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 <labb...@redhat.com>
> Signed-off-by: Ard Biesheuvel <ard.biesheu...@linaro.org>
Reviewed-by: Mark Rutland <mark.rutl...@arm.com>
Tested-by: Mark Rutland <mark.rutl...@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
>
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