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

On 14/03/16 17:18, Shanker Donthineni wrote:
Hi Julien,

Hi Shanker,

On 03/12/2016 10:03 AM, Julien Grall wrote:

On 11/03/2016 20:24, Andrew Cooper wrote:
On 11/03/16 13:13, Jan Beulich wrote:
On 11.03.16 at 13:56, <andrew.cooper3@xxxxxxxxxx> wrote:
On 11/03/16 11:29, Jan Beulich wrote:
On 10.03.16 at 23:00, <shankerd@xxxxxxxxxxxxxx> wrote:
@@ -771,6 +772,7 @@ void __init setup_frametable_mappings(paddr_t ps, paddr_t 
       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.

The frame table size is always aligned to 2MB/32MB. However, the frame table 
may not use all the entries in a level 2 page table (which cover 1GB of 
memory). Those unused entries will be unknown if we don't clear them.

Keeping them unknown is not a problem as long as nobody is trying to access the 
underlying virtual address.

I don't agree keeping a garbage value in PTE is not a problem. The ARMv8 
allows to perform speculative data/instruction read accesses from memory (type 
as along as its PTE valid bit is set.

When you quote the spec, can you give the section/version? It helps the reviewers to find more details.

More generally, having the section/version in the commit message is useful for future reference.

CPU prefetch logic might access garbage PTEs and cause system panic if VA-PA 
happens to be physical address that is not addressable by system BUS.

It would have been nice to see those kind of details in the commit message.


Julien Grall

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