The (1) fw_cfg_write and (2) fw_cfg_read functions in hw/nvram/fw_cfg.c in QEMU before 2.4, when built with the Firmware Configuration device emulation support, allow guest OS users with the CAP_SYS_RAWIO privilege to cause a denial of service (out-of-bounds read or write access and process crash) or possibly execute arbitrary code via an invalid current entry value in a firmware configuration.