[PATCH] powerpc/mm: add devmem_is_allowed() for STRICT_DEVMEM checking

[Scott Wood]

On Mon, 31 Jan 2011 14:16:00 -0500
Steve Best <sfbest at us.ibm.com> wrote:

>     Provide devmem_is_allowed() routine to restrict access to kernel
>     memory from userspace.
>     Set CONFIG_STRICT_DEVMEM config option to switch on checking.
> 
> Signed-off-by: Steve Best <sfbest at us.ibm.com>
> 
> diff --git a/arch/powerpc/Kconfig.debug b/arch/powerpc/Kconfig.debug
> index 2d38a50..6805d5d 100644
> --- a/arch/powerpc/Kconfig.debug
> +++ b/arch/powerpc/Kconfig.debug
> @@ -299,4 +299,16 @@ config PPC_EARLY_DEBUG_CPM_ADDR
>  	  platform probing is done, all platforms selected must
>  	  share the same address.
>  
> +config STRICT_DEVMEM
> +        def_bool y
> +        prompt "Filter access to /dev/mem"
> +        ---help---
> +          This option restricts access to /dev/mem.  If this option is
> +          disabled, you allow userspace access to all memory, including
> +          kernel and userspace memory. Accidental memory access is likely
> +          to be disastrous.
> +          Memory access is required for experts who want to debug the kernel.
> +
> +          If you are unsure, say Y.
> +
>  endmenu
> diff --git a/arch/powerpc/include/asm/page.h b/arch/powerpc/include/asm/page.h
> index 53b64be..f225032 100644
> --- a/arch/powerpc/include/asm/page.h
> +++ b/arch/powerpc/include/asm/page.h
> @@ -262,6 +262,11 @@ extern void copy_user_page(void *to, void *from, unsigned long vaddr,
>  		struct page *p);
>  extern int page_is_ram(unsigned long pfn);
>  
> +static inline int devmem_is_allowed(unsigned long pfn)
> +{
> +        return 0;
> +}
> +

I don't see how this is a sane thing to turn on by default (you're not
restricting it, BTW -- you're completely disabling it with that
implementation of devmem_is_allowed).  It will break anything that
uses /dev/mem to access I/O, possibly including desktoppy stuff like X
servers, as well as lots of stuff that goes on in embedded setups.

You need to be root to access /dev/mem, and root has plenty of
other options for causing "disastrous" results.  You don't just stumble
onto /dev/mem by accident.

-Scott