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

[Mark Rutland]

On Tue, Nov 17, 2015 at 04:34:46PM +0000, Will Deacon 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?

Unfortuantely, it looks like we can guarantee interrupts are _disabled_.

Every function in drivers/firmware/efi/runtime-wrappers.c which uses
efi_call_virt (and hence efi_virtmap_{load,unload}) wraps the call in a
spin_lock_irq{save,restore} pair. Those appear to be the only uses of
efi_call_virt.

Mark.