No reason to allow explicit access to the key (and selected algo) if
> we don't have to.

Makes sense.

> As for per-thread or not, having a "pick a new key now" prctl() sounds
> good, but I'd like to have an eye toward having it just be "automatic"
> on clone().

I thought about that too, but we're out of clone() flags afaict and there's
no arch hook in there. We could add yet another clone syscall, but yuck (and
I reckon viro would kill us).

Or are you saying that we could infer the behaviour from the existing set
of flags?

Will