netdev - Re: [PATCH net-next 03/13] sctp: remove an if() that is always true

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Message-ID: <20180427190329.GC22078@hmswarspite.think-freely.org>
Date:   Fri, 27 Apr 2018 15:03:29 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     netdev@...r.kernel.org, linux-sctp@...r.kernel.org,
        Vlad Yasevich <vyasevich@...il.com>,
        Xin Long <lucien.xin@...il.com>
Subject: Re: [PATCH net-next 03/13] sctp: remove an if() that is always true

On Fri, Apr 27, 2018 at 03:13:49PM -0300, Marcelo Ricardo Leitner wrote:
> On Fri, Apr 27, 2018 at 06:50:50AM -0400, Neil Horman wrote:
> > On Thu, Apr 26, 2018 at 04:58:52PM -0300, Marcelo Ricardo Leitner wrote:
> > > As noticed by Xin Long, the if() here is always true as PMTU can never
> > > be 0.
> > >
> > > Reported-by: Xin Long <lucien.xin@...il.com>
> > > Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
> > > ---
> > >  net/sctp/associola.c | 6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/net/sctp/associola.c b/net/sctp/associola.c
> > > index b3aa95222bd52113295cb246c503c903bdd5c353..c5ed09cfa8423b17546e3d45f6d06db03af66384 100644
> > > --- a/net/sctp/associola.c
> > > +++ b/net/sctp/associola.c
> > > @@ -1397,10 +1397,8 @@ void sctp_assoc_sync_pmtu(struct sctp_association *asoc)
> > >  			pmtu = t->pathmtu;
> > >  	}
> > >
> > > -	if (pmtu) {
> > > -		asoc->pathmtu = pmtu;
> > > -		asoc->frag_point = sctp_frag_point(asoc, pmtu);
> > > -	}
> > > +	asoc->pathmtu = pmtu;
> > > +	asoc->frag_point = sctp_frag_point(asoc, pmtu);
> > >
> > Can you double check this?  Looking at it, it seems far fetched, but if someone
> 
> Sure.
> 
> > sends a crafted icmp dest unreach message to the host, pmtu_sending might be
> > able to get set for an association (which may have no transports established
> > yet), and if so, on the first packet send sctp_assoc_sync_pmtu can be called,
> > leading to a fall through in the loop over all transports, and pmtu being zero.
> > It seems like a far fetched set of circumstances, I know, but if it can happen,
> > I think you might see a crash in sctp_frag_point due to an underflow of the frag
> > value
> 
> If I got you right, this situation would not happen because when
> handling the icmp it will check if there is a transport and ignore it
> otherwise.
> 
Yup, you're right.  sctp_err_lookup returns NULL if there is no transport
associated with the icmp message

Thanks
Neil

>   Marcelo
> 
> >
> > Neil
> >
> > >  	pr_debug("%s: asoc:%p, pmtu:%d, frag_point:%d\n", __func__, asoc,
> > >  		 asoc->pathmtu, asoc->frag_point);
> > > --
> > > 2.14.3
> > >
> > >
> > --
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> >
>