Re: [PATCH] xen: arm: zero EL2 pagetable pages before use

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 11/03/16 13:13, Jan Beulich wrote:
>>>> On 11.03.16 at 13:56, <andrew.cooper3@citrix.com> wrote:
>> On 11/03/16 11:29, Jan Beulich wrote:
>>>>>> On 10.03.16 at 23:00, <shankerd@codeaurora.org> wrote:
>>>> @@ -771,6 +772,7 @@ void __init setup_frametable_mappings(paddr_t ps, paddr_t pe)
>>>>      nr_second = frametable_size >> SECOND_SHIFT;
>>>>      second_base = alloc_boot_pages(nr_second, 1);
>>>>      second = mfn_to_virt(second_base);
>>>> +    memset(second, 0, nr_second * PAGE_SIZE);
>>>>      for ( i = 0; i < nr_second; i++ )
>>>>      {
>>>>          pte = mfn_to_xen_entry(second_base + i, WRITEALLOC);
>>> Along those lines here - use clear_page(), presumably by moving it
>>> into the loop.
>> This need only initialise the entries which are not filled by the loop,
>> which will only be the rounding size up to the next 2M or 32M boundary.
>>
>> Most of the content of 'second' is explicitly initialised, so zeroing it
>> all first is redundant.
> Well, I certainly don't know all the details of how this works on
> ARM, but the way I remember the original problem description
> (sent a few days ago) the problem was with bogus translations
> to be visible transiently. Of course all depends on whether the
> page tables that are being modified here are live ones, which
> I simply don't know.

Looking at the code here, second is hooked into the live pagetables
immediately after the loop.  Therefore, bogus translations will only be
present for the untouched PTEs which make up the alignment space.

However, it is probably best to defer to the ARM maintainers.

~Andrew

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