Introduced in commit f37708f6b8 (2.10). The NBD spec says a client can request export names up to 4096 bytes in length, even though they should not expect success on names longer than 256. However, qemu hard-codes the limit of 256, and fails to filter out a client that probes for a longer name; the result is a stack smash that can potentially give an attacker arbitrary control over the qemu process. The smash can be easily demonstrated with this client: $ qemu-io f raw nbd://localhost:10809/$(printf %3000d 1 | tr ' ' a) If the qemu NBD server binary (whether the standalone qemu-nbd, or the builtin server of QMP nbd-server-start) was compiled with -fstack-protector-strong, the ability to exploit the stack smash into arbitrary execution is a lot more difficult (but still theoretically possible to a determined attacker, perhaps in combination with other CVEs). Still, crashing a running qemu (and losing the VM) is bad enough, even if the attacker did not obtain full execution control.
Related ExploitsTrying to match CVEs (1): CVE-2017-15118
Other Possible E-DB Search Terms: QEMU