kernel-hardening - Re: [PATCH v3 0/7] arm64: Privileged Access Never using TTBR0

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Date: Wed, 14 Sep 2016 11:36:46 +0100
From: Mark Rutland <mark.rutland@....com>
To: Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc: Catalin Marinas <catalin.marinas@....com>,
	"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
	Will Deacon <will.deacon@....com>,
	James Morse <james.morse@....com>,
	Kees Cook <keescook@...omium.org>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH v3 0/7] arm64: Privileged Access Never using TTBR0_EL1
 switching

On Wed, Sep 14, 2016 at 11:30:05AM +0100, Ard Biesheuvel wrote:
> On 14 September 2016 at 11:27, Mark Rutland <mark.rutland@....com> wrote:
> > On Wed, Sep 14, 2016 at 11:13:33AM +0100, Ard Biesheuvel wrote:
> >> On 13 September 2016 at 18:46, Catalin Marinas <catalin.marinas@....com> wrote:
> >> > This is the third version of the arm64 PAN emulation using TTBR0_EL1
> >> > switching.
> >
> >> Given that every __get_user() call now incurs the PAN switch overhead,
> >> I wonder if it would be worth it to stash the real TTBR0_EL1 value in,
> >> e.g., TPIDRRO_EL0 rather than load it from memory each time. We'd have
> >> to reload the real value of TPIDRRO_EL0 at kernel exit every time, but
> >> only for compat tasks, and not nearly as often, obviously.
> >
> > FWIW, my plan for vmap'd stacks involves clobbering TPIDRRO_EL0 early
> > upon kernel entry to reliably detect/handle stack overflow (as we need
> > to free up GPR to detect overflow, and we need to detect that before we
> > try to store to the stack).
> >
> > For non-compat tasks we must restore zero, so either way we'll end up
> > with a load (to determine compat-ness or to load the precise value).
> 
> Are you saying that with vmapped stacks, we'll end up clobbering it
> (and thus restoring it) anyway when entering the kernel, and so we
> could use it for free afterwards while running in the kernel,
> potentially for the real value of TTBR0_EL1?

Yes, assuming that we end up following my current plan for how to
implement that.

Thanks,
Mark.

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