Re: [Xen-devel] [PATCH 2/3] IOMMU/x86: make page type checks consistent when mapping pages

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.03.19 at 14:50, <jgross@xxxxxxxx> wrote:
> On 05/03/2019 14:26, Jan Beulich wrote:
>> There are currently three more or less different checks:
>> - _get_page_type() adjusts the IOMMU mappings according to the new type
>>   alone,
>> - arch_iommu_populate_page_table() wants just the type to be
>>   PGT_writable_page,
>> - iommu_hwdom_init() additionally permits all other types with a type
>>   refcount of zero.
>> The canonical one is in _get_page_type(), and as of XSA-288
>> arch_iommu_populate_page_table() also has no need anymore to deal with
>> PGT_none pages. In the PV Dom0 case, however, PGT_none pages are still
>> necessary to consider, since in that case pages don't get handed to
>> guest_physmap_add_entry(). Furthermore, the function so far also
>> established r/o mappings, which is not in line with the rules set forth
>> by the XSA-288 change.
>> 
>> For arch_iommu_populate_page_table() to not encounter PGT_none pages
>> anymore even in cases where the IOMMU gets enabled for a domain only
>> when it is already running, replace the IOMMU dependency in
>> guest_physmap_add_entry()'s handling of PV guests to check just the
>> system wide state instead of the domain property.
>> 
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> I'm unable to decide whether this patch should make it into 4.12 or not
> with the given information. What happens without this patch (worst
> case)?

I don't think anything really bad can happen, or else this would have
been part of one of the XSAs. The patch is bringing things in line with
what XSA-288 did, without it being obvious what bad could result
from not doing so. The larger part of change here is for hwdom only
anyway.

Jan



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