CVE-2022-48629

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

crypto: qcom-rng - ensure buffer for generate is completely filled

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).

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.