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

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Date:   Thu, 15 Nov 2018 10:25:39 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        linux-arch@...r.kernel.org, Jacob Bramley <jacob.bramley@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-kernel@...r.kernel.org, Adam Wallis <awallis@...eaurora.org>,
        Kristina Martsenko <kristina.martsenko@....com>,
        gorcunov@...nvz.org,
        Ramana Radhakrishnan <ramana.radhakrishnan@....com>,
        Amit Kachhap <Amit.Kachhap@....com>,
        kvmarm@...ts.cs.columbia.edu, linux-arm-kernel@...ts.infradead.org,
        Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH v5 07/17] arm64: add basic pointer authentication support

On Wed, Nov 14, 2018 at 06:11:39PM +0000, Will Deacon wrote:

[...]

> On Fri, Oct 19, 2018 at 12:24:04PM +0100, Will Deacon 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.
> 
> I managed to speak to the CRIU developers at LPC. The good news is that
> their preference is for a ptrace()-based interface for getting and setting
> the keys, so the only prctl() operation we need is to set a random key
> (separately for A and B).

That's good if it works for them, and it seems the cleaner approach.

_If_ they run the new thread up to a checkpoint, restoring the memory
and doing all the setup that requires in-thread syscalls, then stop it
in ptrace to finally inject the regs, then it makes sense to set the
keys at that stop -- i.e., you set the keys atomically* with the final
setting of the thread's PC.

(* with respect to the target thread)

So long as you're confident they've understood the implications of
ptrauth for CRIU, I guess this can work.


(In other news, they will also need to do some work to support SVE, but
that's unrelated to ptrauth.)

Cheers
---Dave