[PATCH v2 3/5] arm64: alternatives: apply boot time fixups via the linear mapping

[Mark Rutland]

On Sat, Feb 11, 2017 at 08:23:04PM +0000, Ard Biesheuvel wrote:
> One important rule of thumb when desiging a secure software system is
> that memory should never be writable and executable at the same time.
> We mostly adhere to this rule in the kernel, except at boot time, when
> regions may be mapped RWX until after we are done applying alternatives
> or making other one-off changes.
> 
> For the alternative patching, we can improve the situation by applying
> the fixups via the linear mapping, which is never mapped with executable
> permissions. So map the linear alias of .text with RW- permissions
> initially, and remove the write permissions as soon as alternative
> patching has completed.
> 
> Reviewed-by: Laura Abbott <labbott at redhat.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>

Reviewed-by: Mark Rutland <mark.rutland at arm.com>
Tested-by: Mark Rutland <mark.rutland at arm.com>

Mark.

> ---
>  arch/arm64/include/asm/mmu.h    |  1 +
>  arch/arm64/kernel/alternative.c |  2 +-
>  arch/arm64/kernel/smp.c         |  1 +
>  arch/arm64/mm/mmu.c             | 22 +++++++++++++++-----
>  4 files changed, 20 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/mmu.h b/arch/arm64/include/asm/mmu.h
> index 47619411f0ff..5468c834b072 100644
> --- a/arch/arm64/include/asm/mmu.h
> +++ b/arch/arm64/include/asm/mmu.h
> @@ -37,5 +37,6 @@ extern void create_pgd_mapping(struct mm_struct *mm, phys_addr_t phys,
>  			       unsigned long virt, phys_addr_t size,
>  			       pgprot_t prot, bool page_mappings_only);
>  extern void *fixmap_remap_fdt(phys_addr_t dt_phys);
> +extern void mark_linear_text_alias_ro(void);
>  
>  #endif
> diff --git a/arch/arm64/kernel/alternative.c b/arch/arm64/kernel/alternative.c
> index 06d650f61da7..8cee29d9bc07 100644
> --- a/arch/arm64/kernel/alternative.c
> +++ b/arch/arm64/kernel/alternative.c
> @@ -128,7 +128,7 @@ static void __apply_alternatives(void *alt_region)
>  
>  		for (i = 0; i < nr_inst; i++) {
>  			insn = get_alt_insn(alt, origptr + i, replptr + i);
> -			*(origptr + i) = cpu_to_le32(insn);
> +			((u32 *)lm_alias(origptr))[i] = cpu_to_le32(insn);
>  		}
>  
>  		flush_icache_range((uintptr_t)origptr,
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index a8ec5da530af..d6307e311a10 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -432,6 +432,7 @@ void __init smp_cpus_done(unsigned int max_cpus)
>  	setup_cpu_features();
>  	hyp_mode_check();
>  	apply_alternatives_all();
> +	mark_linear_text_alias_ro();
>  }
>  
>  void __init smp_prepare_boot_cpu(void)
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 9e0ec1a8cd3b..7ed981c7f4c0 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -398,16 +398,28 @@ static void __init __map_memblock(pgd_t *pgd, phys_addr_t start, phys_addr_t end
>  				     debug_pagealloc_enabled());
>  
>  	/*
> -	 * Map the linear alias of the [_text, __init_begin) interval as
> -	 * read-only/non-executable. This makes the contents of the
> -	 * region accessible to subsystems such as hibernate, but
> -	 * protects it from inadvertent modification or execution.
> +	 * Map the linear alias of the [_text, __init_begin) interval
> +	 * as non-executable now, and remove the write permission in
> +	 * mark_linear_text_alias_ro() below (which will be called after
> +	 * alternative patching has completed). This makes the contents
> +	 * of the region accessible to subsystems such as hibernate,
> +	 * but protects it from inadvertent modification or execution.
>  	 */
>  	__create_pgd_mapping(pgd, kernel_start, __phys_to_virt(kernel_start),
> -			     kernel_end - kernel_start, PAGE_KERNEL_RO,
> +			     kernel_end - kernel_start, PAGE_KERNEL,
>  			     early_pgtable_alloc, debug_pagealloc_enabled());
>  }
>  
> +void __init mark_linear_text_alias_ro(void)
> +{
> +	/*
> +	 * Remove the write permissions from the linear alias of .text/.rodata
> +	 */
> +	create_mapping_late(__pa_symbol(_text), (unsigned long)lm_alias(_text),
> +			    (unsigned long)__init_begin - (unsigned long)_text,
> +			    PAGE_KERNEL_RO);
> +}
> +
>  static void __init map_mem(pgd_t *pgd)
>  {
>  	struct memblock_region *reg;
> -- 
> 2.7.4
>