pcnet: force the buffer access to be in bounds during tx

4096 is the maximum length per TMD and it is also currently the size of
the relay buffer pcnet driver uses for sending the packet data to QEMU
for further processing. With packet spanning multiple TMDs it can
happen that the overall packet size will be bigger than sizeof(buffer),
which results in memory corruption.

Fix this by only allowing to queue maximum sizeof(buffer) bytes.

This is CVE-2015-3209.

[Fixed 3-space indentation to QEMU's 4-space coding standard.
--Stefan]
Signed-off-by: NPetr Matousek <pmatouse@redhat.com>
Reported-by: NMatt Tait <matttait@google.com>
Reviewed-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NStefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/net/pcnet.c

@@ -1241,6 +1241,14 @@ static void pcnet_transmit(PCNetState *s)
         }
 
         bcnt = 4096 - GET_FIELD(tmd.length, TMDL, BCNT);
+
+        /* if multi-tmd packet outsizes s->buffer then skip it silently.
+           Note: this is not what real hw does */
+        if (s->xmit_pos + bcnt > sizeof(s->buffer)) {
+            s->xmit_pos = -1;
+            goto txdone;
+        }
+
         s->phys_mem_read(s->dma_opaque, PHYSADDR(s, tmd.tbadr),
                          s->buffer + s->xmit_pos, bcnt, CSR_BSWP(s));
         s->xmit_pos += bcnt;