From mboxrd@z Thu Jan 1 00:00:00 1970 From: Vegard Nossum Subject: net: uninitialized loopback addr leaks to userspace Date: Sat, 30 May 2009 22:23:24 +0200 Message-ID: <19f34abd0905301323k1498ca3fv31b271de65d60afc@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Ingo Molnar , Pekka Enberg , LKML To: Linux Netdev List Return-path: Received: from mail-fx0-f168.google.com ([209.85.220.168]:50800 "EHLO mail-fx0-f168.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753531AbZE3UXX (ORCPT ); Sat, 30 May 2009 16:23:23 -0400 Sender: netdev-owner@vger.kernel.org List-ID: Hi, It seems that loopback's hardware address is never initialized by the kernel. So if userspace attempts to read this address before it has been set, the kernel will return some uninitialized data (only 6 bytes, though). This can be demonstrated by creating a new network namespace (CLONE_NEWNET), which creates a new loopback device, then call ioctl() with SIOCGIFHWADDR on "lo". If this is done in a loop, with some background load, or by running multiple instances, random data will start to show up in the returned address. [ 406.750329] WARNING: kmemcheck: Caught 16-bit read from uninitialized memory (ffff880007220974) [ 406.753555] 18a2d7060088ffff18a2d7060088ffff00000000010000000100000003000000 [ 406.758862] i i i i i i i i i i i i i i i i i u u u u u u u u u u u u u u u [ 406.766224] ^ [ 406.768792] Modules linked in: [ 406.770416] Pid: 757, comm: ifconfig Not tainted 2.6.30-rc7-next-20090529 #404 [ 406.772876] RIP: 0010:[] [] dev_ioctl+0x5d9/0x600 [ 406.804677] [] sock_ioctl+0x95/0x2a0 [ 406.807242] [] vfs_ioctl+0x1b/0x70 [ 406.809348] [] do_vfs_ioctl+0x8a/0x570 [ 406.811419] [] sys_ioctl+0x99/0xa0 [ 406.813400] [] dev_ifsioc+0x81/0x2f0 [ 406.815424] [] compat_sys_ioctl+0xed/0x3c0 [ 406.817596] [] cstar_dispatch+0x7/0x26 [ 406.819978] [] 0xffffffffffffffff This is the code that triggers the warning, in net/core/dev.c, around line 4150: memcpy(ifr->ifr_hwaddr.sa_data, dev->dev_addr, min(sizeof ifr->ifr_hwaddr.sa_data, (size_t) dev->addr_len)); So it's dev->dev_addr that is the pointer to the uninitialized data. I didn't know how to fix it. Vegard