[kernel-hardening] [PATCH 0/7] arm64: Privileged Access Never using TTBR0_EL1 switching

[Mark Rutland]

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 at arm.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.

Thanks,
Mark.