virtio: error out if guest exceeds virtqueue size
A broken or malicious guest can submit more requests than the virtqueue size permits, causing unbounded memory allocation in QEMU. The guest can submit requests without bothering to wait for completion and is therefore not bound by virtqueue size. This requires reusing vring descriptors in more than one request, which is not allowed by the VIRTIO 1.0 specification. In "3.2.1 Supplying Buffers to The Device", the VIRTIO 1.0 specification says: 1. The driver places the buffer into free descriptor(s) in the descriptor table, chaining as necessary and Note that the above code does not take precautions against the available ring buffer wrapping around: this is not possible since the ring buffer is the same size as the descriptor table, so step (1) will prevent such a condition. This implies that placing more buffers into the virtqueue than the descriptor table size is not allowed. QEMU is missing the check to prevent this case. Processing a request allocates a VirtQueueElement leading to unbounded memory allocation controlled by the guest. Exit with an error if the guest provides more requests than the virtqueue size permits. This bounds memory allocation and makes the buggy guest visible to the user. This patch fixes CVE-2016-5403 and was reported by Zhenhao Hong from 360 Marvel Team, China. Reported-by: NZhenhao Hong <hongzhenhao@360.cn> Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
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