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

[Julien Grall <julien@xen.org>]

Hi Jan,

On 22/09/2021 15:47, Jan Beulich wrote:
> 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.

This would work. I will add it in my todo list but I am not such when I 
will have time to look at it. I am happy if someone else wants to look 
at it.

Cheers,

-- 
Julien Grall