[Xen-devel] [PATCH v2] xen/balloon: Fix mapping PG_offline pages to user space

[David Hildenbrand <david@xxxxxxxxxx>]

The XEN balloon driver - in contrast to other balloon drivers - allows
to map some inflated pages to user space. Such pages are allocated via
alloc_xenballooned_pages() and freed via free_xenballooned_pages().
The pfn space of these allocated pages is used to map other things
by the hypervisor using hypercalls.

Pages marked with PG_offline must never be mapped to user space (as
this page type uses the mapcount field of struct pages).

So what we can do is, clear/set PG_offline when allocating/freeing an
inflated pages. This way, most inflated pages can be excluded by
dumping tools and the "reused for other purpose" balloon pages are
correctly not marked as PG_offline.

Fixes: 77c4adf6a6df (xen/balloon: mark inflated pages PG_offline)
Reported-by: Julien Grall <julien.grall@xxxxxxx>
Tested-by: Julien Grall <julien.grall@xxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---

v1 -> v2:
- Readd the braces dropped by accident :)


 drivers/xen/balloon.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
index 39b229f9e256..d37dd5bb7a8f 100644
--- a/drivers/xen/balloon.c
+++ b/drivers/xen/balloon.c
@@ -604,6 +604,7 @@ int alloc_xenballooned_pages(int nr_pages, struct page 
**pages)
        while (pgno < nr_pages) {
                page = balloon_retrieve(true);
                if (page) {
+                       __ClearPageOffline(page);
                        pages[pgno++] = page;
 #ifdef CONFIG_XEN_HAVE_PVMMU
                        /*
@@ -645,8 +646,10 @@ void free_xenballooned_pages(int nr_pages, struct page 
**pages)
        mutex_lock(&balloon_mutex);
 
        for (i = 0; i < nr_pages; i++) {
-               if (pages[i])
+               if (pages[i]) {
+                       __SetPageOffline(pages[i]);
                        balloon_append(pages[i]);
+               }
        }
 
        balloon_stats.target_unpopulated -= nr_pages;
-- 
2.17.2


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