sctp: potential read out of bounds in sctp_ulpevent_type_enabled()

commit fa5f7b51fc3080c2b195fa87c7eca7c05e56f673 upstream.

This code causes a static checker warning because Smatch doesn't trust
anything that comes from skb->data.  I've reviewed this code and I do
think skb->data can be controlled by the user here.

The sctp_event_subscribe struct has 13 __u8 fields and we want to see
if ours is non-zero.  sn_type can be any value in the 0-USHRT_MAX range.
We're subtracting SCTP_SN_TYPE_BASE which is 1 << 15 so we could read
either before the start of the struct or after the end.

This is a very old bug and it's surprising that it would go undetected
for so long but my theory is that it just doesn't have a big impact so
it would be hard to notice.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/include/net/sctp/ulpevent.h b/include/net/sctp/ulpevent.h
index ca4693b4e09e4bb879c0ef8e483247c913bffb6b..00c0e5bf5d3ea31550101bcb0f8240dbbd7100c9 100644 (file)
--- a/include/net/sctp/ulpevent.h
+++ b/include/net/sctp/ulpevent.h
@@ -143,8 +143,12 @@ __u16 sctp_ulpevent_get_notification_type(const struct sctp_ulpevent *event);
 static inline int sctp_ulpevent_type_enabled(__u16 sn_type,
                                             struct sctp_event_subscribe *mask)
 {
+       int offset = sn_type - SCTP_SN_TYPE_BASE;
        char *amask = (char *) mask;
-       return amask[sn_type - SCTP_SN_TYPE_BASE];
+
+       if (offset >= sizeof(struct sctp_event_subscribe))
+               return 0;
+       return amask[offset];
 }
 
 /* Given an event subscription, is this event enabled? */