kaiser: kaiser_remove_mapping() move along the pgd

When removing the bogus comment from kaiser_remove_mapping(),
I really ought to have checked the extent of its bogosity: as
Neel points out, there is nothing to stop unmap_pud_range_nofree()
from continuing beyond the end of a pud (and starting in the wrong
position on the next).

Fix kaiser_remove_mapping() to constrain the extent and advance pgd
pointer correctly: use pgd_addr_end() macro as used throughout base
mm (but don't assume page-rounded start and size in this case).

But this bug was very unlikely to trigger in this backport: since
any buddy allocation is contained within a single pud extent, and
we are not using vmapped stacks (and are only mapping one page of
stack anyway): the only way to hit this bug here would be when
freeing a large modified ldt.

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/mm/kaiser.c

@@ -297,11 +297,13 @@ void kaiser_remove_mapping(unsigned long start, unsigned long size)
 	extern void unmap_pud_range_nofree(pgd_t *pgd,
 				unsigned long start, unsigned long end);
 	unsigned long end = start + size;
-	unsigned long addr;
+	unsigned long addr, next;
+	pgd_t *pgd;
 
-	for (addr = start; addr < end; addr += PGDIR_SIZE) {
-		pgd_t *pgd = native_get_shadow_pgd(pgd_offset_k(addr));
-		unmap_pud_range_nofree(pgd, addr, end);
+	pgd = native_get_shadow_pgd(pgd_offset_k(start));
+	for (addr = start; addr < end; pgd++, addr = next) {
+		next = pgd_addr_end(addr, end);
+		unmap_pud_range_nofree(pgd, addr, next);
 	}
 }