sctp: frag_point sanity check

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@gmail.com>
Acked-by: Neil Horman <nhorman@tuxdriver.com>
Acked-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

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..d2048de86e7c 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 (%Zu)",
+                                    __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)