Re: [Qemu-devel] [PATCH v3 09/23] util: Add qemu_guest_getrandom and associated routines

[Philippe Mathieu-Daudé]

On 3/15/19 4:26 AM, Richard Henderson wrote:
> 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.
> 
> Cc: Daniel P. Berrangé <address@hidden>
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  include/qemu/guest-random.h | 68 +++++++++++++++++++++++++++
>  util/guest-random.c         | 93 +++++++++++++++++++++++++++++++++++++
>  util/Makefile.objs          |  1 +
>  3 files changed, 162 insertions(+)
>  create mode 100644 include/qemu/guest-random.h
>  create mode 100644 util/guest-random.c
> 
> diff --git a/include/qemu/guest-random.h b/include/qemu/guest-random.h
> new file mode 100644
> index 0000000000..09ff9c2236
> --- /dev/null
> +++ b/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

API documentation appreciated, thanks!

Reviewed-by: Philippe Mathieu-Daudé <address@hidden>

> + *
> + * 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 */
> diff --git a/util/guest-random.c b/util/guest-random.c
> new file mode 100644
> index 0000000000..e8124a3cad
> --- /dev/null
> +++ b/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;
> +    }
> +}
> diff --git a/util/Makefile.objs b/util/Makefile.objs
> index 835fcd69e2..4d4db653cc 100644
> --- a/util/Makefile.objs
> +++ b/util/Makefile.objs
> @@ -53,5 +53,6 @@ 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
>