Re: [PATCH 1/2] gnttab: remove guest_physmap_remove_page() call from gnttab_map_frame()

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

On Mon, Sep 13, 2021 at 08:41:47AM +0200, Jan Beulich wrote:
> Without holding appropriate locks, attempting to remove a prior mapping
> of the underlying page is pointless, as the same (or another) mapping
> could be re-established by a parallel request on another vCPU. Move the
> code to Arm's gnttab_set_frame_gfn(). Of course this new placement
> doesn't improve things in any way as far as the security of grant status
> frame mappings goes (see XSA-379). Proper locking would be needed here
> to allow status frames to be mapped securely.
> 
> In turn this then requires replacing the other use in
> gnttab_unpopulate_status_frames(), which yet in turn requires adjusting
> x86's gnttab_set_frame_gfn(). Note that with proper locking inside
> guest_physmap_remove_page() combined with checking the GFN's mapping
> there against the passed in MFN, there then is no issue with the
> involved multiple gnttab_set_frame_gfn()-s potentially returning varying
> values (due to a racing XENMAPSPACE_grant_table request).
> 
> This, as a side effect, does away with gnttab_map_frame() having a local
> variable "gfn" which shadows a function parameter of the same name.
> 
> Together with XSA-379 this points out that XSA-255's addition to
> gnttab_map_frame() was really useless.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -92,7 +92,7 @@ struct grant_table {
>      struct radix_tree_root maptrack_tree;
>  
>      /* Domain to which this struct grant_table belongs. */
> -    const struct domain *domain;
> +    struct domain *domain;
>  
>      struct grant_table_arch arch;
>  };
> @@ -1795,8 +1795,8 @@ gnttab_unpopulate_status_frames(struct d
>          {
>              int rc = gfn_eq(gfn, INVALID_GFN)
>                       ? 0
> -                     : guest_physmap_remove_page(d, gfn,
> -                                                 page_to_mfn(pg), 0);
> +                     : gnttab_set_frame_gfn(gt, true, i, INVALID_GFN,
> +                                            page_to_mfn(pg));
>  
>              if ( rc )
>              {
> @@ -1806,7 +1806,6 @@ gnttab_unpopulate_status_frames(struct d
>                  domain_crash(d);
>                  return rc;
>              }
> -            gnttab_set_frame_gfn(gt, true, i, INVALID_GFN);
>          }
>  
>          BUG_ON(page_get_owner(pg) != d);
> @@ -4132,6 +4131,9 @@ int gnttab_map_frame(struct domain *d, u
>      struct grant_table *gt = d->grant_table;
>      bool status = false;
>  
> +    if ( gfn_eq(gfn, INVALID_GFN) )
> +        return -EINVAL;
> +
>      grant_write_lock(gt);
>  
>      if ( evaluate_nospec(gt->gt_version == 2) && (idx & 
> XENMAPIDX_grant_table_status) )
> @@ -4144,24 +4146,18 @@ int gnttab_map_frame(struct domain *d, u
>      else
>          rc = gnttab_get_shared_frame_mfn(d, idx, mfn);
>  
> -    if ( !rc && paging_mode_translate(d) )
> -    {
> -        gfn_t gfn = gnttab_get_frame_gfn(gt, status, idx);
> -
> -        if ( !gfn_eq(gfn, INVALID_GFN) )
> -            rc = guest_physmap_remove_page(d, gfn, *mfn, 0);
> -    }
> -
>      if ( !rc )
>      {
> +        struct page_info *pg = mfn_to_page(*mfn);
> +
>          /*
>           * Make sure gnttab_unpopulate_status_frames() won't (successfully)
>           * free the page until our caller has completed its operation.
>           */
> -        if ( get_page(mfn_to_page(*mfn), d) )
> -            gnttab_set_frame_gfn(gt, status, idx, gfn);
> -        else
> +        if ( !get_page(pg, d) )
>              rc = -EBUSY;
> +        else if ( (rc = gnttab_set_frame_gfn(gt, status, idx, gfn, *mfn)) )
> +            put_page(pg);
>      }
>  
>      grant_write_unlock(gt);
> --- a/xen/include/asm-arm/grant_table.h
> +++ b/xen/include/asm-arm/grant_table.h
> @@ -71,11 +71,17 @@ int replace_grant_host_mapping(unsigned
>          XFREE((gt)->arch.status_gfn);                                    \
>      } while ( 0 )
>  
> -#define gnttab_set_frame_gfn(gt, st, idx, gfn)                           \
> -    do {                                                                 \
> -        ((st) ? (gt)->arch.status_gfn : (gt)->arch.shared_gfn)[idx] =    \
> -            (gfn);                                                       \
> -    } while ( 0 )
> +#define gnttab_set_frame_gfn(gt, st, idx, gfn, mfn)                      \
> +    ({                                                                   \
> +        int rc_ = 0;                                                     \
> +        gfn_t ogfn = gnttab_get_frame_gfn(gt, st, idx);                  \

Newline maybe? Not sure whether we decided that macros should also
follow coding style regarding variable definition followed by newline.

> +        if ( gfn_eq(ogfn, INVALID_GFN) || gfn_eq(ogfn, gfn) ||           \

I'm slightly confused by this checks, don't you need to check for
gfn_eq(gfn, INVALID_GFN) (not ogfn) in order to call
guest_physmap_remove_page?

Or assuming that ogfn is not invalid can be used to imply a removal?

Also the check for ogfn == gfn is only used on Arm, while I would
assume a similar one would be needed on x86 to guarantee the same
behavior?

Thanks, Roger.