The generate function in struct rng_alg expects that the destination
buffer is completely filled if the function returns 0. qcom_rng_read()
can run into a situation where the buffer is partially filled with
randomness and the remaining part of the buffer is zeroed since
qcom_rng_generate() doesn't check the return value. This issue can
be reproduced by running the following from libkcapi:

    kcapi-rng -b 9000000 > OUTFILE

The generated OUTFILE will have three huge sections that contain all
zeros, and this is caused by the code where the test
'val & PRNG_STATUS_DATA_AVAIL' fails.

Let's fix this issue by ensuring that qcom_rng_read() always returns
with a full buffer if the function returns success. Let's also have
qcom_rng_generate() return the correct value.

Here's some statistics from the ent project
(https://www.fourmilab.ch/random/) that shows information about the
quality of the generated numbers:

    $ ent -c qcom-random-before
    Value Char Occurrences Fraction
      0           606748   0.067416
      1            33104   0.003678
      2            33001   0.003667
    ...
    253   �        32883   0.003654
    254   �        33035   0.003671
    255   �        33239   0.003693

    Total:       9000000   1.000000

    Entropy = 7.811590 bits per byte.

    Optimum compression would reduce the size
    of this 9000000 byte file by 2 percent.

    Chi square distribution for 9000000 samples is 9329962.81, and
    randomly would exceed this value less than 0.01 percent of the
    times.

    Arithmetic mean value of data bytes is 119.3731 (127.5 = random).
    Monte Carlo value for Pi is 3.197293333 (error 1.77 percent).
    Serial correlation coefficient is 0.159130 (totally uncorrelated =
    0.0).

Without this patch, the results of the chi-square test is 0.01%, and
the numbers are certainly not random according to ent's project page.
The results improve with this patch:

    $ ent -c qcom-random-after
    Value Char Occurrences Fraction
      0            35432   0.003937
      1            35127   0.003903
      2            35424   0.003936
    ...
    253   �        35201   0.003911
    254   �        34835   0.003871
    255   �        35368   0.003930

    Total:       9000000   1.000000

    Entropy = 7.999979 bits per byte.

    Optimum compression would reduce the size
    of this 9000000 byte file by 0 percent.

    Chi square distribution for 9000000 samples is 258.77, and randomly
    would exceed this value 42.24 percent of the times.

    Arithmetic mean value of data bytes is 127.5006 (127.5 = random).
    Monte Carlo value for Pi is 3.141277333 (error 0.01 percent).
    Serial correlation coefficient is 0.000468 (totally uncorrelated =
    0.0).

This change was tested on a Nexus 5 phone (msm8974 SoC).
Signed-off-by: Brian Masney <bmasney@redhat.com>
Fixes: ceec5f5b ("crypto: qcom-rng - Add Qcom prng driver")
Cc: stable@vger.kernel.org # 4.19+
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>