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

[Julien Grall <julien.grall@xxxxxxx>]

Hi Shanker,

On 11/03/2016 05:00, Shanker Donthineni wrote:
> From: Vikram Sethi <vikrams@xxxxxxxxxxxxxx>
>
> arch/arm/mm.c has 2 uses of alloc_boot_pages which are used for
> pagetables, but the allocated pages are not zeroed. This can cause
> crashes on CPUs with aggressive prefetching when they find 'valid'
> entries in the page tables but which are really uninitialized.
> Memset the allocated pages before use.
I first thought the problem was related to break-before-make mandate by 
the ARM architecture (see D4-1732 in ARM DDI 0487A.i) when the page 
tables are modified in a certain way, but neither the frame table noor 
the xen heap are used before the TLBs are nuked.
I would like to see more details in the commit message about the crash 
and why (based on the spec) clearing the page is the right solution.
Note that I think clearing the page is good to avoid polluting the TLBs 
with bogus entries and get better crash log.
> Change-Id: I517ca45ca240766dfbf1d6884c044c377babab7d
What this line for?

> Signed-off-by: Vikram Sethi <vikrams@xxxxxxxxxxxxxx>
> Signed-off-by: Shanker Donthineni <shankerd@xxxxxxxxxxxxxx>
> ---
>   xen/arch/arm/mm.c | 2 ++
>   1 file changed, 2 insertions(+)
>
> diff --git a/xen/arch/arm/mm.c b/xen/arch/arm/mm.c
> index 81f9e2e..215ec93 100644
> --- a/xen/arch/arm/mm.c
> +++ b/xen/arch/arm/mm.c
> @@ -730,6 +730,7 @@ void __init setup_xenheap_mappings(unsigned long base_mfn,
>           else
>           {
>               unsigned long first_mfn = alloc_boot_pages(1, 1);
> +            memset(mfn_to_virt(first_mfn), 0, PAGE_SIZE);
You can move "first = mfn_to_virt(first_mfn)" earlier and re-use first here.

>               pte = mfn_to_xen_entry(first_mfn, WRITEALLOC);
>               pte.pt.table = 1;
>               write_pte(p, pte);
> @@ -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);
Regards,

--
Julien Grall

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