Laurent Vivier <[email protected]> writes:
> From: Kashyap Chamarthy <[email protected]>
>
> 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`.
>
> - - -
>
> 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 <[email protected]>
> Reviewed-by: Daniel P. Berrangé <[email protected]>
> Reviewed-by: Stefan Hajnoczi <[email protected]>
> Signed-off-by: Laurent Vivier <[email protected]>
Suggest to work Daniel's analysis into the commit message, like this:
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` on Linux?
------------------------------------------------------
[...]
What about other OSes?
----------------------
/dev/urandom exists and works on OS-X, FreeBSD, DragonFlyBSD, NetBSD
and OpenBSD, which covers 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`.
[...]
