[Unionfs] bug allowing user to escalate privileges

[Erez Zadok]

Please try this small patch.  It should fix the problem.

Thanks,
Erez.

Unionfs: prevent a privilege escalation during first copyup

Signed-off-by: Erez Zadok <ezk at cs.sunysb.edu>
diff --git a/fs/unionfs/inode.c b/fs/unionfs/inode.c
index 0bd9fab..ce87887 100644
--- a/fs/unionfs/inode.c
+++ b/fs/unionfs/inode.c
@@ -795,10 +795,11 @@ static int unionfs_permission(struct inode *inode, int mask)
 		 * We check basic permissions, but we ignore any conditions
 		 * such as readonly file systems or branches marked as
 		 * readonly, because those conditions should lead to a
-		 * copyup taking place later on.
+		 * copyup taking place later on.  However, if user never had
+		 * access to the file, then no copyup could ever take place.
 		 */
 		err = inode_permission(lower_inode, mask);
-		if (err && bindex > 0) {
+		if (err && err != -EACCES && bindex > 0) {
 			umode_t mode = lower_inode->i_mode;
 			if (is_robranch_super(inode->i_sb, bindex) &&
 			    (S_ISREG(mode) || S_ISDIR(mode) || S_ISLNK(mode)))