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Re: [Qemu-devel] [PATCH v2] VirtIO-RNG: Update default entropy source to
Re: [Qemu-devel] [PATCH v2] VirtIO-RNG: Update default entropy source to `/dev/urandom`
Fri, 10 May 2019 14:03:33 +0200
Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux)
Kashyap Chamarthy <address@hidden> writes:
> 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 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
> And refer to random(7) for a comparison of `/dev/random` and
This is Linux. What about other supported POSIX[*] hosts? If any such
host has /dev/random that works here, but not /dev/urandom, we regress.
*If* there's an actual regression risk: a simple & stupid way to reduce
it risk could be falling back to /dev/random when opening /dev/urandom
fails. Perhaps only when it fails with ENOENT.
Possible implementation: instead of setting a default filename in
rng_random_init(), change rng_random_opened() to try /dev/urandom, then
/dev/random when filename is still null.
Aside: "opened" sounds like a predicate. Goes back to commit
> Given the above, change the entropy source for VirtIO-RNG device to
> Related discussion in these past threads.
>  https://lists.nongnu.org/archive/html/qemu-devel/2018-06/msg08335.html
> -- "RNG: Any reason QEMU doesn't default to `/dev/urandom`?"
>  https://lists.nongnu.org/archive/html/qemu-devel/2018-09/msg02724.html
> -- "[RFC] Virtio RNG: Consider changing the default entropy source to
> Signed-off-by: Kashyap Chamarthy <address@hidden>
[*] POSIX because
common-obj-$(CONFIG_POSIX) += rng-random.o