Revert "ovl: relax permission checking on underlying layers"

This reverts commit 007ea448.

The commit broke some selinux-testsuite cases, and it looks like there's no
straightforward fix keeping the direction of this patch, so revert for now.

The original patch was trying to fix the consistency of permission checks, and
not an observed bug.  So reverting should be safe.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/inode.c

@@ -286,22 +286,13 @@ int ovl_permission(struct inode *inode, int mask)
 	if (err)
 		return err;
 
-	/* No need to do any access on underlying for special files */
-	if (special_file(realinode->i_mode))
-		return 0;
-
-	/* No need to access underlying for execute */
-	mask &= ~MAY_EXEC;
-	if ((mask & (MAY_READ | MAY_WRITE)) == 0)
-		return 0;
-
-	/* Lower files get copied up, so turn write access into read */
-	if (!upperinode && mask & MAY_WRITE) {
+	old_cred = ovl_override_creds(inode->i_sb);
+	if (!upperinode &&
+	    !special_file(realinode->i_mode) && mask & MAY_WRITE) {
 		mask &= ~(MAY_WRITE | MAY_APPEND);
+		/* Make sure mounter can read file for copy up later */
 		mask |= MAY_READ;
 	}
-
-	old_cred = ovl_override_creds(inode->i_sb);
 	err = inode_permission(realinode, mask);
 	revert_creds(old_cred);