Re: [PATCH 2/4] arm64/mm: wire up CONFIG_ARCH_HAS_SET_DIRECT_MAP

[Anshuman Khandual]

On 05/23/2019 03:52 PM, Ard Biesheuvel wrote:
> Wire up the special helper functions to manipulate aliases of vmalloc
> regions in the linear map.

IMHO the commit message here could be bit more descriptive because of the
amount of changes this patch brings in.

> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxx>
> ---
>  arch/arm64/Kconfig                  |  1 +
>  arch/arm64/include/asm/cacheflush.h |  3 ++
>  arch/arm64/mm/pageattr.c            | 48 ++++++++++++++++----
>  mm/vmalloc.c                        | 11 -----
>  4 files changed, 44 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> index ca9c175fb949..4ab32180eabd 100644
> --- a/arch/arm64/Kconfig
> +++ b/arch/arm64/Kconfig
> @@ -26,6 +26,7 @@ config ARM64
>  	select ARCH_HAS_MEMBARRIER_SYNC_CORE
>  	select ARCH_HAS_PTE_SPECIAL
>  	select ARCH_HAS_SETUP_DMA_OPS
> +	select ARCH_HAS_SET_DIRECT_MAP
>  	select ARCH_HAS_SET_MEMORY
>  	select ARCH_HAS_STRICT_KERNEL_RWX
>  	select ARCH_HAS_STRICT_MODULE_RWX
> diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
> index 19844211a4e6..b9ee5510067f 100644
> --- a/arch/arm64/include/asm/cacheflush.h
> +++ b/arch/arm64/include/asm/cacheflush.h
> @@ -187,4 +187,7 @@ static inline void flush_cache_vunmap(unsigned long start, unsigned long end)
>  
>  int set_memory_valid(unsigned long addr, int numpages, int enable);
>  
> +int set_direct_map_invalid_noflush(struct page *page);
> +int set_direct_map_default_noflush(struct page *page);
> +
>  #endif
> diff --git a/arch/arm64/mm/pageattr.c b/arch/arm64/mm/pageattr.c
> index 6cd645edcf35..9c6b9039ec8f 100644
> --- a/arch/arm64/mm/pageattr.c
> +++ b/arch/arm64/mm/pageattr.c
> @@ -159,17 +159,48 @@ int set_memory_valid(unsigned long addr, int numpages, int enable)
>  					__pgprot(PTE_VALID));
>  }
>  
> -#ifdef CONFIG_DEBUG_PAGEALLOC
> +int set_direct_map_invalid_noflush(struct page *page)
> +{
> +	struct page_change_data data = {
> +		.set_mask = __pgprot(0),
> +		.clear_mask = __pgprot(PTE_VALID),
> +	};
> +
> +	if (!rodata_full)
> +		return 0;

Why rodata_full needs to be probed here ? Should not we still require the following
transition even if rodata_full is not enabled. Just wondering whether we can use
VM_FLUSH_RESET_PERMS feature without these required transitions.

        /*
         * Set direct map to something invalid so that it won't be cached if
         * there are any accesses after the TLB flush, then flush the TLB and
         * reset the direct map permissions to the default.
         */
        set_area_direct_map(area, set_direct_map_invalid_noflush);
        _vm_unmap_aliases(start, end, 1);
        set_area_direct_map(area, set_direct_map_default_noflush);

 > +
> +	return apply_to_page_range(&init_mm,
> +				   (unsigned long)page_address(page),
> +				   PAGE_SIZE, change_page_range, &data);
> +}
> +
> +int set_direct_map_default_noflush(struct page *page)
> +{
> +	struct page_change_data data = {
> +		.set_mask = __pgprot(PTE_VALID | PTE_WRITE),
> +		.clear_mask = __pgprot(PTE_RDONLY),

Replace __pgprot(PTE_VALID | PTE_WRITE) with PAGE_KERNEL instead !

> +	};
> +
> +	if (!rodata_full)
> +		return 0;
> +
> +	return apply_to_page_range(&init_mm,
> +				   (unsigned long)page_address(page),
> +				   PAGE_SIZE, change_page_range, &data);
> +}
> +

IIUC set_direct_map_invalid_noflush() and set_direct_map_default_noflush()
should set *appropriate* permissions as seen fit from platform perspective
to implement this transition.

In here set_direct_map_invalid_noflush() makes the entry invalid preventing
further MMU walks (hence new TLB entries). set_direct_map_default_noflush()
makes it a valid write entry. Though it looks similar to PAGE_KERNEL which
is the default permission for linear mapping on arm64 via __map_memblock().
Should not PAGE_KERNEL be used explicitly as suggested above.

>  void __kernel_map_pages(struct page *page, int numpages, int enable)
>  {
> +	if (!debug_pagealloc_enabled() && !rodata_full)
> +		return;
> +

I guess this is not related to CONFIG_ARCH_HAS_SET_DIRECT_MAP here and should
be a fix or an enhancement to CONFIG_DEBUG_PAGEALLOC implementation. Just
curious, !rodata_full check here to ensure that linear mapping does not have
block or contig mappings and should be backed by regular pages only ?

>  	set_memory_valid((unsigned long)page_address(page), numpages, enable);
>  }
> -#ifdef CONFIG_HIBERNATION
> +
>  /*
> - * When built with CONFIG_DEBUG_PAGEALLOC and CONFIG_HIBERNATION, this function
> - * is used to determine if a linear map page has been marked as not-valid by
> - * CONFIG_DEBUG_PAGEALLOC. Walk the page table and check the PTE_VALID bit.
> - * This is based on kern_addr_valid(), which almost does what we need.
> + * This function is used to determine if a linear map page has been marked as
> + * not-valid. Walk the page table and check the PTE_VALID bit. This is based
> + * on kern_addr_valid(), which almost does what we need.
>   *
>   * Because this is only called on the kernel linear map,  p?d_sect() implies
>   * p?d_present(). When debug_pagealloc is enabled, sections mappings are
> @@ -183,6 +214,9 @@ bool kernel_page_present(struct page *page)
>  	pte_t *ptep;
>  	unsigned long addr = (unsigned long)page_address(page);
>  
> +	if (!debug_pagealloc_enabled() && !rodata_full)
> +		return true;
> +

Ditto, not related to CONFIG_ARCH_HAS_SET_DIRECT_MAP.