Fix async deadlock problem

The rand code can aquire locks and then attempt crypto operations. This
can end up in a deadlock if we are using an async engine, because control
returns back to the user code whilst still holding the lock. We need to
force synchronous operation for these sections of code.
Reviewed-by: NRich Salz <rsalz@openssl.org>

crypto/rand/md_rand.c

@@ -130,6 +130,7 @@
 
 #include <openssl/crypto.h>
 #include <openssl/rand.h>
+#include <openssl/async.h>
 #include "rand_lcl.h"
 
 #include <openssl/err.h>
@@ -432,6 +433,11 @@ static int rand_bytes(unsigned char *buf, int num, int pseudo)
      */
 
     CRYPTO_w_lock(CRYPTO_LOCK_RAND);
+    /*
+     * We could end up in an async engine while holding this lock so ensure
+     * we don't pause and cause a deadlock
+     */
+    ASYNC_block_pause();
 
     /* prevent rand_bytes() from trying to obtain the lock again */
     CRYPTO_w_lock(CRYPTO_LOCK_RAND2);
@@ -510,6 +516,7 @@ static int rand_bytes(unsigned char *buf, int num, int pseudo)
 
     /* before unlocking, we must clear 'crypto_lock_rand' */
     crypto_lock_rand = 0;
+    ASYNC_unblock_pause();
     CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
 
     while (num > 0) {
@@ -576,10 +583,15 @@ static int rand_bytes(unsigned char *buf, int num, int pseudo)
         || !MD_Update(&m, local_md, MD_DIGEST_LENGTH))
         goto err;
     CRYPTO_w_lock(CRYPTO_LOCK_RAND);
+    /*
+     * Prevent deadlocks if we end up in an async engine
+     */
+    ASYNC_block_pause();
     if (!MD_Update(&m, md, MD_DIGEST_LENGTH) || !MD_Final(&m, md)) {
         CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
         goto err;
     }
+    ASYNC_unblock_pause();
     CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
 
     EVP_MD_CTX_cleanup(&m);
@@ -635,6 +647,10 @@ static int rand_status(void)
 
     if (!do_not_lock) {
         CRYPTO_w_lock(CRYPTO_LOCK_RAND);
+        /*
+         * Prevent deadlocks in case we end up in an async engine
+         */
+        ASYNC_block_pause();
 
         /*
          * prevent rand_bytes() from trying to obtain the lock again
@@ -656,6 +672,7 @@ static int rand_status(void)
         /* before unlocking, we must clear 'crypto_lock_rand' */
         crypto_lock_rand = 0;
 
+        ASYNC_unblock_pause();
         CRYPTO_w_unlock(CRYPTO_LOCK_RAND);
     }