Date:   Wed, 20 Nov 2019 21:49:48 +0100
From:   Michal Kubecek <mkubecek@...e.cz>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        netdev <netdev@...r.kernel.org>, Firo Yang <firo.yang@...e.com>
Subject: Re: possible race in __inet_lookup_established()

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 <mkubecek@...e.cz> 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 <mkubecek@...e.cz> 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.

Michal

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