x86/mm: relinquish_memory: Grab an extra type ref when setting PGT_partial

x86/mm: relinquish_memory: Grab an extra type ref when setting PGT_partial

The PGT_partial bit in page->type_info holds both a type count and a
general ref count.  During domain tear-down, when free_page_type()
returns -ERESTART, relinquish_memory() correctly handles the general
ref count, but fails to grab an extra type count when setting
PGT_partial.  When this bit is eventually cleared, type_count underflows
and triggers the following BUG in page_alloc.c:free_domheap_pages():

    BUG_ON((pg[i].u.inuse.type_info & PGT_count_mask) != 0);

As far as we can tell, this page underflow cannot be exploited any any
other way: The page can't be used as a pagetable by the dying domain
because it's dying; it can't be used as a pagetable by any other
domain since it belongs to the dying domain; and ownership can't
transfer to any other domain without hitting the BUG_ON() in
free_domheap_pages().

(steal_page() won't work on a page in this state, since it requires
PGC_allocated to be set, and PGC_allocated will already have been
cleared.)

Fix this by grabbing an extra type ref if setting PGT_partial in
relinquish_memory.

This is part of XSA-310.

Signed-off-by: George Dunlap <george.dunlap@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>