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    Subject[PATCH AUTOSEL 4.19 160/192] hwrng: virtio - Avoid repeated init of completion
    Date
    From: David Tolnay <dtolnay@gmail.com>

    [ Upstream commit aef027db48da56b6f25d0e54c07c8401ada6ce21 ]

    The virtio-rng driver uses a completion called have_data to wait for a
    virtio read to be fulfilled by the hypervisor. The completion is reset
    before placing a buffer on the virtio queue and completed by the virtio
    callback once data has been written into the buffer.

    Prior to this commit, the driver called init_completion on this
    completion both during probe as well as when registering virtio buffers
    as part of a hwrng read operation. The second of these init_completion
    calls should instead be reinit_completion because the have_data
    completion has already been inited by probe. As described in
    Documentation/scheduler/completion.txt, "Calling init_completion() twice
    on the same completion object is most likely a bug".

    This bug was present in the initial implementation of virtio-rng in
    f7f510ec1957 ("virtio: An entropy device, as suggested by hpa"). Back
    then the have_data completion was a single static completion rather than
    a member of one of potentially multiple virtrng_info structs as
    implemented later by 08e53fbdb85c ("virtio-rng: support multiple
    virtio-rng devices"). The original driver incorrectly used
    init_completion rather than INIT_COMPLETION to reset have_data during
    read.

    Tested by running `head -c48 /dev/random | hexdump` within crosvm, the
    Chrome OS virtual machine monitor, and confirming that the virtio-rng
    driver successfully produces random bytes from the host.

    Signed-off-by: David Tolnay <dtolnay@gmail.com>
    Tested-by: David Tolnay <dtolnay@gmail.com>
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/char/hw_random/virtio-rng.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/drivers/char/hw_random/virtio-rng.c b/drivers/char/hw_random/virtio-rng.c
    index b89df66ea1ae..7abd604e938c 100644
    --- a/drivers/char/hw_random/virtio-rng.c
    +++ b/drivers/char/hw_random/virtio-rng.c
    @@ -73,7 +73,7 @@ static int virtio_read(struct hwrng *rng, void *buf, size_t size, bool wait)

    if (!vi->busy) {
    vi->busy = true;
    - init_completion(&vi->have_data);
    + reinit_completion(&vi->have_data);
    register_buffer(vi, buf, size);
    }

    --
    2.19.1
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