Re: [Xen-devel] [PATCH] espfix/xen: Fix allocation of pages for paravirt page tables

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Wed, Jul 09, 2014 at 01:18:18PM -0400, Boris Ostrovsky wrote:
> init_espfix_ap() is currently off by one level when informing hypervisor
> that allocated pages will be used for ministacks' page tables.
> 
> The most immediate effect of this on a PV guest is that if
> 'stack_page = __get_free_page()' returns a non-zeroed-out page the hypervisor
> will refuse to use it for a page table (which it shouldn't be anyway). This 
> will
> result in warnings by both Xen and Linux.
> 
> More importantly, a subsequent write to that page (again, by a PV guest) is
> likely to result in fatal page fault.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

<sigh>
> ---
>  arch/x86/kernel/espfix_64.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/espfix_64.c b/arch/x86/kernel/espfix_64.c
> index 6afbb16..94d857f 100644
> --- a/arch/x86/kernel/espfix_64.c
> +++ b/arch/x86/kernel/espfix_64.c
> @@ -175,7 +175,7 @@ void init_espfix_ap(void)
>       if (!pud_present(pud)) {
>               pmd_p = (pmd_t *)__get_free_page(PGALLOC_GFP);
>               pud = __pud(__pa(pmd_p) | (PGTABLE_PROT & ptemask));
> -             paravirt_alloc_pud(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
> +             paravirt_alloc_pmd(&init_mm, __pa(pmd_p) >> PAGE_SHIFT);
>               for (n = 0; n < ESPFIX_PUD_CLONES; n++)
>                       set_pud(&pud_p[n], pud);
>       }
> @@ -185,7 +185,7 @@ void init_espfix_ap(void)
>       if (!pmd_present(pmd)) {
>               pte_p = (pte_t *)__get_free_page(PGALLOC_GFP);
>               pmd = __pmd(__pa(pte_p) | (PGTABLE_PROT & ptemask));
> -             paravirt_alloc_pmd(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
> +             paravirt_alloc_pte(&init_mm, __pa(pte_p) >> PAGE_SHIFT);
>               for (n = 0; n < ESPFIX_PMD_CLONES; n++)
>                       set_pmd(&pmd_p[n], pmd);
>       }
> @@ -193,7 +193,6 @@ void init_espfix_ap(void)
>       pte_p = pte_offset_kernel(&pmd, addr);
>       stack_page = (void *)__get_free_page(GFP_KERNEL);
>       pte = __pte(__pa(stack_page) | (__PAGE_KERNEL_RO & ptemask));
> -     paravirt_alloc_pte(&init_mm, __pa(stack_page) >> PAGE_SHIFT);
>       for (n = 0; n < ESPFIX_PTE_CLONES; n++)
>               set_pte(&pte_p[n*PTE_STRIDE], pte);
>  
> -- 
> 1.8.1.4
> 

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