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.
The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.