From mboxrd@z Thu Jan 1 00:00:00 1970 From: Cong Wang Subject: Re: [Patch net] rds: mark bound socket with SOCK_RCU_FREE Date: Mon, 10 Sep 2018 16:51:11 -0700 Message-ID: References: <20180910222422.19470-1-xiyou.wangcong@gmail.com> <20180910233007.GJ4668@oracle.com> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Cc: Santosh Shilimkar , Linux Kernel Network Developers , rds-devel@oss.oracle.com To: Sowmini Varadhan Return-path: Received: from mail-pl1-f195.google.com ([209.85.214.195]:35960 "EHLO mail-pl1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725945AbeIKEru (ORCPT ); Tue, 11 Sep 2018 00:47:50 -0400 Received: by mail-pl1-f195.google.com with SMTP id p5-v6so2986804plk.3 for ; Mon, 10 Sep 2018 16:51:23 -0700 (PDT) In-Reply-To: <20180910233007.GJ4668@oracle.com> Sender: netdev-owner@vger.kernel.org List-ID: On Mon, Sep 10, 2018 at 4:30 PM Sowmini Varadhan wrote: > > On (09/10/18 15:43), Santosh Shilimkar wrote: > > On 9/10/2018 3:24 PM, Cong Wang wrote: > > >When a rds sock is bound, it is inserted into the bind_hash_table > > >which is protected by RCU. But when releasing rd sock, after it > > >is removed from this hash table, it is freed immediately without > > >respecting RCU grace period. This could cause some use-after-free > > >as reported by syzbot. > > > > > Indeed. > > > > >Mark the rds sock as SOCK_RCU_FREE before inserting it into the > > >bind_hash_table, so that it would be always freed after a RCU grace > > >period. > > So I'm not sure I understand. > > Yes, Cong's fix may eliminate *some* of the syzbot failures, but the > basic problem is not solved. > > To take one example of possible races (one that was discussed in > https://www.spinics.net/lists/netdev/msg475074.html) > rds_recv_incoming->rds_find_bound is being called in rds_send_worker > context and the rds_find_bound code is > > 63 rs = rhashtable_lookup_fast(&bind_hash_table, &key, ht_parms); > 64 if (rs && !sock_flag(rds_rs_to_sk(rs), SOCK_DEAD)) > 65 rds_sock_addref(rs); > 66 else > 67 rs = NULL; > 68 > > After we find an rs at line 63, how can we be sure that the entire > logic of rds_release does not execute on another cpu, and free the rs, > before we hit line 64 with the bad rs? This is a different problem. The RCU grace period should be just extended: diff --git a/net/rds/bind.c b/net/rds/bind.c index 3ab55784b637..fa7592d0760c 100644 --- a/net/rds/bind.c +++ b/net/rds/bind.c @@ -76,11 +76,13 @@ struct rds_sock *rds_find_bound(const struct in6_addr *addr, __be16 port, struct rds_sock *rs; __rds_create_bind_key(key, addr, port, scope_id); - rs = rhashtable_lookup_fast(&bind_hash_table, key, ht_parms); + rcu_read_lock(); + rs = rhashtable_lookup(&bind_hash_table, key, ht_parms); if (rs && !sock_flag(rds_rs_to_sk(rs), SOCK_DEAD)) rds_sock_addref(rs); else rs = NULL; + rcu_read_unlock(); rdsdebug("returning rs %p for %pI6c:%u\n", rs, addr, ntohs(port)); > > Normally synchronize_rcu() or synchronize_net() in rds_release() would > ensure this. How do we ensure this with SOCK_RCU_FREE (or is the > intention to just reduce *some* of the syzbot failures)? Although sock release path is not a very hot path, but blocking it isn't a good idea either, especially when you can use call_rcu(), which has the same effect. I don't see any reason we should prefer synchronize_rcu() here.