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Date:   Tue, 28 Feb 2017 09:58:51 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     network dev <netdev@...r.kernel.org>, linux-sctp@...r.kernel.org,
        davem <davem@...emloft.net>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        Vlad Yasevich <vyasevich@...il.com>,
        Andrey Konovalov <andreyknvl@...gle.com>
Subject: Re: [PATCH net] sctp: call rcu_read_lock before checking for
 duplicate transport nodes

On Tue, Feb 28, 2017 at 10:37:35PM +0800, Xin Long wrote:
> On Tue, Feb 28, 2017 at 10:23 PM, Neil Horman <nhorman@...driver.com> wrote:
> > On Tue, Feb 28, 2017 at 12:41:29PM +0800, Xin Long wrote:
> >> Commit cd2b70875058 ("sctp: check duplicate node before inserting a
> >> new transport") called rhltable_lookup() to check for the duplicate
> >> transport node in transport rhashtable.
> >>
> >> But rhltable_lookup() doesn't call rcu_read_lock inside, it could cause
> >> a use-after-free issue if it tries to dereference the node that another
> >> cpu has freed it. Note that sock lock can not avoid this as it is per
> >> sock.
> >>
> >> This patch is to fix it by calling rcu_read_lock before checking for
> >> duplicate transport nodes.
> >>
> >> Fixes: cd2b70875058 ("sctp: check duplicate node before inserting a new transport")
> >> Reported-by: Andrey Konovalov <andreyknvl@...gle.com>
> >> Signed-off-by: Xin Long <lucien.xin@...il.com>
> >> ---
> >>  net/sctp/input.c | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/net/sctp/input.c b/net/sctp/input.c
> >> index fc45896..2a28ab2 100644
> >> --- a/net/sctp/input.c
> >> +++ b/net/sctp/input.c
> >> @@ -884,14 +884,17 @@ int sctp_hash_transport(struct sctp_transport *t)
> >>       arg.paddr = &t->ipaddr;
> >>       arg.lport = htons(t->asoc->base.bind_addr.port);
> >>
> >> +     rcu_read_lock();
> >>       list = rhltable_lookup(&sctp_transport_hashtable, &arg,
> >>                              sctp_hash_params);
> >>
> >>       rhl_for_each_entry_rcu(transport, tmp, list, node)
> >>               if (transport->asoc->ep == t->asoc->ep) {
> >> +                     rcu_read_unlock();
> >>                       err = -EEXIST;
> >>                       goto out;
> >>               }
> >> +     rcu_read_unlock();
> >>
> >>       err = rhltable_insert_key(&sctp_transport_hashtable, &arg,
> >>                                 &t->node, sctp_hash_params);
> >> --
> >> 2.1.0
> >>
> >>
> >
> > Hmm, rhltable_insert_key I think returns a pointer to the existing object in the
> > hash table if there is a collision (according to the _rhashtable_insert_fast
> > description).  Wouldn't it be better to eliminate the rhl_for_each_entry_rcu
> > entirely, and just check the returned pointer from rhltable_insert_key?
> I wish it could do it, but rhlist(rhltable_insert_key) allows duplicate
> node to be inserted, it will never return the existing node. :(
> 
> I guess the codes you saw are only for the old rhashtable api (they are
> together with rhlist codes in __rhashtable_insert_fast).
> 
> Thanks
> 
yeah, you're right, can't do it that way

Acked-by: Neil Horman <nhorman@...driver.com>

> >
> > Neil
> >
> 

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