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Message-ID: <20160914102659.GC14330@leverpostej>
Date: Wed, 14 Sep 2016 11:27:00 +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 11:13:33AM +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).
Thanks,
Mark.
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