[precise 1/1] UBUNTU: SAUCE: ensure that upper/lower layers are valid before checking permissions

[Andy Whitcroft]

When removing a directory which was only on the lower layer and was empty
on that lower layer we will attempt to confirm we are permitted to write
to the upper layer when we have no upper layer.  Leading to a panic.

  [10531.508838] BUG: unable to handle kernel NULL pointer dereference at 0000000000000030
  [10531.508889] IP: [<ffffffffa08bed80>] ovl_dentry_root_may+0x30/0x60 [overlayfs]

BugLink: http://bugs.launchpad.net/bugs/1465998
Signed-off-by: Andy Whitcroft <apw at canonical.com>
---
 fs/overlayfs/readdir.c | 34 ++++++++++++++++++++--------------
 1 file changed, 20 insertions(+), 14 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index cd55d2d..2dd897a 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -322,13 +322,16 @@ static int ovl_readdir(struct file *file, void *buf, filldir_t filler)
 		ovl_path_lower(file->f_path.dentry, &lowerpath);
 		ovl_path_upper(file->f_path.dentry, &upperpath);
 
-		res = ovl_dentry_root_may(file->f_path.dentry, &upperpath, MAY_READ);
-		if (res)
-			return res;
-		res = ovl_dentry_root_may(file->f_path.dentry, &lowerpath, MAY_READ);
-		if (res)
-			return res;
-
+		if (upperpath.dentry) {
+			res = ovl_dentry_root_may(file->f_path.dentry, &upperpath, MAY_READ);
+			if (res)
+				return res;
+		}
+		if (lowerpath.dentry) {
+			res = ovl_dentry_root_may(file->f_path.dentry, &lowerpath, MAY_READ);
+			if (res)
+				return res;
+		}
 		res = ovl_dir_read_merged(&upperpath, &lowerpath, &rdd);
 		if (res) {
 			ovl_cache_free(rdd.list);
@@ -479,13 +482,16 @@ static int ovl_check_empty_dir(struct dentry *dentry, struct list_head *list)
 	ovl_path_upper(dentry, &upperpath);
 	ovl_path_lower(dentry, &lowerpath);
 
-	err = ovl_dentry_root_may(dentry, &upperpath, MAY_READ);
-	if (err)
-		return err;
-	err = ovl_dentry_root_may(dentry, &lowerpath, MAY_READ);
-	if (err)
-		return err;
-
+	if (upperpath.dentry) {
+		err = ovl_dentry_root_may(dentry, &upperpath, MAY_READ);
+		if (err)
+			return err;
+	}
+	if (lowerpath.dentry) {
+		err = ovl_dentry_root_may(dentry, &lowerpath, MAY_READ);
+		if (err)
+			return err;
+	}
 	err = ovl_dir_read_merged(&upperpath, &lowerpath, &rdd);
 	if (err)
 		return err;
-- 
2.1.4