netdev - Re: [PATCH net] sctp: make sure stream nums can match optlen in sctp_setsockopt_reset

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Date:   Mon, 11 Dec 2017 13:36:39 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Neil Horman <nhorman@...driver.com>
Cc:     Xin Long <lucien.xin@...il.com>,
        network dev <netdev@...r.kernel.org>,
        linux-sctp@...r.kernel.org, davem@...emloft.net,
        syzkaller@...glegroups.com
Subject: Re: [PATCH net] sctp: make sure stream nums can match optlen in
 sctp_setsockopt_reset_streams

Hi,

On Mon, Dec 11, 2017 at 09:54:34AM -0500, Neil Horman wrote:
> On Sun, Dec 10, 2017 at 03:40:51PM +0800, Xin Long wrote:
> > Now in sctp_setsockopt_reset_streams, it only does the check
> > optlen < sizeof(*params) for optlen. But it's not enough, as
> > params->srs_number_streams should also match optlen.
> > 
> > If the streams in params->srs_stream_list are less than stream
> > nums in params->srs_number_streams, later when dereferencing
> > the stream list, it could cause a slab-out-of-bounds crash, as
> > reported by syzbot.
> > 
> > This patch is to fix it by also checking the stream numbers in
> > sctp_setsockopt_reset_streams to make sure at least it's not
> > greater than the streams in the list.
> > 
> > Fixes: 7f9d68ac944e ("sctp: implement sender-side procedures for SSN Reset Request Parameter")
> > Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
> > Signed-off-by: Xin Long <lucien.xin@...il.com>
> > ---
> >  net/sctp/socket.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> > 
> > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > index 014847e..dbf140d 100644
> > --- a/net/sctp/socket.c
> > +++ b/net/sctp/socket.c
> > @@ -3891,13 +3891,17 @@ static int sctp_setsockopt_reset_streams(struct sock *sk,
> >  	struct sctp_association *asoc;
> >  	int retval = -EINVAL;
> >  
> > -	if (optlen < sizeof(struct sctp_reset_streams))
> > +	if (optlen < sizeof(*params))
> >  		return -EINVAL;
> >  
> Is this going to work in all corner cases?  IIRC struct
> sctp_reset_stream has variable length array at the end of it, and so
> sizeof(struct sctp_reset_streams) returns just the size of the
> struct, while sizeof(*params) returns the size of the entire object
> (including the array elements).  If a user space task allocates a

I don't think it can include the array elements as such information
can't be passed from the application to the kernel other than via
optlen parameter. There is no metadata around the struct that could
allow that, and sizeof() is a constant, it can't be assuming different
values in runtime.

Cheers,
Marcelo

> static memory block to hold this struct and the array, and passes it
> in with a shorter optlen (if for example, they do not intend to use
> all the array elements), this will cause a failure, where no failure
> is truly warranted.  It seems the correct check to me should be the
> origional sizeof(struct sctp_reset_streams) check, and the below
> check to ensure that there are at least the same number of array
> elements available as indicated in srs_nuber_streams.
> 
> Regards
> Neil
> 
> >  	params = memdup_user(optval, optlen);
> >  	if (IS_ERR(params))
> >  		return PTR_ERR(params);
> >  
> > +	if (params->srs_number_streams * sizeof(__u16) >
> > +	    optlen - sizeof(*params))
> > +		goto out;
> > +
> >  	asoc = sctp_id2assoc(sk, params->srs_assoc_id);
> >  	if (!asoc)
> >  		goto out;
> > -- 
> > 2.1.0
> > 
> > --
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