Re: [PATCH net] sctp: fix ICMP processing if skb is non-linear

[Xin Long <lucien.xin@gmail.com>]

On Fri, May 19, 2017 at 11:34 PM, Davide Caratti <dcaratti@redhat.com> wrote:
> when the ICMP packet is carried by a paged skb, sctp_err_lookup() may fail
> validation even if the payload contents match an open socket: as a
> consequence, sometimes ICMPs are wrongly ignored. Use skb_header_pointer()
> to retrieve encapsulated SCTP headers, to ensure that ICMP payloads are
> validated correctly, also when skbs are non-linear.
>
> Signed-off-by: Davide Caratti <dcaratti@redhat.com>
> ---
>  include/net/sctp/sctp.h |  2 +-
>  net/sctp/input.c        | 29 +++++++++++++++++++----------
>  net/sctp/ipv6.c         |  2 +-
>  3 files changed, 21 insertions(+), 12 deletions(-)
>
> diff --git a/include/net/sctp/sctp.h b/include/net/sctp/sctp.h
> index 069582e..1b8c16b 100644
> --- a/include/net/sctp/sctp.h
> +++ b/include/net/sctp/sctp.h
> @@ -152,7 +152,7 @@ void sctp_v4_err(struct sk_buff *skb, u32 info);
>  void sctp_hash_endpoint(struct sctp_endpoint *);
>  void sctp_unhash_endpoint(struct sctp_endpoint *);
>  struct sock *sctp_err_lookup(struct net *net, int family, struct sk_buff *,
> -                            struct sctphdr *, struct sctp_association **,
> +                            struct sctp_association **,
>                              struct sctp_transport **);
>  void sctp_err_finish(struct sock *, struct sctp_transport *);
>  void sctp_icmp_frag_needed(struct sock *, struct sctp_association *,
> diff --git a/net/sctp/input.c b/net/sctp/input.c
> index 0e06a27..7f3f983 100644
> --- a/net/sctp/input.c
> +++ b/net/sctp/input.c
> @@ -469,19 +469,19 @@ void sctp_icmp_proto_unreachable(struct sock *sk,
>
>  /* Common lookup code for icmp/icmpv6 error handler. */
>  struct sock *sctp_err_lookup(struct net *net, int family, struct sk_buff *skb,
> -                            struct sctphdr *sctphdr,
>                              struct sctp_association **app,
>                              struct sctp_transport **tpp)
>  {
> +       struct sctp_init_chunk _chunkhdr, *chunkhdr;
> +       struct sctphdr _sctphdr, *sctphdr;
>         union sctp_addr saddr;
>         union sctp_addr daddr;
>         struct sctp_af *af;
>         struct sock *sk = NULL;
>         struct sctp_association *asoc;
>         struct sctp_transport *transport = NULL;
> -       struct sctp_init_chunk *chunkhdr;
> -       __u32 vtag = ntohl(sctphdr->vtag);
> -       int len = skb->len - ((void *)sctphdr - (void *)skb->data);
> +       int offset;
> +       __u32 vtag;
>
>         *app = NULL; *tpp = NULL;
>
> @@ -515,14 +515,23 @@ struct sock *sctp_err_lookup(struct net *net, int family, struct sk_buff *skb,
>          * or the chunk type or the Initiate Tag does not match, silently
>          * discard the packet.
>          */
> +       offset = skb_transport_offset(skb);
> +       sctphdr = skb_header_pointer(skb, offset, sizeof(_sctphdr), &_sctphdr);
> +       if (unlikely(!sctphdr))
> +               goto out;
> +
> +       vtag = ntohl(sctphdr->vtag);
>         if (vtag == 0) {
> -               chunkhdr = (void *)sctphdr + sizeof(struct sctphdr);
> -               if (len < sizeof(struct sctphdr) + sizeof(sctp_chunkhdr_t)
> -                         + sizeof(__be32) ||
> +               offset += sizeof(_sctphdr);
will be nice to delete this line, and use
> +               /* chunk header + first 4 octects of init header */
> +               chunkhdr = skb_header_pointer(skb, offset,
chunkhdr = skb_header_pointer(skb, offset + sizeof(_sctphdr), ;)
wdyt?

> +                                             sizeof(struct sctp_chunkhdr) +
> +                                             sizeof(__be32), &_chunkhdr);
> +               if (!chunkhdr ||
>                     chunkhdr->chunk_hdr.type != SCTP_CID_INIT ||
> -                   ntohl(chunkhdr->init_hdr.init_tag) != asoc->c.my_vtag) {
> +                   ntohl(chunkhdr->init_hdr.init_tag) != asoc->c.my_vtag)
>                         goto out;
> -               }
> +
>         } else if (vtag != asoc->c.peer_vtag) {
>                 goto out;
>         }
> @@ -585,7 +594,7 @@ void sctp_v4_err(struct sk_buff *skb, __u32 info)
>         savesctp = skb->transport_header;
>         skb_reset_network_header(skb);
>         skb_set_transport_header(skb, ihlen);
> -       sk = sctp_err_lookup(net, AF_INET, skb, sctp_hdr(skb), &asoc, &transport);
> +       sk = sctp_err_lookup(net, AF_INET, skb, &asoc, &transport);
>         /* Put back, the original values. */
>         skb->network_header = saveip;
>         skb->transport_header = savesctp;
> diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
> index 142b70e..d72c8d5 100644
> --- a/net/sctp/ipv6.c
> +++ b/net/sctp/ipv6.c
> @@ -157,7 +157,7 @@ static void sctp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>         savesctp = skb->transport_header;
>         skb_reset_network_header(skb);
>         skb_set_transport_header(skb, offset);
> -       sk = sctp_err_lookup(net, AF_INET6, skb, sctp_hdr(skb), &asoc, &transport);
> +       sk = sctp_err_lookup(net, AF_INET6, skb, &asoc, &transport);
>         /* Put back, the original pointers. */
>         skb->network_header   = saveip;
>         skb->transport_header = savesctp;
> --
> 2.7.4
>