ovl: During copy up, first copy up data and then xattrs

If a file with capability set (and hence security.capability xattr) is
written kernel clears security.capability xattr. For overlay, during file
copy up if xattrs are copied up first and then data is, copied up. This
means data copy up will result in clearing of security.capability xattr
file on lower has. And this can result into surprises. If a lower file has
CAP_SETUID, then it should not be cleared over copy up (if nothing was
actually written to file).

This also creates problems with chown logic where it first copies up file
and then tries to clear setuid bit. But by that time security.capability
xattr is already gone (due to data copy up), and caller gets -ENODATA.
This has been reported by Giuseppe here.

https://github.com/containers/libpod/issues/2015#issuecomment-447824842

Fix this by copying up data first and then metadta. This is a regression
which has been introduced by my commit as part of metadata only copy up
patches.

TODO: There will be some corner cases where a file is copied up metadata
only and later data copy up happens and that will clear security.capability
xattr. Something needs to be done about that too.

Fixes: bd64e575 ("ovl: During copy up, first copy up metadata and then data")
Cc: <stable@vger.kernel.org> # v4.19+
Reported-by: Giuseppe Scrivano <gscrivan@redhat.com>
Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/overlayfs/copy_up.c

@@ -443,6 +443,24 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 {
 	int err;
 
+	/*
+	 * Copy up data first and then xattrs. Writing data after
+	 * xattrs will remove security.capability xattr automatically.
+	 */
+	if (S_ISREG(c->stat.mode) && !c->metacopy) {
+		struct path upperpath, datapath;
+
+		ovl_path_upper(c->dentry, &upperpath);
+		if (WARN_ON(upperpath.dentry != NULL))
+			return -EIO;
+		upperpath.dentry = temp;
+
+		ovl_path_lowerdata(c->dentry, &datapath);
+		err = ovl_copy_up_data(&datapath, &upperpath, c->stat.size);
+		if (err)
+			return err;
+	}
+
 	err = ovl_copy_xattr(c->lowerpath.dentry, temp);
 	if (err)
 		return err;
@@ -460,19 +478,6 @@ static int ovl_copy_up_inode(struct ovl_copy_up_ctx *c, struct dentry *temp)
 			return err;
 	}
 
-	if (S_ISREG(c->stat.mode) && !c->metacopy) {
-		struct path upperpath, datapath;
-
-		ovl_path_upper(c->dentry, &upperpath);
-		BUG_ON(upperpath.dentry != NULL);
-		upperpath.dentry = temp;
-
-		ovl_path_lowerdata(c->dentry, &datapath);
-		err = ovl_copy_up_data(&datapath, &upperpath, c->stat.size);
-		if (err)
-			return err;
-	}
-
 	if (c->metacopy) {
 		err = ovl_check_setxattr(c->dentry, temp, OVL_XATTR_METACOPY,
 					 NULL, 0, -EOPNOTSUPP);