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

[will.deacon@arm.com (Will Deacon)]

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@linaro.org>
> 
> From digging into the way the ASID allocator works, I believe this is
> correct. FWIW:
> 
> Reviewed-by: Mark Rutland <mark.rutland@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.

Will