Re: [Qemu-devel] [PATCH v2] VirtIO-RNG: Update default entropy source to `/dev/urandom`

[Markus Armbruster <armbru@redhat.com>]

Kashyap Chamarthy <kchamart@redhat.com> 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
>     available."
>
> And refer to random(7) for a comparison of `/dev/random` and
> `/dev/urandom`.

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
a9b7b2ad7b0.

> 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 <kchamart@redhat.com>

[*] POSIX because
common-obj-$(CONFIG_POSIX) += rng-random.o