[PATCH 4.9 47/49] xen-blkback: fix error handling in xen_blkbk_map()

[Greg Kroah-Hartman]

From: Jan Beulich <jbeulich@xxxxxxxx>

commit 871997bc9e423f05c7da7c9178e62dde5df2a7f8 upstream.

The function uses a goto-based loop, which may lead to an earlier error
getting discarded by a later iteration. Exit this ad-hoc loop when an
error was encountered.

The out-of-memory error path additionally fails to fill a structure
field looked at by xen_blkbk_unmap_prepare() before inspecting the
handle which does get properly set (to BLKBACK_INVALID_HANDLE).

Since the earlier exiting from the ad-hoc loop requires the same field
filling (invalidation) as that on the out-of-memory path, fold both
paths. While doing so, drop the pr_alert(), as extra log messages aren't
going to help the situation (the kernel will log oom conditions already
anyway).

This is XSA-365.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
Reviewed-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Julien Grall <julien@xxxxxxx>
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 drivers/block/xen-blkback/blkback.c |   24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -843,8 +843,11 @@ again:
 			pages[i]->page = persistent_gnt->page;
 			pages[i]->persistent_gnt = persistent_gnt;
 		} else {
-			if (get_free_page(ring, &pages[i]->page))
-				goto out_of_memory;
+			if (get_free_page(ring, &pages[i]->page)) {
+				put_free_pages(ring, pages_to_gnt, segs_to_map);
+				ret = -ENOMEM;
+				goto out;
+			}
 			addr = vaddr(pages[i]->page);
 			pages_to_gnt[segs_to_map] = pages[i]->page;
 			pages[i]->persistent_gnt = NULL;
@@ -928,17 +931,18 @@ next:
 	}
 	segs_to_map = 0;
 	last_map = map_until;
-	if (map_until != num)
+	if (!ret && map_until != num)
 		goto again;
 
-	return ret;
-
-out_of_memory:
-	pr_alert("%s: out of memory\n", __func__);
-	put_free_pages(ring, pages_to_gnt, segs_to_map);
-	for (i = last_map; i < num; i++)
+out:
+	for (i = last_map; i < num; i++) {
+		/* Don't zap current batch's valid persistent grants. */
+		if(i >= last_map + segs_to_map)
+			pages[i]->persistent_gnt = NULL;
 		pages[i]->handle = BLKBACK_INVALID_HANDLE;
-	return -ENOMEM;
+	}
+
+	return ret;
 }
 
 static int xen_blkbk_map_seg(struct pending_req *pending_req)