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)

4 files changed, 59 insertions, 1 deletions

diff --git a/include/internal/rand.h b/include/internal/rand.h
index 07f141d6cc..feda9beaa1 100644
--- a/include/internal/rand.h
+++ b/include/internal/rand.h
@@ -14,6 +14,17 @@
 #define RAND_DRBG_FLAG_CTR_USE_DF            0x2
 
 /*
+ * Default security strength (in the sense of [NIST SP 800-90Ar1])
+ * of the default OpenSSL DRBG, and the corresponding NID.
+ *
+ * Currently supported values: 128, 192, 256
+ *
+ * TODO(DRBG): would be nice to have the strength configurable
+ */
+# define RAND_DRBG_STRENGTH             128
+# define RAND_DRBG_NID                  NID_aes_128_ctr
+
+/*
  * Object lifetime functions.
  */
 RAND_DRBG *RAND_DRBG_new(int type, unsigned int flags, RAND_DRBG *parent);
@@ -64,4 +75,38 @@ int RAND_DRBG_set_callbacks(RAND_DRBG *dctx,
                             RAND_DRBG_get_nonce_fn get_nonce,
                             RAND_DRBG_cleanup_nonce_fn cleanup_nonce);
 
+/*
+ * RAND_POOL functions
+ */
+RAND_POOL *RAND_POOL_new(int entropy_requested, size_t min_len, size_t max_len);
+void RAND_POOL_free(RAND_POOL *pool);
+
+const unsigned char *RAND_POOL_buffer(RAND_POOL *pool);
+unsigned char *RAND_POOL_detach(RAND_POOL *pool);
+
+size_t RAND_POOL_entropy(RAND_POOL *pool);
+size_t RAND_POOL_length(RAND_POOL *pool);
+
+size_t RAND_POOL_entropy_available(RAND_POOL *pool);
+size_t RAND_POOL_entropy_needed(RAND_POOL *pool);
+size_t RAND_POOL_bytes_needed(RAND_POOL *pool, unsigned int entropy_per_byte);
+size_t RAND_POOL_bytes_remaining(RAND_POOL *pool);
+
+size_t RAND_POOL_add(RAND_POOL *pool,
+                     const unsigned char *buffer, size_t len, size_t entropy);
+unsigned char *RAND_POOL_add_begin(RAND_POOL *pool, size_t len);
+size_t RAND_POOL_add_end(RAND_POOL *pool, size_t len, size_t entropy);
+
+
+/*
+ * Add random bytes to the pool to acquire requested amount of entropy
+ *
+ * This function is platform specific and tries to acquire the requested
+ * amount of entropy by polling platform specific entropy sources.
+ *
+ * If the function succeeds in acquiring at least |entropy_requested| bits
+ * of entropy, the total entropy count is returned. If it fails, it returns
+ * an entropy count of 0.
+ */
+size_t RAND_POOL_acquire_entropy(RAND_POOL *pool);
 #endif
diff --git a/include/openssl/ossl_typ.h b/include/openssl/ossl_typ.h
index b00777694f..f5214ab1e5 100644
--- a/include/openssl/ossl_typ.h
+++ b/include/openssl/ossl_typ.h
@@ -115,6 +115,7 @@ typedef struct ec_key_method_st EC_KEY_METHOD;
 
 typedef struct rand_meth_st RAND_METHOD;
 typedef struct rand_drbg_st RAND_DRBG;
+typedef struct rand_pool_st RAND_POOL;
 
 typedef struct ssl_dane_st SSL_DANE;
 typedef struct x509_st X509;
diff --git a/include/openssl/rand.h b/include/openssl/rand.h
index 82e3762690..2dec35ea53 100644
--- a/include/openssl/rand.h
+++ b/include/openssl/rand.h
@@ -64,7 +64,6 @@ int RAND_egd_bytes(const char *path, int bytes);
 typedef void (*RAND_poll_cb)(void *arg,
                              const void *buf, int num, double randomness);
 int RAND_poll(void);
-int RAND_poll_ex(RAND_poll_cb rand_add, void *arg);
 
 # if defined(_WIN32) && (defined(BASETYPES) || defined(_WINDEF_H))
 /* application has to include <windows.h> in order to use these */
diff --git a/include/openssl/randerr.h b/include/openssl/randerr.h
index 79c652fcdf..b07ea233eb 100644
--- a/include/openssl/randerr.h
+++ b/include/openssl/randerr.h
@@ -24,14 +24,21 @@ int ERR_load_RAND_strings(void);
  */
 # define RAND_F_DRBG_BYTES                                101
 # define RAND_F_DRBG_GET_ENTROPY                          105
+# define RAND_F_DRBG_SETUP                                117
 # define RAND_F_GET_ENTROPY                               106
 # define RAND_F_RAND_BYTES                                100
 # define RAND_F_RAND_DRBG_GENERATE                        107
 # define RAND_F_RAND_DRBG_INSTANTIATE                     108
 # define RAND_F_RAND_DRBG_NEW                             109
 # define RAND_F_RAND_DRBG_RESEED                          110
+# define RAND_F_RAND_DRBG_RESTART                         102
 # define RAND_F_RAND_DRBG_SET                             104
 # define RAND_F_RAND_LOAD_FILE                            111
+# define RAND_F_RAND_POOL_ADD                             103
+# define RAND_F_RAND_POOL_ADD_BEGIN                       113
+# define RAND_F_RAND_POOL_ADD_END                         114
+# define RAND_F_RAND_POOL_BYTES_NEEDED                    115
+# define RAND_F_RAND_POOL_NEW                             116
 # define RAND_F_RAND_WRITE_FILE                           112
 
 /*
@@ -39,13 +46,18 @@ int ERR_load_RAND_strings(void);
  */
 # define RAND_R_ADDITIONAL_INPUT_TOO_LONG                 102
 # define RAND_R_ALREADY_INSTANTIATED                      103
+# define RAND_R_ARGUMENT_OUT_OF_RANGE                     105
 # define RAND_R_CANNOT_OPEN_FILE                          121
 # define RAND_R_DRBG_NOT_INITIALISED                      104
+# define RAND_R_ENTROPY_INPUT_TOO_LONG                    106
+# define RAND_R_ENTROPY_OUT_OF_RANGE                      124
+# define RAND_R_ERROR_ENTROPY_POOL_WAS_IGNORED            127
 # define RAND_R_ERROR_INITIALISING_DRBG                   107
 # define RAND_R_ERROR_INSTANTIATING_DRBG                  108
 # define RAND_R_ERROR_RETRIEVING_ADDITIONAL_INPUT         109
 # define RAND_R_ERROR_RETRIEVING_ENTROPY                  110
 # define RAND_R_ERROR_RETRIEVING_NONCE                    111
+# define RAND_R_FAILED_TO_CREATE_LOCK                     126
 # define RAND_R_FUNC_NOT_IMPLEMENTED                      101
 # define RAND_R_FWRITE_ERROR                              123
 # define RAND_R_GENERATE_ERROR                            112
@@ -55,6 +67,7 @@ int ERR_load_RAND_strings(void);
 # define RAND_R_NOT_INSTANTIATED                          115
 # define RAND_R_PERSONALISATION_STRING_TOO_LONG           116
 # define RAND_R_PRNG_NOT_SEEDED                           100
+# define RAND_R_RANDOM_POOL_OVERFLOW                      125
 # define RAND_R_REQUEST_TOO_LARGE_FOR_DRBG                117
 # define RAND_R_RESEED_ERROR                              118
 # define RAND_R_SELFTEST_FAILURE                          119