cadence_gem: Avoid infinite loops with a misconfigured buffer

A guest can write zero to the DMACFG resulting in an infinite loop when
it reaches the while(bytes_to_copy) loop.

To avoid this issue enforce a minimum size for the RX buffer. Hardware
does not have this enforcement and relies on the guest to set a non-zero
value.

Signed-off-by: Alistair Francis <alistair.francis@xilinx.com>
Reported-by: Li Qiang <liqiang6-s@360.cn>
Reported-by: P J P <ppandit@redhat.com>
Message-id: 84bb1c391b833275da3f573d4972920cea34c188.1466539342.git.alistair.francis@xilinx.com
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

hw/net/cadence_gem.c

@@ -664,6 +664,13 @@ static ssize_t gem_receive(NetClientState *nc, const uint8_t *buf, size_t size)
                  GEM_DMACFG_RBUFSZ_S) * GEM_DMACFG_RBUFSZ_MUL;
     bytes_to_copy = size;
 
+    /* Hardware allows a zero value here but warns against it. To avoid QEMU
+     * indefinite loops we enforce a minimum value here
+     */
+    if (rxbufsize < GEM_DMACFG_RBUFSZ_MUL) {
+        rxbufsize = GEM_DMACFG_RBUFSZ_MUL;
+    }
+
     /* Pad to minimum length. Assume FCS field is stripped, logic
      * below will increment it to the real minimum of 64 when
      * not FCS stripping