x86/mm: Adjust linear uses / entries when a page loses validation

x86/mm: Adjust linear uses / entries when a page loses validation

"Linear pagetables" is a technique which involves either pointing a
pagetable at itself, or to another pagetable the same or higher level.
Xen has limited support for linear pagetables: A page may either point
to itself, or point to another page of the same level (i.e., L2 to L2,
L3 to L3, and so on).

XSA-240 introduced an additional restriction that limited the "depth"
of such chains by allowing pages to either *point to* other pages of
the same level, or *be pointed to* by other pages of the same level,
but not both.  To implement this, we keep track of the number of
outstanding times a page points to or is pointed to another page
table, to prevent both from happening at the same time.

Additionally, XSA-299 introduced a mode whereby if a page was known to
have been only partially validated, _put_page_type() would be called
with PTF_partial_set, indicating that if the page had been
de-validated by someone else, the type count should be left alone.

Unfortunately, this change did not account for the required accounting
for linear page table uses and entries; in the case that a previously
partially-devalidated pagetable was fully-devalidated by someone else,
the linear_pt_counts are not updated.

This could happen in one of two places:

1. In the case a partially-devalidated page was re-validated by
someone else

2. During domain tear-down, when pages are force-invalidated while
leaving the type count intact.

The second could be ignored, since at that point the pages can no
longer be abused; but the first requires handling.  Note however that
this would not be a security issue: having the counts be too high is
overly strict (i.e., will prevent a page from being used in a way
which is perfectly safe), but shouldn't cause any other issues.

Fix this by adjusting the linear counts when a page loses validation,
regardless of whether the de-validation completed or was only partial.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-acked-by: Juergen Gross <jgross@suse.com>