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

[Ard Biesheuvel]

On 15 August 2016 at 12:06, Mark Rutland <mark.rutland at arm.com> wrote:
> On Mon, Aug 15, 2016 at 12:02:33PM +0200, Ard Biesheuvel wrote:
>> On 15 August 2016 at 11:58, Mark Rutland <mark.rutland at arm.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 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.
>>
>> ... 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.
>
> That's a good point, and a better approach.
>
> Unfortunately, we're still left with the issue that TCR_EL1.* can be cached in
> a TLB, as Catalin pointed out. Which at minimum would require a TLBI ASIDE1,
> and may require something stronger, given the precise rules for TLB-cached
> fields isn't clear.
>

So how exactly would EPDn = 1 be cached in a TLB, given that the bit
specifically means that TTBRn_ELn is ignored on a TLB *miss*. Doesn't
that mean 'not covered by a TLB entry'? Or does it mean 'not covered
by a TLB entry describing a valid translation'?

I guess it indeed makes sense to get this clarified ...

As to Will's point, I suppose there is a window where a speculative
TLB fill could occur, so I suppose that means updating TTBR0_EL1.ASID
first, then TCR_EL1.EPD0, and finally perform the TLBI ASIDE1 on the
reserved ASID.