Re: [Xen-devel] [PATCH v4] gnttab: don't expose host physical address without need

[Julien Grall <julien@xxxxxxx>]

Hi,

On 05/12/2019 15:34, Jan Beulich wrote:
> Translated domains shouldn't see host physical addresses. While the
> address is also not supposed to be handed back even to non-translated
> domains when GNTMAP_device_map is not set (as explicitly stated by a
> comment in the public header), PV kernels (Linux at least) assume the
> field to get populated nevertheless. (Similarly mapkind() should check
> only GNTMAP_device_map.)
>
> Along these lines split the paging mode related check near the top of
> map_grant_ref() to handle the "external" and "translated" cases
> separately (GNTMAP_device_map use getting tied to being non-translated
> rather than non-external).
>
> Still along these lines in the unmapping case there's no point checking
> ->dev_bus_addr when GNTMAP_device_map isn't set (and hence the field
> isn't going to be consumed).
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> v4: Re-base over dropped patches.
> v3: New.
>
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -967,10 +967,16 @@ map_grant_ref(
>       }
>   
>       if ( unlikely(paging_mode_external(ld) &&
> -                  (op->flags & (GNTMAP_device_map|GNTMAP_application_map|
> -                            GNTMAP_contains_pte))) )
> +                  (op->flags & (GNTMAP_application_map|GNTMAP_contains_pte))) )
>       {
> -        gdprintk(XENLOG_INFO, "No device mapping in HVM domain\n");
> +        gdprintk(XENLOG_INFO, "No app/pte mapping in HVM domain\n");
> +        op->status = GNTST_general_error;
> +        return;
> +    }
> +
> +    if ( paging_mode_translate(ld) && unlikely(op->flags & GNTMAP_device_map) )
There is at least one instance in Linux where GNTMAP_device_map may be 
given regardless the type of the guest. See dmabuf_exp_from_refs() in 
drivers/xen/gntdev-dmabuf.c.
How are you going to deal with them?

> +    {
> +        gdprintk(XENLOG_INFO, "No device mapping in translated domain\n");
>           op->status = GNTST_general_error;
>           return;
>       }
> @@ -1213,7 +1219,8 @@ map_grant_ref(
>       if ( need_iommu )
>           double_gt_unlock(lgt, rgt);
>   
> -    op->dev_bus_addr = mfn_to_maddr(mfn);
> +    op->dev_bus_addr = paging_mode_translate(ld) ? op->host_addr
> +                                                 : mfn_to_maddr(mfn);
The "host_addr" is pretty confusing. I first thought it would be a "Host 
Physical Address" but it seems to be a "Guest Physical address"
If so, this is going to break Linux Dom0 on Arm where it is expected to 
return the machine physical address to have a DMA fully working.
I don't abide with the current behavior on Arm, but I don't think we 
should break them when there are no replacement for it.
So it would be better if we look at a different approach (i.e a new flag 
or strict mode) in order to avoid breakage.
Cheers,

--
Julien Grall

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