scsi: fix buffer overflow in scsi_req_parse_cdb (CVE-2015-5158)

This is a guest-triggerable buffer overflow present in QEMU 2.2.0
and newer.  scsi_cdb_length returns -1 as an error value, but the
caller does not check it.

Luckily, the massive overflow means that QEMU will just SIGSEGV,
making the impact much smaller.

Reported-by: Zhu Donghai (朱东海) <donghai.zdh@alibaba-inc.com>
Fixes: 1894df02811f6b79ea3ffbf1084599d96f316173
Reviewed-by: Fam Zheng <famz@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

hw/scsi/scsi-bus.c

@@ -1239,10 +1239,15 @@ int scsi_cdb_length(uint8_t *buf) {
 int scsi_req_parse_cdb(SCSIDevice *dev, SCSICommand *cmd, uint8_t *buf)
 {
     int rc;
+    int len;
 
     cmd->lba = -1;
-    cmd->len = scsi_cdb_length(buf);
+    len = scsi_cdb_length(buf);
+    if (len < 0) {
+        return -1;
+    }
 
+    cmd->len = len;
     switch (dev->type) {
     case TYPE_TAPE:
         rc = scsi_req_stream_xfer(cmd, dev, buf);