[SRU][F/aws][PATCH 0/5] AWS: fix out of entropy on Graviton 2 instances types (mg6.*)

[Andrea Righi]

BugLink: https://bugs.launchpad.net/bugs/1927692

[Impact]

AWS Graviton 2 instances do not have enough entropy available at boot,
so any task that require entropy (even reading few bytes from
/dev/random) will be stuck forever.

[Fix]

The proper fix for this problem is to correctly refill the entropy pool
with some real random data using some hardware-generated randomness.

In the meantime a reasonable workaround can be to apply the following
upstream commits:

 30c08efec888 random: make /dev/random be almost like /dev/urandom
 48446f198f9a random: ignore GRND_RANDOM in getentropy(2)
 75551dbf112c random: add GRND_INSECURE to return best-effort non-cryptographic bytes
 c6f1deb15878 random: Add a urandom_read_nowait() for random APIs that don't warn
 4c8d062186d9 random: Don't wake crng_init_wait when crng_init == 1

In this way the system will not run out of entropy and will be able to
provide best-effort randomness in any case, preventing the out of
entropy issue on the AWS Gravion 2 instances.

[Test plan]

Execute the following command on any m6g instance:

  dd bs=32 count=1 if=/dev/random of=/dev/null

This should return quickly, if not it means that the system does not
have enough entropy available. When the problem happens this command
hangs forever.

[Where problems could occur]

This changes affect the read semantics of /dev/random to be the same as
/dev/urandom except that reads will block until the CRNG is ready. This
should not materially break any API. Any code that worked without these
changes should work at least as well as before. However, applications
that have strict randomness requirements might be affected by the
provided best-effort randomness, so we may need to apply more
commits/changes to introduce a proper hardware entropy support on
Graviton 2 instances to provide a better quality of randomness. In the
meantime these upstream changes consist a reasonable workaround to
prevent applications from hanging forever on the mg6.* instances.

----------------------------------------------------------------
Andy Lutomirski (5):
      random: add GRND_INSECURE to return best-effort non-cryptographic bytes
      random: Don't wake crng_init_wait when crng_init == 1
      random: Add a urandom_read_nowait() for random APIs that don't warn
      random: ignore GRND_RANDOM in getentropy(2)
      random: make /dev/random be almost like /dev/urandom

 drivers/char/random.c       | 81 +++++++++++++++++++++++++++++++++------------------------------------------------
 include/uapi/linux/random.h |  4 +++-
 2 files changed, 36 insertions(+), 49 deletions(-)