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    • Herbert Xu's avatar
      hwrng: virtio - Fix race on data_avail and actual data · ac52578d
      Herbert Xu authored
      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 data_avail with smp_store_release and reading
      it with smp_load_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 data_idx in random_recv_done
      (data_idx must already be zero at this point) and data_avail in
      request_entropy (ditto).
      
      Reported-by: syzbot+726dc8c62c3536431ceb@syzkaller.appspotmail.com
      Fixes: f7f510ec ("virtio: An entropy device, as suggested by hpa.")
      Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
      Acked-by: default avatarMichael S. Tsirkin <mst@redhat.com>
      Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
      ac52578d