netdev - Re: [PATCH net] sctp: frag

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Date:   Tue, 4 Dec 2018 14:39:46 -0500
From:   Neil Horman <nhorman@...driver.com>
To:     Jakub Audykowicz <jakub.audykowicz@...il.com>
Cc:     linux-sctp@...r.kernel.org, vyasevich@...il.com,
        marcelo.leitner@...il.com, davem@...emloft.net,
        netdev@...r.kernel.org
Subject: Re: [PATCH net] sctp: frag_point sanity check

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 <jakub.audykowicz@...il.com>
> ---
>  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