Re: [Xen-devel] [PATCH 2/2] xen/x86: hap: Clean-up and harden hap_enable()

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 04, 2020 at 09:34:11AM +0000, Julien Grall wrote:
> From: Julien Grall <jgrall@xxxxxxxxxx>
> 
> Unlike shadow_enable(), hap_enable() can only be called once during
> domain creation and with the mode equal to mode equal to
                                    ^ equals to
> PG_external | PG_translate | PG_refcounts.
> 
> If it were called twice, then we might have something interesting
                                              ^ a problem
> problem as the p2m tables would be re-allocated (and therefore all the
> mappings would be lost).
> 
> Add code to sanity check the mode and that the function is only called
> once. Take the opportunity to an if checking that PG_translate is set.
                                ^ add an if
> 
> Signed-off-by: Julien Grall <jgrall@xxxxxxxxxx>
> 
> ---
> 
> It is not entirely clear when PG_translate was enforced.
> ---
>  xen/arch/x86/mm/hap/hap.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/xen/arch/x86/mm/hap/hap.c b/xen/arch/x86/mm/hap/hap.c
> index 31362a31b6..b734e2e6d3 100644
> --- a/xen/arch/x86/mm/hap/hap.c
> +++ b/xen/arch/x86/mm/hap/hap.c
> @@ -445,6 +445,13 @@ int hap_enable(struct domain *d, u32 mode)
>      unsigned int i;
>      int rv = 0;
>  
> +    if ( mode != (PG_external | PG_translate | PG_refcounts) )
> +        return -EINVAL;
> +
> +    /* The function can only be called once */
> +    if ( d->arch.paging.mode != 0 )
> +        return -EINVAL;

If you want to return EINVAL for both they can be merged into a single
if. Also note that this would usually be written as
if ( d->arch.paging.mode ) to keep it shorter.

Albeit I think you might want to return EEXIST instead of EINVAL if
mode is already set.

> +
>      domain_pause(d);
>  
>      old_pages = d->arch.paging.hap.total_pages;
> @@ -465,13 +472,10 @@ int hap_enable(struct domain *d, u32 mode)
>      d->arch.paging.alloc_page = hap_alloc_p2m_page;
>      d->arch.paging.free_page = hap_free_p2m_page;
>  
> -    /* allocate P2m table */
> -    if ( mode & PG_translate )
> -    {
> -        rv = p2m_alloc_table(p2m_get_hostp2m(d));
> -        if ( rv != 0 )
> -            goto out;
> -    }
> +    /* allocate P2M table */
> +    rv = p2m_alloc_table(p2m_get_hostp2m(d));
> +    if ( rv != 0 )

I would also avoid comparing against 0 here.

Thanks, Roger.

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