[RFC PATCH] arm64: use non-global mappings for UEFI runtime regions

[Ard Biesheuvel]

On 17 November 2015 at 17:34, Will Deacon <will.deacon at arm.com> wrote:
> On Tue, Nov 17, 2015 at 03:25:58PM +0000, Mark Rutland wrote:
>> On Tue, Nov 17, 2015 at 09:53:31AM +0100, Ard Biesheuvel wrote:
>> > As pointed out by Russell King in response to the proposed ARM version
>> > of this code, the sequence to switch between the UEFI runtime mapping
>> > and current's actual userland mapping (and vice versa) is potentially
>> > unsafe, since it leaves a time window between the switch to the new
>> > page tables and the TLB flush where speculative accesses may hit on
>> > stale global TLB entries.
>>
>> Wow, annoying that we missed that.
>>
>> > So instead, use non-global mappings, and perform the switch via the
>> > ordinary ASID-aware context switch routines.
>> >
>> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>>
>> From digging into the way the ASID allocator works, I believe this is
>> correct. FWIW:
>>
>> Reviewed-by: Mark Rutland <mark.rutland at arm.com>
>>
>> For backporting, I'm not sure that this is necessarily safe prior to
>> Will's rework of the ASID allocator. I think we can IPI in this context,
>> and it looks like the cpu_set_reserved_ttbr0() in flush_context() would
>> save us from the problem described above, but I may have missed
>> something.
>>
>> Will, are you aware of anything that could bite us here?
>
> Can we guarantee that efi_virtmap_{load,unload} are called with interrupts
> enabled? Also, the old rollover code seems to rely on current->active_mm
> being the thing to switch to, so an incoming rollover might break things
> if we're running with the efi_mm.
>

OK, but not the current rollover code, right? I.e., if we go with this
approach (after fixing the interupts issue), it will carry over to ARM
as well?