Re: [PATCH v3 3/4] x86, mpx: add prctl commands PR_MPX_INIT, PR_MPX_RELEASE

[Ren Qiaowei <qiaowei.ren@intel.com>]

On 01/26/2014 11:14 PM, Ingo Molnar wrote:
>
> * Ren, Qiaowei <qiaowei.ren@intel.com> wrote:
>
>> The size of one bound table is 4M bytes for 64bit, and 16K bytes for
>> 32bit. It can not be accessed by user-space, and it will be accessed
>> automatically by hardware.
>
> So, here's the bound-table allocation AFAICS:
>
> +static bool allocate_bt(unsigned long bd_entry)
> +{
> +       unsigned long bt_size = 1UL << (MPX_L2_BITS+MPX_L2_SHIFT);
> +       unsigned long bt_addr, old_val = 0;
> +
> +       bt_addr = sys_mmap_pgoff(0, bt_size, PROT_READ | PROT_WRITE,
> +                       MAP_ANONYMOUS | MAP_PRIVATE | MAP_POPULATE, -1, 0);
>
> What ensures that user-space cannot access (and in particular, modify)
> the pages at bt_addr? It's a read-write anonymous mapping AFAICS.
>
Looks like that we can not be able to ensure this. I just mean that 
user-space doesn't know the bound tables, and it should not access them 
also.

Thanks,
Qiaowei