net: add checks to validate ring buffer pointers(CVE-2015-5279)

Ne2000 NIC uses ring buffer of NE2000_MEM_SIZE(49152)
bytes to process network packets. While receiving packets
via ne2000_receive() routine, a local 'index' variable
could exceed the ring buffer size, which could lead to a
memory buffer overflow. Added other checks at initialisation.
Reported-by: NQinghao Tang <luodalongde@gmail.com>
Signed-off-by: NP J P <pjp@fedoraproject.org>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>

hw/net/ne2000.c

@@ -221,6 +221,9 @@ ssize_t ne2000_receive(NetClientState *nc, const uint8_t *buf, size_t size_)
     }
 
     index = s->curpag << 8;
+    if (index >= NE2000_PMEM_END) {
+        index = s->start;
+    }
     /* 4 bytes for header */
     total_len = size + 4;
     /* address for next packet (4 bytes for CRC) */
@@ -306,13 +309,19 @@ static void ne2000_ioport_write(void *opaque, uint32_t addr, uint32_t val)
         offset = addr | (page << 4);
         switch(offset) {
         case EN0_STARTPG:
+            if (val << 8 <= NE2000_PMEM_END) {
                 s->start = val << 8;
+            }
             break;
         case EN0_STOPPG:
+            if (val << 8 <= NE2000_PMEM_END) {
                 s->stop = val << 8;
+            }
             break;
         case EN0_BOUNDARY:
+            if (val << 8 < NE2000_PMEM_END) {
                 s->boundary = val;
+            }
             break;
         case EN0_IMR:
             s->imr = val;
@@ -353,7 +362,9 @@ static void ne2000_ioport_write(void *opaque, uint32_t addr, uint32_t val)
             s->phys[offset - EN1_PHYS] = val;
             break;
         case EN1_CURPAG:
+            if (val << 8 < NE2000_PMEM_END) {
                 s->curpag = val;
+            }
             break;
         case EN1_MULT ... EN1_MULT + 7:
             s->mult[offset - EN1_MULT] = val;