A stack based buffer overflow was found in the virtio-net device of QEMU. The flaw occurs while copying data to mhdr, a local variable of type virtio_net_hdr_mrg_rxbuf, when flushing TX in the virtio_net_flush_tx function. If guest features VIRTIO_NET_F_HASH_REPORT, VIRTIO_F_VERSION_1 and VIRTIO_NET_F_MRG_RXBUF are enabled, `n->guest_hdr_len` is set to sizeof(struct virtio_net_hdr_v1_hash), which is bigger than sizeof(virtio_net_hdr_mrg_rxbuf). This vulnerability could potentially allow a malicious user to overwrite local variables adjacent to mhdr allocated on the stack. Specifically, the out_sg variable could be used to read some part of process memory and send it to the wire:

ret = qemu_sendv_packet_async(qemu_get_subqueue(n->nic, queue_index), out_sg, out_num, virtio_net_tx_complete);