xen/blkfront: fix leaking data in shared pages

When allocating pages to be used for shared communication with the
backend always zero them, this avoids leaking unintended data present
on the pages.

This is CVE-2022-26365, part of XSA-403.
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/block/xen-blkfront.c

@@ -311,7 +311,7 @@ static int fill_grant_buffer(struct blkfront_ring_info *rinfo, int num)
 			goto out_of_memory;
 
 		if (info->feature_persistent) {
-			granted_page = alloc_page(GFP_NOIO);
+			granted_page = alloc_page(GFP_NOIO | __GFP_ZERO);
 			if (!granted_page) {
 				kfree(gnt_list_entry);
 				goto out_of_memory;
@@ -2183,7 +2183,8 @@ static int blkfront_setup_indirect(struct blkfront_ring_info *rinfo)
 
 		BUG_ON(!list_empty(&rinfo->indirect_pages));
 		for (i = 0; i < num; i++) {
-			struct page *indirect_page = alloc_page(GFP_KERNEL);
+			struct page *indirect_page = alloc_page(GFP_KERNEL |
+								__GFP_ZERO);
 			if (!indirect_page)
 				goto out_of_memory;
 			list_add(&indirect_page->lru, &rinfo->indirect_pages);