[PATCH v3 03/13] arm64: improve CONFIG_STRICT_DEVMEM handling

[Dave Young]

On 11/18/14 at 01:57pm, Ard Biesheuvel wrote:
> Improve the handling of /dev/mem mappings under CONFIG_STRICT_DEVMEM by:
> - allowing read-only access to parts of System RAM that are not
>   considered memory by the kernel, this is mainly intended for exposing
>   UEFI Configuration tables to userland;

Ard, can you elabrate a bit? Are this for the acpi case because no dtb in
procfs? 
 
Anyway I also think using /dev/mem looks not a good way to expose
infomation to userspace.

> - avoid using non-cached mappings for those parts of System RAM, as it
>   may result in mismatched attributes.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
> ---
>  arch/arm64/mm/mmap.c |  5 +++--
>  arch/arm64/mm/mmu.c  | 15 ++++++++++++++-
>  2 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmap.c b/arch/arm64/mm/mmap.c
> index 1d73662f00ff..802cdf0df921 100644
> --- a/arch/arm64/mm/mmap.c
> +++ b/arch/arm64/mm/mmap.c
> @@ -127,14 +127,15 @@ int valid_mmap_phys_addr_range(unsigned long pfn, size_t size)
>  /*
>   * devmem_is_allowed() checks to see if /dev/mem access to a certain address
>   * is valid. The argument is a physical page number.  We mimic x86 here by
> - * disallowing access to system RAM as well as device-exclusive MMIO regions.
> + * disallowing access to system RAM that is in active use by the kernel, as
> + * well as device-exclusive MMIO regions.
>   * This effectively disable read()/write() on /dev/mem.
>   */
>  int devmem_is_allowed(unsigned long pfn)
>  {
>  	if (iomem_is_exclusive(pfn << PAGE_SHIFT))
>  		return 0;
> -	if (!page_is_ram(pfn))
> +	if (!pfn_valid(pfn))
>  		return 1;
>  	return 0;
>  }
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 4d583aa9ff4e..93ba10838359 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -121,7 +121,7 @@ early_param("cachepolicy", early_cachepolicy);
>  pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  			      unsigned long size, pgprot_t vma_prot)
>  {
> -	if (!pfn_valid(pfn))
> +	if (!page_is_ram(pfn))
>  		return pgprot_noncached(vma_prot);
>  	else if (file->f_flags & O_SYNC)
>  		return pgprot_writecombine(vma_prot);
> @@ -129,6 +129,19 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  }
>  EXPORT_SYMBOL(phys_mem_access_prot);
>  
> +/*
> + * This definition of phys_mem_access_prot_allowed() overrides
> + * the __weak definition in drivers/char/mem.c
> + */
> +int phys_mem_access_prot_allowed(struct file *file, unsigned long pfn,
> +				 unsigned long size, pgprot_t *prot)
> +{
> +	/* Disallow read-write access to reserved system RAM */
> +	if ((pgprot_val(*prot) & PTE_WRITE) && page_is_ram(pfn))
> +		return 0;
> +	return 1;
> +}
> +
>  static void __init *early_alloc(unsigned long sz)
>  {
>  	void *ptr = __va(memblock_alloc(sz, sz));
> -- 
> 1.8.3.2
>