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

[Julien Grall <julien@xxxxxxx>]

Hi,

On 22/09/2021 14:42, 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."
>
> > TBH I think it would be in our best interest to try to make
> > xenmem_add_to_physmap_one behave as close as possible between arches.
> > This discrepancy between x86 and Arm regarding page removal is just
> > going to bring more trouble in the long term, and hiding the
> > differences inside gnttab_set_frame_gfn just makes it even more
> > obscure.
> Stefano, Julien?
This would be ideal as I don't particular like the approach taken in 
this patch. But AFAICT, this would require us to implement an M2P. Or is 
there another way to do it?
In another context, I saw the suggestion to bring an M2P on Arm. But I 
am still somewhat split whether this is really worth it for the current use.
Cheers,

--
Julien Grall