crypto: add "none" random provider

In case of not using random-number needing feature, it makes sense to
skip RNG init too. This is especially helpful when QEMU is sandboxed in
Stubdomain under Xen, where there is very little entropy so initial
getrandom() call delays the startup several seconds. In that setup, no
random bytes are needed at all.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

configure

@@ -509,6 +509,7 @@ libpmem=""
 default_devices="yes"
 plugins="no"
 fuzzing="no"
+rng_none="no"
 
 supported_cpu="no"
 supported_os="no"
@@ -1601,6 +1602,10 @@ for opt do
   ;;
   --gdb=*) gdb_bin="$optarg"
   ;;
+  --enable-rng-none) rng_none=yes
+  ;;
+  --disable-rng-none) rng_none=no
+  ;;
   *)
       echo "ERROR: unknown option $opt"
       echo "Try '$0 --help' for more information"
@@ -1898,6 +1903,7 @@ disabled with --disable-FEATURE, default is enabled if available:
   debug-mutex     mutex debugging support
   libpmem         libpmem support
   xkbcommon       xkbcommon support
+  rng-none        dummy RNG, avoid using /dev/(u)random and getrandom()
 
 NOTE: The object files are built at the place where configure is launched
 EOF
@@ -6767,6 +6773,7 @@ echo "default devices   $default_devices"
 echo "plugin support    $plugins"
 echo "fuzzing support   $fuzzing"
 echo "gdb               $gdb_bin"
+echo "rng-none          $rng_none"
 
 if test "$supported_cpu" = "no"; then
     echo
@@ -7744,6 +7751,10 @@ if test "$edk2_blobs" = "yes" ; then
   echo "DECOMPRESS_EDK2_BLOBS=y" >> $config_host_mak
 fi
 
+if test "$rng_none" = "yes"; then
+  echo "CONFIG_RNG_NONE=y" >> $config_host_mak
+fi
+
 # use included Linux headers
 if test "$linux" = "yes" ; then
   mkdir -p linux-headers

crypto/Makefile.objs

@@ -35,5 +35,6 @@ crypto-obj-y += block-luks.o
 
 util-obj-$(CONFIG_GCRYPT) += random-gcrypt.o
 util-obj-$(if $(CONFIG_GCRYPT),n,$(CONFIG_GNUTLS)) += random-gnutls.o
-util-obj-$(if $(CONFIG_GCRYPT),n,$(if $(CONFIG_GNUTLS),n,y)) += random-platform.o
+util-obj-$(if $(CONFIG_GCRYPT),n,$(if $(CONFIG_GNUTLS),n,$(CONFIG_RNG_NONE))) += random-none.o
+util-obj-$(if $(CONFIG_GCRYPT),n,$(if $(CONFIG_GNUTLS),n,$(if $(CONFIG_RNG_NONE),n,y))) += random-platform.o
 util-obj-y += aes.o init.o

crypto/random-none.c

@@ -0,0 +1,38 @@
+/*
+ * QEMU Crypto "none" random number provider
+ *
+ * Copyright (c) 2020 Marek Marczykowski-Górecki
+ *                      <marmarek@invisiblethingslab.com>
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#include "qemu/osdep.h"
+
+#include "crypto/random.h"
+#include "qapi/error.h"
+
+int qcrypto_random_init(Error **errp)
+{
+    return 0;
+}
+
+int qcrypto_random_bytes(void *buf,
+                         size_t buflen,
+                         Error **errp)
+{
+    error_setg(errp, "Random bytes not available with \"none\" rng");
+    return -1;
+}