[Qemu-commits] [qemu/qemu] a2230b: VirtIO-RNG: Update default entropy source to `/dev...

[Peter Maydell]

  Branch: refs/heads/master
  Home:   https://github.com/qemu/qemu
  Commit: a2230bd778d8445231d2120b87be3ed262bb4f8a
      
https://github.com/qemu/qemu/commit/a2230bd778d8445231d2120b87be3ed262bb4f8a
  Author: Kashyap Chamarthy <address@hidden>
  Date:   2019-07-03 (Wed, 03 Jul 2019)

  Changed paths:
    M backends/rng-random.c
    M qemu-options.hx

  Log Message:
  -----------
  VirtIO-RNG: Update default entropy source to `/dev/urandom`

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>
Message-Id: <address@hidden>
Signed-off-by: Laurent Vivier <address@hidden>


  Commit: 2cd925dac80d708d01f124b26de52fb447a333aa
      
https://github.com/qemu/qemu/commit/2cd925dac80d708d01f124b26de52fb447a333aa
  Author: Liam Merwick <address@hidden>
  Date:   2019-07-03 (Wed, 03 Jul 2019)

  Changed paths:
    M docs/devel/testing.rst

  Log Message:
  -----------
  docs/devel/testing: Fix typo in dockerfile path

Signed-off-by: Liam Merwick <address@hidden>
Reviewed-by: Philippe Mathieu-Daudé <address@hidden>
Reviewed-by: Alex Bennée <address@hidden>
Reviewed-by: Li Qiang <address@hidden>
Message-Id: <address@hidden>
Signed-off-by: Laurent Vivier <address@hidden>


  Commit: d2c5f91ca944aaade642624397e1853801bbc744
      
https://github.com/qemu/qemu/commit/d2c5f91ca944aaade642624397e1853801bbc744
  Author: Peter Maydell <address@hidden>
  Date:   2019-07-05 (Fri, 05 Jul 2019)

  Changed paths:
    M backends/rng-random.c
    M docs/devel/testing.rst
    M qemu-options.hx

  Log Message:
  -----------
  Merge remote-tracking branch 
'remotes/vivier2/tags/trivial-branch-pull-request' into staging

Fix typo, change virtio-rng default to urandom

# gpg: Signature made Wed 03 Jul 2019 16:24:53 BST
# gpg:                using RSA key CD2F75DDC8E3A4DC2E4F5173F30C38BD3F2FBE3C
# gpg:                issuer "address@hidden"
# gpg: Good signature from "Laurent Vivier <address@hidden>" [full]
# gpg:                 aka "Laurent Vivier <address@hidden>" [full]
# gpg:                 aka "Laurent Vivier (Red Hat) <address@hidden>" [full]
# Primary key fingerprint: CD2F 75DD C8E3 A4DC 2E4F  5173 F30C 38BD 3F2F BE3C

* remotes/vivier2/tags/trivial-branch-pull-request:
  docs/devel/testing: Fix typo in dockerfile path
  VirtIO-RNG: Update default entropy source to `/dev/urandom`

Signed-off-by: Peter Maydell <address@hidden>


Compare: https://github.com/qemu/qemu/compare/dba519a2980a...d2c5f91ca944