util: Add qemu_guest_getrandom and associated routines

This routine is intended to produce high-quality random numbers to the
guest.  Normally, such numbers are crypto quality from the host, but a
command-line option can force the use of a fully deterministic sequence
for use while debugging.

Reviewed-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

include/qemu/guest-random.h

@@ -0,0 +1,68 @@
+/*
+ * QEMU guest-visible random functions
+ *
+ * Copyright 2019 Linaro, Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#ifndef QEMU_GUEST_RANDOM_H
+#define QEMU_GUEST_RANDOM_H
+
+/**
+ * qemu_guest_random_seed_main(const char *optarg, Error **errp)
+ * @optarg: a non-NULL pointer to a C string
+ * @errp: an error indicator
+ *
+ * The @optarg value is that which accompanies the -seed argument.
+ * This forces qemu_guest_getrandom into deterministic mode.
+ *
+ * Returns 0 on success, < 0 on failure while setting *errp.
+ */
+int qemu_guest_random_seed_main(const char *optarg, Error **errp);
+
+/**
+ * qemu_guest_random_seed_thread_part1(void)
+ *
+ * If qemu_getrandom is in deterministic mode, returns an
+ * independent seed for the new thread.  Otherwise returns 0.
+ */
+uint64_t qemu_guest_random_seed_thread_part1(void);
+
+/**
+ * qemu_guest_random_seed_thread_part2(uint64_t seed)
+ * @seed: a value for the new thread.
+ *
+ * If qemu_guest_getrandom is in deterministic mode, this stores an
+ * independent seed for the new thread.  Otherwise a no-op.
+ */
+void qemu_guest_random_seed_thread_part2(uint64_t seed);
+
+/**
+ * qemu_guest_getrandom(void *buf, size_t len, Error **errp)
+ * @buf: a buffer of bytes to be written
+ * @len: the number of bytes in @buf
+ * @errp: an error indicator
+ *
+ * Fills len bytes in buf with random data.  This should only be used
+ * for data presented to the guest.  Host-side crypto services should
+ * use qcrypto_random_bytes.
+ *
+ * Returns 0 on success, < 0 on failure while setting *errp.
+ */
+int qemu_guest_getrandom(void *buf, size_t len, Error **errp);
+
+/**
+ * qemu_guest_getrandom_nofail(void *buf, size_t len)
+ * @buf: a buffer of bytes to be written
+ * @len: the number of bytes in @buf
+ *
+ * Like qemu_guest_getrandom, but will assert for failure.
+ * Use this when there is no reasonable recovery.
+ */
+void qemu_guest_getrandom_nofail(void *buf, size_t len);
+
+#endif /* QEMU_GUEST_RANDOM_H */

util/Makefile.objs

@@ -54,5 +54,6 @@ util-obj-y += iova-tree.o
 util-obj-$(CONFIG_INOTIFY1) += filemonitor-inotify.o
 util-obj-$(CONFIG_LINUX) += vfio-helpers.o
 util-obj-$(CONFIG_OPENGL) += drm.o
+util-obj-y += guest-random.o
 
 stub-obj-y += filemonitor-stub.o

util/guest-random.c

@@ -0,0 +1,93 @@
+/*
+ * QEMU guest-visible random functions
+ *
+ * Copyright 2019 Linaro, Ltd.
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License as published by the Free
+ * Software Foundation; either version 2 of the License, or (at your option)
+ * any later version.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu-common.h"
+#include "qemu/cutils.h"
+#include "qapi/error.h"
+#include "qemu/guest-random.h"
+#include "crypto/random.h"
+
+
+static __thread GRand *thread_rand;
+static bool deterministic;
+
+
+static int glib_random_bytes(void *buf, size_t len)
+{
+    GRand *rand = thread_rand;
+    size_t i;
+    uint32_t x;
+
+    if (unlikely(rand == NULL)) {
+        /* Thread not initialized for a cpu, or main w/o -seed.  */
+        thread_rand = rand = g_rand_new();
+    }
+
+    for (i = 0; i + 4 <= len; i += 4) {
+        x = g_rand_int(rand);
+        __builtin_memcpy(buf + i, &x, 4);
+    }
+    if (i < len) {
+        x = g_rand_int(rand);
+        __builtin_memcpy(buf + i, &x, i - len);
+    }
+    return 0;
+}
+
+int qemu_guest_getrandom(void *buf, size_t len, Error **errp)
+{
+    if (unlikely(deterministic)) {
+        /* Deterministic implementation using Glib's Mersenne Twister.  */
+        return glib_random_bytes(buf, len);
+    } else {
+        /* Non-deterministic implementation using crypto routines.  */
+        return qcrypto_random_bytes(buf, len, errp);
+    }
+}
+
+void qemu_guest_getrandom_nofail(void *buf, size_t len)
+{
+    qemu_guest_getrandom(buf, len, &error_fatal);
+}
+
+uint64_t qemu_guest_random_seed_thread_part1(void)
+{
+    if (deterministic) {
+        uint64_t ret;
+        glib_random_bytes(&ret, sizeof(ret));
+        return ret;
+    }
+    return 0;
+}
+
+void qemu_guest_random_seed_thread_part2(uint64_t seed)
+{
+    g_assert(thread_rand == NULL);
+    if (deterministic) {
+        thread_rand =
+            g_rand_new_with_seed_array((const guint32 *)&seed,
+                                       sizeof(seed) / sizeof(guint32));
+    }
+}
+
+int qemu_guest_random_seed_main(const char *optarg, Error **errp)
+{
+    unsigned long long seed;
+    if (parse_uint_full(optarg, &seed, 0)) {
+        error_setg(errp, "Invalid seed number: %s", optarg);
+        return -1;
+    } else {
+        deterministic = true;
+        qemu_guest_random_seed_thread_part2(seed);
+        return 0;
+    }
+}