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Wed, 20 Nov 2019 12:58:00 -0800 (PST) MIME-Version: 1.0 References: <20191120083919.GH27852@unicorn.suse.cz> <20191120181046.GA29650@unicorn.suse.cz> <20191120195226.GB29650@unicorn.suse.cz> <20191120204948.GC29650@unicorn.suse.cz> In-Reply-To: <20191120204948.GC29650@unicorn.suse.cz> From: Eric Dumazet Date: Wed, 20 Nov 2019 12:57:48 -0800 Message-ID: Subject: Re: possible race in __inet_lookup_established() To: Michal Kubecek Cc: Eric Dumazet , netdev , Firo Yang Content-Type: text/plain; charset="UTF-8" Sender: netdev-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org On Wed, Nov 20, 2019 at 12:49 PM Michal Kubecek wrote: > > On Wed, Nov 20, 2019 at 12:04:53PM -0800, Eric Dumazet wrote: > > > > > > On 11/20/19 11:52 AM, Michal Kubecek wrote: > > > On Wed, Nov 20, 2019 at 11:13:09AM -0800, Eric Dumazet wrote: > > >> On Wed, Nov 20, 2019 at 10:10 AM Michal Kubecek wrote: > > >>> > > >>> On Wed, Nov 20, 2019 at 08:12:10AM -0800, Eric Dumazet wrote: > > >>>> On Wed, Nov 20, 2019 at 12:39 AM Michal Kubecek wrote: > > >>>> > > >>>>> Hello Eric, > > >>>>> > > >>>>> we are investigating a crash in socket lookup in a distribution kernel > > >>>>> based on v4.12 but the possible problem we found seems to also apply to > > >>>>> current mainline (or net) code. > > >>>>> > > >>>>> The common pattern is: > > >>>>> > > >>>>> - the crash always happens in __inet_lookup_established() in > > >>>>> > > >>>>> sk_nulls_for_each_rcu(sk, node, &head->chain) { > > >>>>> if (sk->sk_hash != hash) <----------------- > > >>>>> continue; > > >>>>> > > >>>>> as sk is an invalid pointer; in particular, &sk->sk_nulls_node is null > > >>>>> so dereferencing sk->sk_hash faults > > >>>>> > > >>>>> - the reason is that previous sk value pointed to a listening socket > > >>>>> rather than an established one; as listening socket uses sk_node, end > > >>>>> of the chain is marked by a null pointer which is not detected as > > >>>>> a chain end by sk_nulls_for_each_rcu() > > >>>>> > > >>>>> - there is no socket matching skb which is a TCP pure ACK having > > >>>>> 127.0.0.1 as both source and destination > > >>>>> > > >>>>> - the chain pointed to by head variable is empty > > >>>>> > > >>>>> Firo Yang came with the theory that this could be a race between socket > > >>>>> lookup and freing the socket and replacing it with a listening one: > > >>>>> > > >>>>> 1. CPU A gets a pointer to an established socket as sk in the > > >>>>> sk_nulls_for_each_rcu() loop in __inet_lookup_established() but does not > > >>>>> thake a reference to it. > > >>>>> > > >>>>> 2. CPU B frees the socket > > >>>>> > > >>>>> 3. Slab object pointed to by sk is reused for a new listening socket. > > >>>>> This socket has null sk->sk_node->next which uses the same spot as > > >>>>> sk->sk_nulls_node->next > > >>>>> > > >>>>> 4. CPU A tests sk->sk_nulls_node->next with is_a_nulls() (false) and > > >>>>> follows the pointer, resulting in a fault dereferencing sk->sk_hash. > > >>>>> > > >>>>> Unless we missed something, there is no protection against established > > >>>>> socket being freed and replaced by a new listening one while > > >>>>> __inet_lookup_established() has a pointer to it. The RCU loop only > > >>>>> prevents the slab object being reused for a different slab cache or > > >>>>> something completely different but as established and listening sockets > > >>>>> share the same slab cache, it does not protect us from switching from > > >>>>> established to listening. > > >>>>> > > >>>>> As far as I can say, this kind of race could have happened for quite > > >>>>> long but before your commit ou3b24d854cb35 ("tcp/dccp: do not touch > > >>>>> listener sk_refcnt under synflood"), the worst that could happen would > > >>>>> be switching to a chain in listener lookup table, following it to its > > >>>>> end and then (most likely) restarting the lookup or failing. Now that > > >>>>> established and listening sockets use different list types, replacing > > >>>>> one with the other can be deadly. > > >>>>> > > >>>>> Do you agree that this race is possible or is there something we missed > > >>>>> that would prevent it? > > >>>>> > > >>>> A listener is hashed on icsk_listen_portaddr_node, so I do not see how a > > >>>> listener could be found in the establish chain ? > > >>> > > >>> It is not really in the chain. What we suspect is that between sk is > > >>> assigned pointer to an established socket in __inet_lookup_established() > > >>> and using sk->sk_nulls_node->next to go to the next (or stop if it's odd > > >>> nulls value), this established socket could be freed and its slab object > > >>> reused for a listening socket. As listening sockets no longer use a > > >>> nulls hashlist but a normal hashlist, in the most common case where the > > >>> socket is last in the chain, sk->sk_node->next (which occupies the same > > >>> place as sk->sk_nulls_node->next) would be NULL so that is_a_nulls() > > >>> does not recognize the chain end and the loop would go on to next socket > > >>> in the chain. > > >>> > > >> > > >> I hear you, but where is the sk->sk_nulls_node->next would be set to > > >> NULL exactly ? > > > > > > In __inet_hash() when the new listening socket is inserted into the > > > listening hashtable: > > > > > > if (IS_ENABLED(CONFIG_IPV6) && sk->sk_reuseport && > > > sk->sk_family == AF_INET6) > > > hlist_add_tail_rcu(&sk->sk_node, &ilb->head); > > > else > > > hlist_add_head_rcu(&sk->sk_node, &ilb->head); > > > > > > If the chain is empty, sk->sk_node->next will be set to NULL by either > > > branch. And even if it's not, the loop in __inet_lookup_established() > > > would follow the chain from listening hashtable and still get to the > > > NULL end marker eventually. > > > > > > Oh right, I was confused by icsk_listen_portaddr_node, but listener use two > > hashes... > > > > Do you have a patch, or do you want me to work on a fix ? > > Firo suggested something like > > ------------------------------------------------------------------------ > --- a/net/ipv4/inet_hashtables.c > +++ b/net/ipv4/inet_hashtables.c > @@ -362,6 +362,8 @@ struct sock *__inet_lookup_established(struct net *net, > > begin: > sk_nulls_for_each_rcu(sk, node, &head->chain) { > + if (unlikely(!node)) > + goto begin; > if (sk->sk_hash != hash) > continue; > if (likely(INET_MATCH(sk, net, acookie, > ------------------------------------------------------------------------ > > It depends on implementation details but I believe it would work. It > would be nicer if we could detect the switch to a listening socket but > I don't see how to make such test race free without introducing > unacceptable performance penalty. No, we do not want to add more checks in the fast path really. I was more thinking about not breaking the RCU invariants. (ie : adding back the nulls stuff that I removed in 3b24d854cb35 ("tcp/dccp: do not touch listener sk_refcnt under synflood")