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Date: Mon, 15 Aug 2016 12:02:33 +0200
From: Ard Biesheuvel <ard.biesheuvel@...aro.org>
To: Mark Rutland <mark.rutland@....com>
Cc: Catalin Marinas <catalin.marinas@....com>, Kees Cook <keescook@...omium.org>,
kernel-hardening@...ts.openwall.com, Will Deacon <will.deacon@....com>,
Julien Grall <julien.grall@....com>, James Morse <james.morse@....com>,
"linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 0/7] arm64: Privileged Access Never
using TTBR0_EL1 switching
On 15 August 2016 at 11:58, Mark Rutland <mark.rutland@....com> wrote:
> On Mon, Aug 15, 2016 at 10:48:42AM +0100, Catalin Marinas wrote:
>> On Sat, Aug 13, 2016 at 11:13:58AM +0200, Ard Biesheuvel wrote:
>> > On 12 August 2016 at 17:27, Catalin Marinas <catalin.marinas@....com> wrote:
>> > > This is the first (public) attempt at emulating PAN by disabling
>> > > TTBR0_EL1 accesses on arm64.
>> >
>> > I take it using TCR_EL1.EPD0 is too expensive?
>>
>> It would require full TLB invalidation on entering/exiting the kernel
>> and again for any user access. That's because the architecture allows
>> this bit to be cached in the TLB so without TLBI we wouldn't have any
>> guarantee that the actual PAN was toggled. I'm not sure it's even clear
>> whether a TLBI by ASID or a local one would suffice (likely OK for the
>> latter).
>
> It's worth noting that even ignoring the TLB-caching of TCR_EL1.EPD0, the
> control only affects the behaviour on a TLB miss. Thus to use EPD0 we'd at
> least need TLB invalidation by ASID to remove previously-allocated entries from
> TLBs.
>
... or update the ASID to the reserved ASID in TTBR0_EL1, but leave
the actual TTBR address alone.
This would remove the need for a zero page, and for recording the
original TTBR address in a per-cpu variable.
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