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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Wed, Sep 22, 2021 at 11:42:30AM +0200, Jan Beulich wrote:
> On 22.09.2021 11:26, Roger Pau Monné wrote:
> > On Tue, Sep 21, 2021 at 12:12:05PM +0200, Jan Beulich wrote:
> >> On 21.09.2021 10:32, Roger Pau Monné wrote:
> >>> On Mon, Sep 20, 2021 at 05:27:17PM +0200, Jan Beulich wrote:
> >>>> On 20.09.2021 12:20, Roger Pau Monné wrote:
> >>>>> On Mon, Sep 13, 2021 at 08:41:47AM +0200, Jan Beulich wrote:
> >>>>>> --- a/xen/include/asm-arm/grant_table.h
> >>>>>> +++ b/xen/include/asm-arm/grant_table.h
> >>>>>> +        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?
> >>>>
> >>>> Why? It's ogfn which gets passed to the function. And it indeed is the
> >>>> prior GFN's mapping that we want to remove here.
> >>>>
> >>>>> Or assuming that ogfn is not invalid can be used to imply a removal?
> >>>>
> >>>> That implication can be (and on x86 is) used for the incoming argument,
> >>>> i.e. "gfn". I don't think "ogfn" can serve this purpose.
> >>>
> >>> I guess I'm confused due to the ogfn checks done on the Arm side that
> >>> are not performed on x86. So on Arm you always need to explicitly
> >>> unhook the previous GFN before attempting to setup a new mapping,
> >>> while on x86 you only need to do this when it's a removal in order to
> >>> clear the entry?
> >>
> >> The difference isn't with guest_physmap_add_entry() (both x86 and
> >> Arm only insert a new mapping there), but with
> >> xenmem_add_to_physmap_one(): Arm's variant doesn't care about prior
> >> mappings. And gnttab_map_frame() gets called only from there. This
> >> is effectively what the first paragraph of the description is about.
> > 
> > OK, sorry, it wasn't clear to me from the description. Could you
> > explicitly mention in the description that the removal is moved into
> > gnttab_set_frame_gfn on Arm in order to cope with the fact that
> > xenmem_add_to_physmap_one doesn't perform it.
> 
> Well, it's not really "in order to cope" - that's true for the placement
> prior to this change as well, so not a justification for the change.
> Nevertheless I've tried to make this more clear by changing the 1st
> paragraph to:
> 
> "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(); it cannot be dropped there since
>  xenmem_add_to_physmap_one() doesn't call it either (unlike on x86). 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."

Thanks, this is indeed much clearer IMO:

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Albeit I still think we need to fix Arm side to do the removal in
xenmem_add_to_physmap_one (or the x86 side to not do it).

Thanks, Roger.