CVE-2023-53998

In the Linux kernel, the following vulnerability has been resolved:

hwrng: virtio - Fix race on data_avail and actual data

The virtio rng device kicks off a new entropy request whenever the data available reaches zero. When a new request occurs at the end of a read operation, that is, when the result of that request is only needed by the next reader, then there is a race between the writing of the new data and the next reader.

This is because there is no synchronisation whatsoever between the writer and the reader.

Fix this by writing dataavail with smpstorerelease and reading it with smpload_acquire when we first enter read. The subsequent reads are safe because they're either protected by the first load acquire, or by the completion mechanism.

Also remove the redundant zeroing of dataidx in randomrecvdone (dataidx must already be zero at this point) and dataavail in requestentropy (ditto).