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

[Will Deacon]

Hi all,

On Fri, Oct 19, 2018 at 12:24:04PM +0100, Will Deacon wrote:
> On Fri, Oct 19, 2018 at 12:15:43PM +0100, Catalin Marinas wrote:
> > On Fri, Oct 05, 2018 at 09:47:44AM +0100, Kristina Martsenko wrote:
> > > diff --git a/arch/arm64/include/asm/pointer_auth.h b/arch/arm64/include/asm/pointer_auth.h
> > > new file mode 100644
> > > index 000000000000..2aefedc31d9e
> > > --- /dev/null
> > > +++ b/arch/arm64/include/asm/pointer_auth.h
> > > @@ -0,0 +1,63 @@
> > > +// SPDX-License-Identifier: GPL-2.0
> > > +#ifndef __ASM_POINTER_AUTH_H
> > > +#define __ASM_POINTER_AUTH_H
> > > +
> > > +#include <linux/random.h>
> > > +
> > > +#include <asm/cpufeature.h>
> > > +#include <asm/sysreg.h>
> > > +
> > > +#ifdef CONFIG_ARM64_PTR_AUTH
> > > +/*
> > > + * Each key is a 128-bit quantity which is split across a pair of 64-bit
> > > + * registers (Lo and Hi).
> > > + */
> > > +struct ptrauth_key {
> > > +	unsigned long lo, hi;
> > > +};
> > > +
> > > +/*
> > > + * We give each process its own instruction A key (APIAKey), which is shared by
> > > + * all threads. This is inherited upon fork(), and reinitialised upon exec*().
> > > + * All other keys are currently unused, with APIBKey, APDAKey, and APBAKey
> > > + * instructions behaving as NOPs.
> > > + */
> > 
> > I don't remember the past discussions but I assume the tools guys are ok
> > with a single key shared by multiple threads. Ramana, could you ack this
> > part, FTR?
> > 
> > (and it would help if someone from the Android and Chrome camps can
> > confirm)
> 
> 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).

Will