Re: [PATCH] crypto/gcrypt: prefer kernel as direct source of entropy

[Daniel P . Berrangé]

On Fri, Jan 19, 2024 at 05:39:40PM -0300, Cristian Rodríguez wrote:
> gcrypt by default uses an userspace RNG, which cannot know
> when it is time to discard/invalidate its buffer
> (suspend, resume, vm forks, other corner cases)
> as a "when to discard" event is unavailable to userspace.

So in this scenario QEMU is impacted when QEMU is running inside
another VM. ie the L0 QEMU "forks" the guest, and the L1 QEMU
needs to re-init its RNG.

> Set GCRYCTL_SET_PREFERRED_RNG_TYPE to GCRY_RNG_TYPE_SYSTEM
> which must be done before the first call to gcry_check_version()

QEMU is just one out of many applications that use libgcrypt and
I see no reason why QEMU should be special cased in this respect.

Updating each application to hardcode use of GCRY_RNG_TYPE_SYSTEM
does not feel like a good solution. If this was a good default
to use everywhere, then gcrypt should have set this default
already, rather than requiring every app to solve the forking
problem itself.

Updating every app that uses gcrypt is not really practical
in terms of time investment anyway.

If gcrypt doesn't want to make this its global default, then
I would suggest that gcrypt should make its default be
configurable. I see from its docs:

https://gnupg.org/documentation/manuals/gcrypt/Configuration.html#Configuration

that it already supports a /etc/gcrypt/random.conf file.
Perhaps they would extend that to allow selection of the
default RNG backend, system-wide.

> 
> Signed-off-by: Cristian Rodríguez <cristian@rodriguez.im>
> ---
>  crypto/init.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/crypto/init.c b/crypto/init.c
> index fb7f1bff10..0c3fe6a841 100644
> --- a/crypto/init.c
> +++ b/crypto/init.c
> @@ -60,6 +60,7 @@ int qcrypto_init(Error **errp)
>  #endif
>  
>  #ifdef CONFIG_GCRYPT
> +    gcry_control(GCRYCTL_SET_PREFERRED_RNG_TYPE, GCRY_RNG_TYPE_SYSTEM);
>      if (!gcry_check_version(NULL)) {
>          error_setg(errp, "Unable to initialize gcrypt");
>          return -1;
> -- 
> 2.43.0
> 

With regards,
Daniel
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