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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.09.2021 12:28, Julien Grall wrote:
> 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?

For the purpose of just XENMAPSPACE_grant_table the "restricted" M2P
(to just grant table pages), as introduced by an on-list patch, would
do afaict. That wouldn't remove all the differences between the two
implementations, but at least the one affecting the difference in how
gnttab_map_frame() needs to behave.

Jan