Fix reseeding issues of the public RAND_DRBG

Reseeding is handled very differently by the classic RAND_METHOD API
and the new RAND_DRBG api. These differences led to some problems when
the new RAND_DRBG was made the default OpenSSL RNG. In particular,
RAND_add() did not work as expected anymore. These issues are discussed
on the thread '[openssl-dev] Plea for a new public OpenSSL RNG API'
and in Pull Request #4328. This commit fixes the mentioned issues,
introducing the following changes:

- Replace the fixed size RAND_BYTES_BUFFER by a new RAND_POOL API which
  facilitates collecting entropy by the get_entropy() callback.
- Don't use RAND_poll()/RAND_add() for collecting entropy from the
  get_entropy() callback anymore. Instead, replace RAND_poll() by
  RAND_POOL_acquire_entropy().
- Add a new function rand_drbg_restart() which tries to get the DRBG
  in an instantiated state by all means, regardless of the current
  state (uninstantiated, error, ...) the DRBG is in. If the caller
  provides entropy or additional input, it will be used for reseeding.
- Restore the original documented behaviour of RAND_add() and RAND_poll()
  (namely to reseed the DRBG immediately) by a new implementation based
  on rand_drbg_restart().
- Add automatic error recovery from temporary failures of the entropy
  source to RAND_DRBG_generate() using the rand_drbg_restart() function.

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Kurt Roeckx <kurt@roeckx.be>
Reviewed-by: Rich Salz <rsalz@openssl.org>
Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/4328)

1 files changed, 73 insertions, 16 deletions

diff --git a/doc/man3/RAND_add.pod b/doc/man3/RAND_add.pod
index ea81492c2c..62048e6792 100644
--- a/doc/man3/RAND_add.pod
+++ b/doc/man3/RAND_add.pod
@@ -2,8 +2,7 @@
 
 =head1 NAME
 
-RAND_add, RAND_poll, RAND_poll_ex, RAND_poll_cb,
-RAND_seed, RAND_status, RAND_event, RAND_screen
+RAND_add, RAND_poll, RAND_seed, RAND_status, RAND_event, RAND_screen
 - add randomness to the PRNG or get its status
 
 =head1 SYNOPSIS
@@ -11,10 +10,6 @@ RAND_seed, RAND_status, RAND_event, RAND_screen
  #include <openssl/rand.h>
 
  int RAND_status(void);
-
- typedef void (*RAND_poll_cb)(void *arg,
-                              const void *buf, int num, double randomness);
- int RAND_poll_ex(RAND_poll_cb cb, void *arg);
  int RAND_poll();
 
  void RAND_add(const void *buf, int num, double randomness);
@@ -40,16 +35,12 @@ and network packet timings, can be reasonable sources of seeding material.
 RAND_status() indicates whether or not the CSPRNG has been sufficiently
 seeded. If not, functions such as RAND_bytes(3) will fail.
 
-RAND_poll_ex() uses the system's capabilities to obtain a buffer
-containing random bits which can then be used to seed a CSPRNG. The
-exact features used depends on how OpenSSL was configured, and a summary
-can be displayed with the OpenSSL L<version(1)> command.  This function
-is normally called as needed by the CSPRNG.  The B<arg> parameter is an
-arbitrary pointer which will be passed as an argument to the callback.
-The B<cb> function is called each time there is data to add.
-
-RAND_poll() invokes RAND_poll_ex() with B<cb> and B<arg> set so that it
-will call RAND_add(), to add the randomness to the global CSPRNG.
+RAND_poll() uses the system's capabilities to seed the CSPRNG using
+random input obtained from polling various trusted entropy sources.
+The default choice of the entropy source can be modified at build time
+using the --with-rand-seed configure option, see also the B<NOTES> section.
+A summary of the configure options can be displayed with the OpenSSL
+L<version(1)> command.
 
 RAND_add() mixes the B<num> bytes at B<buf> into the PRNG state.
 The B<randomness> argument is an estimate of how much randomness is
@@ -78,6 +69,72 @@ RAND_event() returns RAND_status().
 
 The other functions do not return values.
 
+=head1 NOTES
+
+The new OpenSSL DRBG has some peculiarities which need to be taken
+into account when it is selected as the default OpenSSL CSPRNG, i.e.,
+when RAND_get_rand_method() == RAND_OpenSSL().
+This applies in particular to the way reseeding is done by the DRBG:
+
+=over 2
+
+=item *
+
+The DRBG seeds itself automatically, pulling random input from trusted
+entropy sources.
+Automatic reseeding occurs after a predefined number of generate requests.
+The selection of the trusted entropy sources is configured at build
+time using the --with-rand-seed option.
+
+=item *
+
+The DRBG distinguishes two different types of random input:
+'entropy', which comes from a trusted source, and 'additional input',
+which can optionally be added by the user and is considered untrusted.
+
+=back
+
+Automatic seeding can be disabled using the --with-rand-seed=none option.
+
+=head2 DRBG with automatic seeding enabled
+
+Calling RAND_poll() or RAND_add() is not necessary, because the DRBG
+polls the entropy source automatically.
+However, both calls are permitted, and do reseed the RNG.
+
+RAND_add() can be used to add both kinds of random input, depending on the
+value of the B<randomness> argument:
+
+=over 4
+
+=item randomness == 0:
+
+The random bytes are mixed as additional input into the current state of
+the DRBG.
+Mixing in additional input is not considered a full reseeding, hence the
+reseed counter is not reset.
+
+
+=item randomness > 0:
+
+The random bytes are used as entropy input for a full reseeding
+(resp. reinstantiation) if the DRBG is instantiated
+(resp. uninstantiated or in an error state).
+A reseeding requires 16 bytes (128 bits) of randomness.
+It is possible to provide less randomness than required.
+In this case the missing randomness will be obtained by pulling random input
+from the trusted entropy sources.
+
+=back
+
+=head2 DRBG with automatic seeding disabled (--with-rand-seed=none)
+
+Calling RAND_poll() will always fail.
+
+RAND_add() needs to be called for initial seeding and periodic reseeding.
+At least 16 bytes (128 bits) of randomness have to be provided, otherwise
+the (re-)seeding of the DRBG will fail.
+
 =head1 HISTORY
 
 RAND_event() and RAND_screen() were deprecated in OpenSSL 1.1.0 and should