Subject: Re: [PATCH net] sctp: frag_point sanity check

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Tue, 4 Dec 2018 17:52:02 -0200

On Tue, Dec 04, 2018 at 02:39:46PM -0500, Neil Horman wrote:
> On Tue, Dec 04, 2018 at 08:27:41PM +0100, Jakub Audykowicz wrote:
> > If for some reason an association's fragmentation point is zero,
> > sctp_datamsg_from_user will try to endlessly try to divide a message
> > into zero-sized chunks. This eventually causes kernel panic due to
> > running out of memory.
> > 
> > Although this situation is quite unlikely, it has occurred before as
> > reported. I propose to add this simple last-ditch sanity check due to
> > the severity of the potential consequences.
> > 
> > Signed-off-by: Jakub Audykowicz <[email-masked]>
> > ---
> >  include/net/sctp/sctp.h | 5 +++++
> >  net/sctp/chunk.c        | 6 ++++++
> >  net/sctp/socket.c       | 3 +--
> >  3 files changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
> > index ab9242e51d9e..2abbc15824af 100644
> > --- a/include/net/sctp/sctp.h
> > +++ b/include/net/sctp/sctp.h
> > @@ -620,4 +620,9 @@ static inline bool sctp_transport_pmtu_check(struct sctp_transport *t)
> >  	return false;
> >  }
> >  
> > +static inline __u32 sctp_min_frag_point(struct sctp_sock *sp, __u16 datasize)
> > +{
> > +	return sctp_mtu_payload(sp, SCTP_DEFAULT_MINSEGMENT, datasize);
> > +}
> > +
> >  #endif /* __net_sctp_h__ */
> > diff --git a/net/sctp/chunk.c b/net/sctp/chunk.c
> > index ce8087846f05..d5b91bc8a377 100644
> > --- a/net/sctp/chunk.c
> > +++ b/net/sctp/chunk.c
> > @@ -191,6 +191,12 @@ struct sctp_datamsg *sctp_datamsg_from_user(struct sctp_association *asoc,
> >  	 * the packet
> >  	 */
> >  	max_data = asoc->frag_point;
> > +	if (unlikely(!max_data)) {
> > +		max_data = sctp_min_frag_point(sctp_sk(asoc->base.sk),
> > +					       sctp_datachk_len(&asoc->stream));
> > +		pr_warn_ratelimited("%s: asoc:%p frag_point is zero, forcing max_data to default minimum (%d)",
> > +				    __func__, asoc, max_data);
> > +	}
> >  
> >  	/* If the the peer requested that we authenticate DATA chunks
> >  	 * we need to account for bundling of the AUTH chunks along with
> > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > index bf618d1b41fd..b8cebd5a87e5 100644
> > --- a/net/sctp/socket.c
> > +++ b/net/sctp/socket.c
> > @@ -3324,8 +3324,7 @@ static int sctp_setsockopt_maxseg(struct sock *sk, char __user *optval, unsigned
> >  		__u16 datasize = asoc ? sctp_datachk_len(&asoc->stream) :
> >  				 sizeof(struct sctp_data_chunk);
> >  
> > -		min_len = sctp_mtu_payload(sp, SCTP_DEFAULT_MINSEGMENT,
> > -					   datasize);
> > +		min_len = sctp_min_frag_point(sp, datasize);
> >  		max_len = SCTP_MAX_CHUNK_LEN - datasize;
> >  
> >  		if (val < min_len || val > max_len)
> > -- 
> > 2.17.1
> > 
> > 
> Why not just prevent the frag point from ever going below
> SCTP_DEFAULT_MINSEGMENT in the first place in sctp_assoc_update_frag_point?
> Something like:
> 
> asoc->frag_point = SCTP_TRUNC4(frag) < SCTP_DEFAILT_MINSEGMENT) ? \
> SCTP_DEFAILT_MINSEGMENT : SCTP_TRUNC4(frag);
> 
> Should do the trick I would think
> Neil

This is in the light of "sctp: update frag_point when
stream_interleave is set".

Because of
https://www.mail-archive.com/[email-masked]/msg256575.html
This wouldn't have helped because sctp_assoc_update_frag_point()
didn't get called. The issue is not that the calc issued a bad value,
but that it wasn't done.
 
  Marcelo