[PATCH 1/2] arm64: SW PAN: Point saved ttbr0 at the zero page when switching to init_mm

[Catalin Marinas]

On Wed, Dec 06, 2017 at 11:16:07AM +0000, Will Deacon wrote:
> update_saved_ttbr0 mandates that mm->pgd is not swapper, since swapper
> contains kernel mappings and should never be installed into ttbr0. However,
> this means that callers must avoid passing the init_mm to update_saved_ttbr0
> which in turn can cause the saved ttbr0 value to be out-of-date in the context
> of the idle thread. For example, EFI runtime services may leave the saved ttbr0
> pointing at the EFI page table, and kernel threads may end up with stale
> references to freed page tables.
> 
> This patch changes update_saved_ttbr0 so that the init_mm points the saved
> ttbr0 value to the empty zero page, which always exists and never contains
> valid translations. EFI and switch can then call into update_saved_ttbr0
> unconditionally.
> 
> Cc: Mark Rutland <mark.rutland at arm.com>
> Cc: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> Cc: Vinayak Menon <vinmenon at codeaurora.org>
> Reported-by: Vinayak Menon <vinmenon at codeaurora.org>
> Signed-off-by: Will Deacon <will.deacon at arm.com>

Reviewed-by: Catalin Marinas <catalin.marinas at arm.com>