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Re: [Qemu-trivial] [Qemu-devel] [PATCH v7 1/4] VirtIO-RNG: Update defaul
From: |
Laurent Vivier |
Subject: |
Re: [Qemu-trivial] [Qemu-devel] [PATCH v7 1/4] VirtIO-RNG: Update default entropy source to `/dev/urandom` |
Date: |
Tue, 2 Jul 2019 09:41:30 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.7.0 |
Le 29/05/2019 à 16:31, Laurent Vivier a écrit :
> From: Kashyap Chamarthy <address@hidden>
>
> When QEMU exposes a VirtIO-RNG device to the guest, that device needs a
> source of entropy, and that source needs to be "non-blocking", like
> `/dev/urandom`. However, currently QEMU defaults to the problematic
> `/dev/random`, which on Linux is "blocking" (as in, it waits until
> sufficient entropy is available).
>
> Why prefer `/dev/urandom` over `/dev/random`?
> ---------------------------------------------
>
> The man pages of urandom(4) and random(4) state:
>
> "The /dev/random device is a legacy interface which dates back to a
> time where the cryptographic primitives used in the implementation
> of /dev/urandom were not widely trusted. It will return random
> bytes only within the estimated number of bits of fresh noise in the
> entropy pool, blocking if necessary. /dev/random is suitable for
> applications that need high quality randomness, and can afford
> indeterminate delays."
>
> Further, the "Usage" section of the said man pages state:
>
> "The /dev/random interface is considered a legacy interface, and
> /dev/urandom is preferred and sufficient in all use cases, with the
> exception of applications which require randomness during early boot
> time; for these applications, getrandom(2) must be used instead,
> because it will block until the entropy pool is initialized.
>
> "If a seed file is saved across reboots as recommended below (all
> major Linux distributions have done this since 2000 at least), the
> output is cryptographically secure against attackers without local
> root access as soon as it is reloaded in the boot sequence, and
> perfectly adequate for network encryption session keys. Since reads
> from /dev/random may block, users will usually want to open it in
> nonblocking mode (or perform a read with timeout), and provide some
> sort of user notification if the desired entropy is not immediately
> available."
>
> And refer to random(7) for a comparison of `/dev/random` and
> `/dev/urandom`.
>
> What about other OSes?
> ----------------------
>
> `/dev/urandom` exists and works on OS-X, FreeBSD, DragonFlyBSD, NetBSD
> and OpenBSD, which cover all the non-Linux platforms we explicitly
> support, aside from Windows.
>
> On Windows `/dev/random` doesn't work either so we don't regress.
> This is actually another argument in favour of using the newly
> proposed 'rng-builtin' backend by default, as that will work on
> Windows.
>
> - - -
>
> Given the above, change the entropy source for VirtIO-RNG device to
> `/dev/urandom`.
>
> Related discussion in these[1][2] past threads.
>
> [1] https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html
> -- "RNG: Any reason QEMU doesn't default to `/dev/urandom`?"
> [2] https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html
> -- "[RFC] Virtio RNG: Consider changing the default entropy source to
> /dev/urandom"
>
> Signed-off-by: Kashyap Chamarthy <address@hidden>
> Reviewed-by: Daniel P. Berrangé <address@hidden>
> Reviewed-by: Stefan Hajnoczi <address@hidden>
> Reviewed-by: Markus Armbruster <address@hidden>
> Signed-off-by: Laurent Vivier <address@hidden>
> ---
> backends/rng-random.c | 2 +-
> qemu-options.hx | 2 +-
> 2 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/backends/rng-random.c b/backends/rng-random.c
> index e2a49b0571d7..eff36ef14084 100644
> --- a/backends/rng-random.c
> +++ b/backends/rng-random.c
> @@ -112,7 +112,7 @@ static void rng_random_init(Object *obj)
> rng_random_set_filename,
> NULL);
>
> - s->filename = g_strdup("/dev/random");
> + s->filename = g_strdup("/dev/urandom");
> s->fd = -1;
> }
>
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 39dc17042967..f6e9bd1d9c42 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -4328,7 +4328,7 @@ Creates a random number generator backend which obtains
> entropy from
> a device on the host. The @option{id} parameter is a unique ID that
> will be used to reference this entropy backend from the @option{virtio-rng}
> device. The @option{filename} parameter specifies which file to obtain
> -entropy from and if omitted defaults to @option{/dev/random}.
> +entropy from and if omitted defaults to @option{/dev/urandom}.
>
> @item -object rng-egd,id=@var{id},chardev=@var{chardevid}
>
>
As this patch is trivial and reviewed by at least 3 persons, and it
seems no one wants to take care of it, I will push it through my next
trivial pull request.
Thanks,
Laurent
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Laurent Vivier <=