From patchwork Thu Oct 13 16:52:15 2016 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Jiri Bohac X-Patchwork-Id: 681874 X-Patchwork-Delegate: davem@davemloft.net Return-Path: X-Original-To: patchwork-incoming@ozlabs.org Delivered-To: patchwork-incoming@ozlabs.org Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by ozlabs.org (Postfix) with ESMTP id 3svxcf6P7Yz9s8x for ; Fri, 14 Oct 2016 03:53:06 +1100 (AEDT) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933735AbcJMQxC (ORCPT ); Thu, 13 Oct 2016 12:53:02 -0400 Received: from mx2.suse.de ([195.135.220.15]:57214 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933409AbcJMQw5 (ORCPT ); Thu, 13 Oct 2016 12:52:57 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (charybdis-ext.suse.de [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id D465CADB2; Thu, 13 Oct 2016 16:52:15 +0000 (UTC) Date: Thu, 13 Oct 2016 18:52:15 +0200 From: Jiri Bohac To: "David S. Miller" , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy Cc: netdev@vger.kernel.org Subject: [PATCH 2/2] IPv6: fix DESYNC_FACTOR Message-ID: <20161013165215.52jxdlkh7v5octl7@dwarf.suse.cz> MIME-Version: 1.0 Content-Disposition: inline User-Agent: Mutt/1.6.2-neo (2016-06-11) Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org The IPv6 temporary address generation uses a variable called DESYNC_FACTOR to prevent hosts updating the addresses at the same time. Quoting RFC 4941: ... The value DESYNC_FACTOR is a random value (different for each client) that ensures that clients don't synchronize with each other and generate new addresses at exactly the same time ... DESYNC_FACTOR is defined as: DESYNC_FACTOR -- A random value within the range 0 - MAX_DESYNC_FACTOR. It is computed once at system start (rather than each time it is used) and must never be greater than (TEMP_VALID_LIFETIME - REGEN_ADVANCE). First, I believe the RFC has a typo in it and meant to say: "and must never be greater than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE)" The reason is that at various places in the RFC, DESYNC_FACTOR is used in a calculation like (TEMP_PREFERRED_LIFETIME - DESYNC_FACTOR) or (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE - DESYNC_FACTOR). It needs to be smaller than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE) for the result of these calculations to be larger than zero. It's never used in a calculation together with TEMP_VALID_LIFETIME. I already submitted an errata to the rfc-editor: https://www.rfc-editor.org/errata_search.php?rfc=4941 The Linux implementation of DESYNC_FACTOR is very wrong: max_desync_factor is used in places DESYNC_FACTOR should be used. max_desync_factor is initialized to the RFC-recommended value for MAX_DESYNC_FACTOR (600) but the whole point is to get a _random_ value. And nothing ensures that the value used is not greater than (TEMP_PREFERRED_LIFETIME - REGEN_ADVANCE), which leads to underflows. The effect can easily be observed when setting the temp_prefered_lft sysctl e.g. to 60. The preferred lifetime of the temporary addresses will be bogus. TEMP_PREFERRED_LIFETIME and REGEN_ADVANCE are not constants and can be influenced by these three sysctls: regen_max_retry, dad_transmits and temp_prefered_lft. Thus, the upper bound for desync_factor needs to be re-calculated each time a new address is generated and if desync_factor is larger than the new upper bound, a new random value needs to be re-generated. And since we already have max_desync_factor configurable per interface, we also need to calculate and store desync_factor per interface. Signed-off-by: Jiri Bohac --- include/net/if_inet6.h | 1 + net/ipv6/addrconf.c | 39 +++++++++++++++++++++++++++++++-------- 2 files changed, 32 insertions(+), 8 deletions(-) diff --git a/include/net/if_inet6.h b/include/net/if_inet6.h index ae70735..b0576cb 100644 --- a/include/net/if_inet6.h +++ b/include/net/if_inet6.h @@ -190,6 +190,7 @@ struct inet6_dev { __u32 if_flags; int dead; + u32 desync_factor; u8 rndid[8]; struct list_head tempaddr_list; diff --git a/net/ipv6/addrconf.c b/net/ipv6/addrconf.c index 63fc857..eca15f3 100644 --- a/net/ipv6/addrconf.c +++ b/net/ipv6/addrconf.c @@ -422,6 +422,7 @@ static struct inet6_dev *ipv6_add_dev(struct net_device *dev) #endif INIT_LIST_HEAD(&ndev->tempaddr_list); + ndev->desync_factor = U32_MAX; if ((dev->flags&IFF_LOOPBACK) || dev->type == ARPHRD_TUNNEL || dev->type == ARPHRD_TUNNEL6 || @@ -1183,6 +1184,7 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, struct inet6_ifaddr *i int ret = 0; u32 addr_flags; unsigned long now = jiffies; + long max_desync_factor; write_lock_bh(&idev->lock); if (ift) { @@ -1218,20 +1220,41 @@ static int ipv6_create_tempaddr(struct inet6_ifaddr *ifp, struct inet6_ifaddr *i ipv6_try_regen_rndid(idev, tmpaddr); memcpy(&addr.s6_addr[8], idev->rndid, 8); age = (now - ifp->tstamp) / HZ; + + regen_advance = idev->cnf.regen_max_retry * + idev->cnf.dad_transmits * + NEIGH_VAR(idev->nd_parms, RETRANS_TIME) / HZ; + + /* recalculate max_desync_factor each time and update + * idev->desync_factor if it's larger + */ + max_desync_factor = min_t(__u32, + idev->cnf.max_desync_factor, + idev->cnf.temp_prefered_lft - regen_advance); + + if (unlikely(idev->desync_factor > max_desync_factor)) { + if (max_desync_factor > 0) { + get_random_bytes(&idev->desync_factor, + sizeof(idev->desync_factor)); + idev->desync_factor %= max_desync_factor; + } else { + idev->desync_factor = 0; + } + } + tmp_valid_lft = min_t(__u32, ifp->valid_lft, idev->cnf.temp_valid_lft + age); - tmp_prefered_lft = min_t(__u32, - ifp->prefered_lft, - idev->cnf.temp_prefered_lft + age - - idev->cnf.max_desync_factor); + tmp_prefered_lft = idev->cnf.temp_prefered_lft + age - + idev->desync_factor; + /* guard against underflow in case of concurrent updates to cnf */ + if (unlikely(tmp_prefered_lft < 0)) + tmp_prefered_lft = 0; + tmp_prefered_lft = min_t(__u32, ifp->prefered_lft, tmp_prefered_lft); tmp_plen = ifp->prefix_len; tmp_tstamp = ifp->tstamp; spin_unlock_bh(&ifp->lock); - regen_advance = idev->cnf.regen_max_retry * - idev->cnf.dad_transmits * - NEIGH_VAR(idev->nd_parms, RETRANS_TIME) / HZ; write_unlock_bh(&idev->lock); /* A temporary address is created only if this calculated Preferred @@ -2316,7 +2339,7 @@ static void manage_tempaddrs(struct inet6_dev *idev, max_valid = 0; max_prefered = idev->cnf.temp_prefered_lft - - idev->cnf.max_desync_factor - age; + idev->desync_factor - age; if (max_prefered < 0) max_prefered = 0;