Re: [Xen-devel] [PATCH] x86/pv: Fill all Xen slots in init_guest_l4_table()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.08.17 at 12:24, <andrew.cooper3@xxxxxxxxxx> wrote:
> There is a bug when using highmem-start= where some L4 directmap slots are not
> audited in alloc_l4_table(), and not overwritten by init_guest_l4_table().
> 
> As highmem_start is only available in debug builds of the hypervisor, this
> does not constitute a security issue.
> 
> Ensure that init_guest_l4_table() writes to all of the Xen slots.
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
albeit ...

> @@ -1604,9 +1608,19 @@ void init_guest_l4_table(l4_pgentry_t l4tab[], const 
> struct domain *d,
>             &idle_pg_table[ROOT_PAGETABLE_FIRST_XEN_SLOT],
>             root_pgt_pv_xen_slots * sizeof(l4_pgentry_t));
>  #ifndef NDEBUG
> -    if ( l4e_get_intpte(split_l4e) )
> -        l4tab[ROOT_PAGETABLE_FIRST_XEN_SLOT + root_pgt_pv_xen_slots] =
> -            split_l4e;
> +    if ( unlikely(root_pgt_pv_xen_slots < ROOT_PAGETABLE_PV_XEN_SLOTS) )
> +    {
> +        l4_pgentry_t *next = &l4tab[ROOT_PAGETABLE_FIRST_XEN_SLOT +
> +                                    root_pgt_pv_xen_slots];
> +
> +        if ( l4e_get_intpte(split_l4e) )
> +            *next++ = split_l4e;
> +
> +        memset(next, 0,
> +               _p(&l4tab[ROOT_PAGETABLE_LAST_XEN_SLOT + 1]) - _p(next));

... these disguised casts. But all alternatives I can think of make the
code more difficult to read.

Jan


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