Never allow OpenSSL engines to replace the RAND_SSLeay method

This fixes bug 10402, where the rdrand engine would use the rdrand
instruction, not as an additional entropy source, but as a replacement
for the entire userspace PRNG.  That's obviously stupid: even if you
don't think that RDRAND is a likely security risk, the right response
to an alleged new alleged entropy source is never to throw away all
previously used entropy sources.

Thanks to coderman and rl1987 for diagnosing and tracking this down.

changes/bug10402

@@ -0,0 +1,11 @@
+  o Major bugfixes:
+    - Do not allow OpenSSL engines to replace the PRNG, even when
+      HardwareAccel is set. The only default builtin PRNG engine uses
+      the Intel RDRAND instruction to replace the entire PRNG, and
+      ignores all attempts to seed it with more entropy. That's
+      cryptographically stupid: the right response to a new alleged
+      entropy source is never to discard all previously used entropy
+      sources. Fixes bug 10402; works around behavior introduced in
+      OpenSSL 1.0.0. Diagnosis and investigation thanks to "coderman"
+      and "rl1987".
+

src/common/crypto.c

@@ -169,8 +169,8 @@ log_engine(const char *fn, ENGINE *e)
     const char *name, *id;
     name = ENGINE_get_name(e);
     id = ENGINE_get_id(e);
-    log_notice(LD_CRYPTO, "Using OpenSSL engine %s [%s] for %s",
-        name?name:"?", id?id:"?", fn);
+    log_notice(LD_CRYPTO, "Default OpenSSL engine for %s is %s [%s]",
+               fn, name?name:"?", id?id:"?");
   } else {
     log_info(LD_CRYPTO, "Using default implementation for %s", fn);
   }
@@ -288,7 +288,7 @@ crypto_global_init(int useAccel, const char *accelName, const char *accelDir)
       }
       log_engine("RSA", ENGINE_get_default_RSA());
       log_engine("DH", ENGINE_get_default_DH());
-      log_engine("RAND", ENGINE_get_default_RAND());
+      log_engine("RAND (which we will not use)", ENGINE_get_default_RAND());
       log_engine("SHA1", ENGINE_get_digest_engine(NID_sha1));
       log_engine("3DES", ENGINE_get_cipher_engine(NID_des_ede3_ecb));
       log_engine("AES", ENGINE_get_cipher_engine(NID_aes_128_ecb));
@@ -297,6 +297,13 @@ crypto_global_init(int useAccel, const char *accelName, const char *accelDir)
       log_info(LD_CRYPTO, "NOT using OpenSSL engine support.");
     }
 
+    if (RAND_get_rand_method() != RAND_SSLeay()) {
+      log_notice(LD_CRYPTO, "It appears that one of our engines has provided "
+                 "a replacement the OpenSSL RNG. Resetting it to the default "
+                 "implementation.");
+      RAND_set_rand_method(RAND_SSLeay());
+    }
+
     evaluate_evp_for_aes(-1);
     evaluate_ctr_for_aes();