Fix flexible array buffer overflow

This is kind of a DoS, possibly flexible array in the protocol
causes the network size check to be ignored due to integer overflows.

The size of flexible array is computed as (message_end - position),
then this size is added to the number of bytes before the array and
this number is used to check if we overflow initial message.

An example is:

    message {
        uint32 dummy[2];
        uint8 data[] @end;
    } LenMessage;

which generated this (simplified remove useless code) code:

    { /* data */
        data__nelements = message_end - (start + 8);

        data__nw_size = data__nelements;
    }

    nw_size = 8 + data__nw_size;

    /* Check if message fits in reported side */
    if (nw_size > (uintptr_t) (message_end - start)) {
        return NULL;
    }

Following code:
- data__nelements == message_end - (start + 8)
- data__nw_size == data__nelements == message_end - (start + 8)
- nw_size == 8 + data__nw_size == 8 + message_end - (start + 8) ==
  8 + message_end - start - 8 == message_end -start
- the check for overflow is (nw_size > (message_end - start)) but
  nw_size == message_end - start so the check is doing
  ((message_end - start) > (message_end - start)) which is always false.

If message_end - start < 8 then data__nelements (number of element
on the array above) computation generate an integer underflow that
later create a buffer overflow.

Add a check to make sure that the array starts before the message ends
to avoid the overflow.

Difference is:
    diff -u save/generated_client_demarshallers1.c common/generated_client_demarshallers1.c
    --- save/generated_client_demarshallers1.c	2018-06-22 22:13:48.626793919 +0100
    +++ common/generated_client_demarshallers1.c	2018-06-22 22:14:03.408163291 +0100
    @@ -225,6 +225,9 @@
         uint64_t data__nelements;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 0) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 0);

             data__nw_size = data__nelements;
    @@ -243,6 +246,9 @@
         *free_message = nofree;
         return data;

    +   error:
    +    free(data);
    +    return NULL;
     }

     static uint8_t * parse_msg_set_ack(uint8_t *message_start, uint8_t *message_end, SPICE_GNUC_UNUSED int minor, size_t *size, message_destructor_t *free_message)
    @@ -301,6 +307,9 @@
         SpiceMsgPing *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 12) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 12);

             data__nw_size = data__nelements;
    @@ -5226,6 +5235,9 @@
             uint64_t cursor_data__nw_size;
             uint64_t cursor_data__nelements;
             { /* data */
    +            if (SPICE_UNLIKELY((start2 + 22) > message_end)) {
    +                goto error;
    +            }
                 cursor_data__nelements = message_end - (start2 + 22);

                 cursor_data__nw_size = cursor_data__nelements;
    @@ -5305,6 +5317,9 @@
             uint64_t cursor_data__nw_size;
             uint64_t cursor_data__nelements;
             { /* data */
    +            if (SPICE_UNLIKELY((start2 + 22) > message_end)) {
    +                goto error;
    +            }
                 cursor_data__nelements = message_end - (start2 + 22);

                 cursor_data__nw_size = cursor_data__nelements;
    @@ -5540,6 +5555,9 @@
         SpiceMsgPlaybackPacket *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 4) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 4);

             data__nw_size = data__nelements;
    @@ -5594,6 +5612,9 @@
         SpiceMsgPlaybackMode *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 8) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 8);

             data__nw_size = data__nelements;
    diff -u save/generated_client_demarshallers.c common/generated_client_demarshallers.c
    --- save/generated_client_demarshallers.c	2018-06-22 22:13:48.626793919 +0100
    +++ common/generated_client_demarshallers.c	2018-06-22 22:14:03.004153195 +0100
    @@ -225,6 +225,9 @@
         uint64_t data__nelements;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 0) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 0);

             data__nw_size = data__nelements;
    @@ -243,6 +246,9 @@
         *free_message = nofree;
         return data;

    +   error:
    +    free(data);
    +    return NULL;
     }

     static uint8_t * parse_msg_set_ack(uint8_t *message_start, uint8_t *message_end, SPICE_GNUC_UNUSED int minor, size_t *size, message_destructor_t *free_message)
    @@ -301,6 +307,9 @@
         SpiceMsgPing *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 12) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 12);

             data__nw_size = data__nelements;
    @@ -6574,6 +6583,9 @@
             }

             { /* data */
    +            if (SPICE_UNLIKELY((start2 + 2 + cursor_u__nw_size) > message_end)) {
    +                goto error;
    +            }
                 cursor_data__nelements = message_end - (start2 + 2 + cursor_u__nw_size);

                 cursor_data__nw_size = cursor_data__nelements;
    @@ -6670,6 +6682,9 @@
             }

             { /* data */
    +            if (SPICE_UNLIKELY((start2 + 2 + cursor_u__nw_size) > message_end)) {
    +                goto error;
    +            }
                 cursor_data__nelements = message_end - (start2 + 2 + cursor_u__nw_size);

                 cursor_data__nw_size = cursor_data__nelements;
    @@ -6907,6 +6922,9 @@
         SpiceMsgPlaybackPacket *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 4) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 4);

             data__nw_size = data__nelements;
    @@ -6961,6 +6979,9 @@
         SpiceMsgPlaybackMode *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 6) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 6);

             data__nw_size = data__nelements;
    @@ -7559,6 +7580,9 @@
         SpiceMsgTunnelSocketData *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 2) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 2);

             data__nw_size = data__nelements;
    @@ -7840,6 +7864,9 @@
         }

         { /* compressed_data */
    +        if (SPICE_UNLIKELY((start + 1 + u__nw_size) > message_end)) {
    +            goto error;
    +        }
             compressed_data__nelements = message_end - (start + 1 + u__nw_size);

             compressed_data__nw_size = compressed_data__nelements;
    diff -u save/generated_server_demarshallers.c common/generated_server_demarshallers.c
    --- save/generated_server_demarshallers.c	2018-06-22 22:13:48.627793944 +0100
    +++ common/generated_server_demarshallers.c	2018-06-22 22:14:05.231208847 +0100
    @@ -306,6 +306,9 @@
         uint64_t data__nelements;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 0) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 0);

             data__nw_size = data__nelements;
    @@ -324,6 +327,9 @@
         *free_message = nofree;
         return data;

    +   error:
    +    free(data);
    +    return NULL;
     }

     static uint8_t * parse_msgc_disconnecting(uint8_t *message_start, uint8_t *message_end, SPICE_GNUC_UNUSED int minor, size_t *size, message_destructor_t *free_message)
    @@ -1259,6 +1265,9 @@
         SpiceMsgcRecordPacket *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 4) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 4);

             data__nw_size = data__nelements;
    @@ -1313,6 +1322,9 @@
         SpiceMsgcRecordMode *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 6) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 6);

             data__nw_size = data__nelements;
    @@ -1841,6 +1853,9 @@
         SpiceMsgcTunnelSocketData *out;

         { /* data */
    +        if (SPICE_UNLIKELY((start + 2) > message_end)) {
    +            goto error;
    +        }
             data__nelements = message_end - (start + 2);

             data__nw_size = data__nelements;
    @@ -2057,6 +2072,9 @@
         }

         { /* compressed_data */
    +        if (SPICE_UNLIKELY((start + 1 + u__nw_size) > message_end)) {
    +            goto error;
    +        }
             compressed_data__nelements = message_end - (start + 1 + u__nw_size);

             compressed_data__nw_size = compressed_data__nelements;

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/python_modules/demarshal.py b/python_modules/demarshal.py
index 7b53361..5a237a6 100644
--- a/python_modules/demarshal.py
+++ b/python_modules/demarshal.py
@@ -331,6 +331,7 @@ def write_validate_array_item(writer, container, item, scope, parent_scope, star
         writer.assign(nelements, array.size)
     elif array.is_remaining_length():
         if element_type.is_fixed_nw_size():
+            writer.error_check("%s > message_end" % item.get_position())
             if element_type.get_fixed_nw_size() == 1:
                 writer.assign(nelements, "message_end - %s" % item.get_position())
             else: