crypto: drbg - make reseeding from get_random_bytes() synchronous

get_random_bytes() usually hasn't full entropy available by the time DRBG
instances are first getting seeded from it during boot. Thus, the DRBG
implementation registers random_ready_callbacks which would in turn
schedule some work for reseeding the DRBGs once get_random_bytes() has
sufficient entropy available.

For reference, the relevant history around handling DRBG (re)seeding in
the context of a not yet fully seeded get_random_bytes() is:

  commit 16b369a91d0d ("random: Blocking API for accessing
                        nonblocking_pool")
  commit 4c7879907edd ("crypto: drbg - add async seeding operation")

  commit 205a525c3342 ("random: Add callback API for random pool
                        readiness")
  commit 57225e679788 ("crypto: drbg - Use callback API for random
                        readiness")
  commit c2719503f5e1 ("random: Remove kernel blocking API")

However, some time later, the initialization state of get_random_bytes()
has been made queryable via rng_is_initialized() introduced with commit
9a47249d444d ("random: Make crng state queryable"). This primitive now
allows for streamlining the DRBG reseeding from get_random_bytes() by
replacing that aforementioned asynchronous work scheduling from
random_ready_callbacks with some simpler, synchronous code in
drbg_generate() next to the related logic already present therein. Apart
from improving overall code readability, this change will also enable DRBG
users to rely on wait_for_random_bytes() for ensuring that the initial
seeding has completed, if desired.

The previous patches already laid the grounds by making drbg_seed() to
record at each DRBG instance whether it was being seeded at a time when
rng_is_initialized() still had been false as indicated by
->seeded == DRBG_SEED_STATE_PARTIAL.

All that remains to be done now is to make drbg_generate() check for this
condition, determine whether rng_is_initialized() has flipped to true in
the meanwhile and invoke a reseed from get_random_bytes() if so.

Make this move:
- rename the former drbg_async_seed() work handler, i.e. the one in charge
  of reseeding a DRBG instance from get_random_bytes(), to
  "drbg_seed_from_random()",
- change its signature as appropriate, i.e. make it take a struct
  drbg_state rather than a work_struct and change its return type from
  "void" to "int" in order to allow for passing error information from
  e.g. its __drbg_seed() invocation onwards to callers,
- make drbg_generate() invoke this drbg_seed_from_random() once it
  encounters a DRBG instance with ->seeded == DRBG_SEED_STATE_PARTIAL by
  the time rng_is_initialized() has flipped to true and
- prune everything related to the former, random_ready_callback based
  mechanism.

As drbg_seed_from_random() is now getting invoked from drbg_generate() with
the ->drbg_mutex being held, it must not attempt to recursively grab it
once again. Remove the corresponding mutex operations from what is now
drbg_seed_from_random(). Furthermore, as drbg_seed_from_random() can now
report errors directly to its caller, there's no need for it to temporarily
switch the DRBG's ->seeded state to DRBG_SEED_STATE_UNSEEDED so that a
failure of the subsequently invoked __drbg_seed() will get signaled to
drbg_generate(). Don't do it then.

Signed-off-by: Nicolai Stange <nstange@suse.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

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