fuse: listxattr: verify xattr list

Make sure userspace filesystem is returning a well formed list of xattr
names (zero or more nonzero length, null terminated strings).

[Michael Theall: only verify in the nonzero size case]
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>

fs/fuse/dir.c

@@ -1801,6 +1801,23 @@ static ssize_t fuse_getxattr(struct dentry *entry, struct inode *inode,
 	return ret;
 }
 
+static int fuse_verify_xattr_list(char *list, size_t size)
+{
+	size_t origsize = size;
+
+	while (size) {
+		size_t thislen = strnlen(list, size);
+
+		if (!thislen || thislen == size)
+			return -EIO;
+
+		size -= thislen + 1;
+		list += thislen + 1;
+	}
+
+	return origsize;
+}
+
 static ssize_t fuse_listxattr(struct dentry *entry, char *list, size_t size)
 {
 	struct inode *inode = d_inode(entry);
@@ -1836,6 +1853,8 @@ static ssize_t fuse_listxattr(struct dentry *entry, char *list, size_t size)
 	ret = fuse_simple_request(fc, &args);
 	if (!ret && !size)
 		ret = outarg.size;
+	if (ret > 0 && size)
+		ret = fuse_verify_xattr_list(list, ret);
 	if (ret == -ENOSYS) {
 		fc->no_listxattr = 1;
 		ret = -EOPNOTSUPP;