clone: fix directory traversal

This was introduced in the initial version of this code, way back when
in 2008.

$ curl http://127.0.0.1/cgit/repo/objects/?path=../../../../../../../../../etc/passwd
root❌0:0:root:/root:/bin/sh
...

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reported-by: Jann Horn <jannh@google.com>

ui-clone.c

@@ -92,17 +92,32 @@ void cgit_clone_info(void)
 
 void cgit_clone_objects(void)
 {
-	if (!ctx.qry.path) {
-		cgit_print_error_page(400, "Bad request", "Bad request");
-		return;
-	}
+	char *p;
+
+	if (!ctx.qry.path)
+		goto err;
 
 	if (!strcmp(ctx.qry.path, "info/packs")) {
 		print_pack_info();
 		return;
 	}
 
+	/* Avoid directory traversal by forbidding "..", but also work around
+	 * other funny business by just specifying a fairly strict format. For
+	 * example, now we don't have to stress out about the Cygwin port.
+	 */
+	for (p = ctx.qry.path; *p; ++p) {
+		if (*p == '.' && *(p + 1) == '.')
+			goto err;
+		if (!isalnum(*p) && *p != '/' && *p != '.' && *p != '-')
+			goto err;
+	}
+
 	send_file(git_path("objects/%s", ctx.qry.path));
+	return;
+
+err:
+	cgit_print_error_page(400, "Bad request", "Bad request");
 }
 
 void cgit_clone_head(void)