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

Will Deacon
2015-11-17T17:08:17Z

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.
>
> 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>
> ---
> arch/arm64/include/asm/mmu_context.h | 2 +-
> arch/arm64/kernel/efi.c | 14 +++++---------
> 2 files changed, 6 insertions(+), 10 deletions(-)

Acked-by: Will Deacon <will.deacon@arm.com>

Please do *not* tag this for stable! ;)

Will

> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index c0e87898ba96..24165784b803 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -101,7 +101,7 @@ static inline void cpu_set_default_tcr_t0sz(void)
> #define destroy_context(mm) do { } while(0)
> void check_and_switch_context(struct mm_struct *mm, unsigned int cpu);
>
> -#define init_new_context(tsk,mm) ({ atomic64_set(&mm->context.id, 0); 0; })
> +#define init_new_context(tsk,mm) ({ atomic64_set(&(mm)->context.id, 0); 0; })
>
> /*
> * This is called when "tsk" is about to enter lazy TLB mode.
> diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
> index de46b50f4cdf..fc5508e0df57 100644
> --- a/arch/arm64/kernel/efi.c
> +++ b/arch/arm64/kernel/efi.c
> @@ -224,6 +224,8 @@ static bool __init efi_virtmap_init(void)
> {
> efi_memory_desc_t *md;
>
> + init_new_context(NULL, &efi_mm);
> +
> for_each_efi_memory_desc(&memmap, md) {
> u64 paddr, npages, size;
> pgprot_t prot;
> @@ -254,7 +256,8 @@ static bool __init efi_virtmap_init(void)
> else
> prot = PAGE_KERNEL;
>
> - create_pgd_mapping(&efi_mm, paddr, md->virt_addr, size, prot);
> + create_pgd_mapping(&efi_mm, paddr, md->virt_addr, size,
> + __pgprot(pgprot_val(prot) | PTE_NG));
> }
> return true;
> }
> @@ -329,14 +332,7 @@ core_initcall(arm64_dmi_init);
>
> static void efi_set_pgd(struct mm_struct *mm)
> {
> - if (mm == &init_mm)
> - cpu_set_reserved_ttbr0();
> - else
> - cpu_switch_mm(mm->pgd, mm);
> -
> - local_flush_tlb_all();
> - if (icache_is_aivivt())
> - __local_flush_icache_all();
> + switch_mm(NULL, mm, NULL);
> }
>
> void efi_virtmap_load(void)
> --
> 1.9.1
>