linux-kernel - Re: [PATCH] hwrng: core - don't block in add_early

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Date:   Mon, 8 Apr 2019 08:55:29 +0200
From:   Laurent Vivier <lvivier@...hat.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        Matt Mackall <mpm@...enic.com>
Subject: Re: [PATCH] hwrng: core - don't block in add_early_randomness()

On 08/04/2019 08:31, Herbert Xu wrote:
> On Fri, Mar 29, 2019 at 05:30:11PM +0100, Laurent Vivier wrote:
>> If the device is not ready to provide data the kernel will
>> be stuck indefinitely in the init function.
>>
>> This is not a problem if the device is driven using a module,
>> but if the driver is linked directly into the kernel then the
>> kernel boot sequence hangs.
>>
>> This can happen with virtio-rng device with rng-egd backend
>> with no data provider, for instance with QEMU command line parameters:
>>
>> ...
>>     -chardev socket,id=charrng0,host=localhost,port=2345,server,nowait \
>>     -object rng-egd,id=objrng0,chardev=charrng0 \
>>     -device virtio-rng-pci,rng=objrng0,id=rng0
>>
>> To avoid that, we can call rng_get_data() in non blocking mode because
>> the function already manages the case where byte_read is
>> 0 (if the device is not already initialized).
>>
>> See also commit d3cc7996473a
>> ("hwrng: fetch randomness only after device init")
>>
>> Signed-off-by: Laurent Vivier <lvivier@...hat.com>
> 
> I think this is either not a bug (make sure that the user supplies
> a RNG source) or it should be addressed in virtio-rng.

Yes, I did more investigation and I agree.

Thanks,
Laurent