net: sctp: fix information leaks in ulpevent layer

While working on some other SCTP code, I noticed that some
structures shared with user space are leaking uninitialized
stack or heap buffer. In particular, struct sctp_sndrcvinfo
has a 2 bytes hole between .sinfo_flags and .sinfo_ppid that
remains unfilled by us in sctp_ulpevent_read_sndrcvinfo() when
putting this into cmsg. But also struct sctp_remote_error
contains a 2 bytes hole that we don't fill but place into a skb
through skb_copy_expand() via sctp_ulpevent_make_remote_error().

Both structures are defined by the IETF in RFC6458:

* Section 5.3.2. SCTP Header Information Structure:

  The sctp_sndrcvinfo structure is defined below:

  struct sctp_sndrcvinfo {
    uint16_t sinfo_stream;
    uint16_t sinfo_ssn;
    uint16_t sinfo_flags;
    <-- 2 bytes hole  -->
    uint32_t sinfo_ppid;
    uint32_t sinfo_context;
    uint32_t sinfo_timetolive;
    uint32_t sinfo_tsn;
    uint32_t sinfo_cumtsn;
    sctp_assoc_t sinfo_assoc_id;
  };

* 6.1.3. SCTP_REMOTE_ERROR:

  A remote peer may send an Operation Error message to its peer.
  This message indicates a variety of error conditions on an
  association. The entire ERROR chunk as it appears on the wire
  is included in an SCTP_REMOTE_ERROR event. Please refer to the
  SCTP specification [RFC4960] and any extensions for a list of
  possible error formats. An SCTP error notification has the
  following format:

  struct sctp_remote_error {
    uint16_t sre_type;
    uint16_t sre_flags;
    uint32_t sre_length;
    uint16_t sre_error;
    <-- 2 bytes hole  -->
    sctp_assoc_t sre_assoc_id;
    uint8_t  sre_data[];
  };

Fix this by setting both to 0 before filling them out. We also
have other structures shared between user and kernel space in
SCTP that contains holes (e.g. struct sctp_paddrthlds), but we
copy that buffer over from user space first and thus don't need
to care about it in that cases.

While at it, we can also remove lengthy comments copied from
the draft, instead, we update the comment with the correct RFC
number where one can look it up.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 15 insertions, 107 deletions

diff --git a/net/sctp/ulpevent.c b/net/sctp/ulpevent.c
index 85c64658bd0b..b6842fdb53d4 100644
--- a/net/sctp/ulpevent.c
+++ b/net/sctp/ulpevent.c
@@ -366,9 +366,10 @@ fail:
  * specification [SCTP] and any extensions for a list of possible
  * error formats.
  */
-struct sctp_ulpevent *sctp_ulpevent_make_remote_error(
-        const struct sctp_association *asoc, struct sctp_chunk *chunk,
-        __u16 flags, gfp_t gfp)
+struct sctp_ulpevent *
+sctp_ulpevent_make_remote_error(const struct sctp_association *asoc,
+                                struct sctp_chunk *chunk, __u16 flags,
+                                gfp_t gfp)
 {
         struct sctp_ulpevent *event;
         struct sctp_remote_error *sre;
@@ -387,8 +388,7 @@ struct sctp_ulpevent *sctp_ulpevent_make_remote_error(
         /* Copy the skb to a new skb with room for us to prepend
          * notification with.
          */
-        skb = skb_copy_expand(chunk->skb, sizeof(struct sctp_remote_error),
-                              0, gfp);
+        skb = skb_copy_expand(chunk->skb, sizeof(*sre), 0, gfp);
 
         /* Pull off the rest of the cause TLV from the chunk.  */
         skb_pull(chunk->skb, elen);
@@ -399,62 +399,21 @@ struct sctp_ulpevent *sctp_ulpevent_make_remote_error(
         event = sctp_skb2event(skb);
         sctp_ulpevent_init(event, MSG_NOTIFICATION, skb->truesize);
 
-        sre = (struct sctp_remote_error *)
-                skb_push(skb, sizeof(struct sctp_remote_error));
+        sre = (struct sctp_remote_error *) skb_push(skb, sizeof(*sre));
 
         /* Trim the buffer to the right length.  */
-        skb_trim(skb, sizeof(struct sctp_remote_error) + elen);
+        skb_trim(skb, sizeof(*sre) + elen);
 
-        /* Socket Extensions for SCTP
-         * 5.3.1.3 SCTP_REMOTE_ERROR
-         *
-         * sre_type:
-         *   It should be SCTP_REMOTE_ERROR.
-         */
+        /* RFC6458, Section 6.1.3. SCTP_REMOTE_ERROR */
+        memset(sre, 0, sizeof(*sre));
         sre->sre_type = SCTP_REMOTE_ERROR;
-
-        /*
-         * Socket Extensions for SCTP
-         * 5.3.1.3 SCTP_REMOTE_ERROR
-         *
-         * sre_flags: 16 bits (unsigned integer)
-         *   Currently unused.
-         */
         sre->sre_flags = 0;
-
-        /* Socket Extensions for SCTP
-         * 5.3.1.3 SCTP_REMOTE_ERROR
-         *
-         * sre_length: sizeof (__u32)
-         *
-         * This field is the total length of the notification data,
-         * including the notification header.
-         */
         sre->sre_length = skb->len;
-
-        /* Socket Extensions for SCTP
-         * 5.3.1.3 SCTP_REMOTE_ERROR
-         *
-         * sre_error: 16 bits (unsigned integer)
-         * This value represents one of the Operational Error causes defined in
-         * the SCTP specification, in network byte order.
-         */
         sre->sre_error = cause;
-
-        /* Socket Extensions for SCTP
-         * 5.3.1.3 SCTP_REMOTE_ERROR
-         *
-         * sre_assoc_id: sizeof (sctp_assoc_t)
-         *
-         * The association id field, holds the identifier for the association.
-         * All notifications for a given association have the same association
-         * identifier.  For TCP style socket, this field is ignored.
-         */
         sctp_ulpevent_set_owner(event, asoc);
         sre->sre_assoc_id = sctp_assoc2id(asoc);
 
         return event;
-
 fail:
         return NULL;
 }
@@ -899,7 +858,9 @@ __u16 sctp_ulpevent_get_notification_type(const struct sctp_ulpevent *event)
         return notification->sn_header.sn_type;
 }
 
-/* Copy out the sndrcvinfo into a msghdr.  */
+/* RFC6458, Section 5.3.2. SCTP Header Information Structure
+ * (SCTP_SNDRCV, DEPRECATED)
+ */
 void sctp_ulpevent_read_sndrcvinfo(const struct sctp_ulpevent *event,
                                    struct msghdr *msghdr)
 {
@@ -908,74 +869,21 @@ void sctp_ulpevent_read_sndrcvinfo(const struct sctp_ulpevent *event,
         if (sctp_ulpevent_is_notification(event))
                 return;
 
-        /* Sockets API Extensions for SCTP
-         * Section 5.2.2 SCTP Header Information Structure (SCTP_SNDRCV)
-         *
-         * sinfo_stream: 16 bits (unsigned integer)
-         *
-         * For recvmsg() the SCTP stack places the message's stream number in
-         * this value.
-        */
+        memset(&sinfo, 0, sizeof(sinfo));
         sinfo.sinfo_stream = event->stream;
-        /* sinfo_ssn: 16 bits (unsigned integer)
-         *
-         * For recvmsg() this value contains the stream sequence number that
-         * the remote endpoint placed in the DATA chunk.  For fragmented
-         * messages this is the same number for all deliveries of the message
-         * (if more than one recvmsg() is needed to read the message).
-         */
         sinfo.sinfo_ssn = event->ssn;
-        /* sinfo_ppid: 32 bits (unsigned integer)
-         *
-         * In recvmsg() this value is
-         * the same information that was passed by the upper layer in the peer
-         * application.  Please note that byte order issues are NOT accounted
-         * for and this information is passed opaquely by the SCTP stack from
-         * one end to the other.
-         */
         sinfo.sinfo_ppid = event->ppid;
-        /* sinfo_flags: 16 bits (unsigned integer)
-         *
-         * This field may contain any of the following flags and is composed of
-         * a bitwise OR of these values.
-         *
-         * recvmsg() flags:
-         *
-         * SCTP_UNORDERED - This flag is present when the message was sent
-         *                 non-ordered.
-         */
         sinfo.sinfo_flags = event->flags;
-        /* sinfo_tsn: 32 bit (unsigned integer)
-         *
-         * For the receiving side, this field holds a TSN that was
-         * assigned to one of the SCTP Data Chunks.
-         */
         sinfo.sinfo_tsn = event->tsn;
-        /* sinfo_cumtsn: 32 bit (unsigned integer)
-         *
-         * This field will hold the current cumulative TSN as
-         * known by the underlying SCTP layer.  Note this field is
-         * ignored when sending and only valid for a receive
-         * operation when sinfo_flags are set to SCTP_UNORDERED.
-         */
         sinfo.sinfo_cumtsn = event->cumtsn;
-        /* sinfo_assoc_id: sizeof (sctp_assoc_t)
-         *
-         * The association handle field, sinfo_assoc_id, holds the identifier
-         * for the association announced in the COMMUNICATION_UP notification.
-         * All notifications for a given association have the same identifier.
-         * Ignored for one-to-one style sockets.
-         */
         sinfo.sinfo_assoc_id = sctp_assoc2id(event->asoc);
-
-        /* context value that is set via SCTP_CONTEXT socket option. */
+        /* Context value that is set via SCTP_CONTEXT socket option. */
         sinfo.sinfo_context = event->asoc->default_rcv_context;
-
         /* These fields are not used while receiving. */
         sinfo.sinfo_timetolive = 0;
 
         put_cmsg(msghdr, IPPROTO_SCTP, SCTP_SNDRCV,
-                 sizeof(struct sctp_sndrcvinfo), (void *)&sinfo);
+                 sizeof(sinfo), &sinfo);
 }
 
 /* Do accounting for bytes received and hold a reference to the association