linux-kernel - Re: [PATCH 1/2 v2] hwrng: Allow drivers to disable reading during probe

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Date:	Wed, 2 Jul 2014 18:56:35 +0530
From:	Amit Shah <amit.shah@...hat.com>
To:	Jason Cooper <jason@...edaemon.net>
Cc:	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Kees Cook <keescook@...omium.org>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] hwrng: Allow drivers to disable reading during
 probe

Hi Jason,

On (Wed) 02 Jul 2014 [13:00:19], Jason Cooper wrote:
> Commit d9e7972619334 "hwrng: add randomness to system from rng sources"
> added a call to rng_get_data() from the hwrng_register() function.
> However, some rng devices need initialization before data can be read
> from them.
> 
> Also, the virtio-rng device does not behave properly when this call is
> made in its probe() routine - the virtio core sets the DRIVER_OK status
> bit only on a successful probe, which means the host ignores all
> communication from the guest, and the guest insmod or boot process just
> sits there doing nothing.
> 
> [ jac: Modify the API to allow drivers to disable reading at probe, new
> patch, copied Amit's commit message. ]
> 
> CC: Kees Cook <keescook@...omium.org>
> CC: Jason Cooper <jason@...edaemon.net>
> CC: Herbert Xu <herbert@...dor.apana.org.au>
> CC: <stable@...r.kernel.org> # v3.15+
> Signed-off-by: Amit Shah <amit.shah@...hat.com>
> Signed-off-by: Jason Cooper <jason@...edaemon.net>
> ---
>  drivers/char/hw_random/core.c | 8 +++++---
>  include/linux/hw_random.h     | 4 ++++
>  2 files changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> index 334601cc81cf..b7b6c48ca682 100644
> --- a/drivers/char/hw_random/core.c
> +++ b/drivers/char/hw_random/core.c
> @@ -347,9 +347,11 @@ int hwrng_register(struct hwrng *rng)
>  	INIT_LIST_HEAD(&rng->list);
>  	list_add_tail(&rng->list, &rng_list);
>  
> -	bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> -	if (bytes_read > 0)
> -		add_device_randomness(bytes, bytes_read);
> +	if (!(rng->flags & HWRNG_NO_READ_AT_PROBE)) {
> +		bytes_read = rng_get_data(rng, bytes, sizeof(bytes), 1);
> +		if (bytes_read > 0)
> +			add_device_randomness(bytes, bytes_read);
> +	}

But this has the inverse problem: if there are two hwrngs in the
system, one will be initialized and probed.  The other will not be
initialized, but still be probed.

My version was more conservative while this one keeps the bug from the
current kernels.

		Amit
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