Re: [PATCH] Allow hwrng to initialize crng.

[Louis Collard <louiscollard@chromium.org>]

On Sun, Nov 18, 2018 at 4:15 AM Michael Niewöhner <linux@mniewoehner.de> wrote:
>
> Hi Louis,
>
> On Wed, 2018-09-26 at 11:24 +0800, Louis Collard wrote:
> > Some systems, for example embedded systems, do not generate
> > enough entropy on boot through interrupts, and boot may be blocked for
> > several minutes waiting for a call to getrandom to complete.
> >
> > Currently, random data is read from a hwrng when it is registered,
> > and is loaded into primary_crng. This data is treated in the same
> > way as data that is device-specific but otherwise unchanging, and
> > so primary_crng cannot become initialized with the data from the
> > hwrng.
> >
> > This change causes the data initially read from the hwrng to be
> > treated the same as subsequent data that is read from the hwrng if
> > it's quality score is non-zero.
> >
> > The implications of this are:
> >
> > The data read from hwrng can cause primary_crng to become
> > initialized, therefore avoiding problems of getrandom blocking
> > on boot.
> >
> > Calls to getrandom (with GRND_RANDOM) may be using entropy
> > exclusively (or in practise, almost exclusively) from the hwrng.
> >
> > Regarding the latter point; this behavior is the same as if a
> > user specified a quality score of 1 (bit of entropy per 1024 bits)
> > so hopefully this is not too scary a change to make.
> >
> > This change is the result of the discussion here:
> > https://patchwork.kernel.org/patch/10453893/
> >
> > Signed-off-by: Louis Collard <louiscollard@chromium.org>
> > Acked-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
> > ---
> >  drivers/char/hw_random/core.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
> > index aaf9e5afaad4..47f358aa0c3d 100644
> > --- a/drivers/char/hw_random/core.c
> > +++ b/drivers/char/hw_random/core.c
> > @@ -24,6 +24,7 @@
> >  #include <linux/sched.h>
> >  #include <linux/slab.h>
> >  #include <linux/uaccess.h>
> > +#include <crypto/chacha20.h>
> >
> >  #define RNG_MODULE_NAME              "hw_random"
> >
> > @@ -64,13 +65,17 @@ static size_t rng_buffer_size(void)
> >  static void add_early_randomness(struct hwrng *rng)
> >  {
> >       int bytes_read;
> > -     size_t size = min_t(size_t, 16, rng_buffer_size());
> > +     /* Read enough to initialize crng. */
> > +     size_t size = 2*CHACHA20_KEY_SIZE;
> >
> >       mutex_lock(&reading_mutex);
> >       bytes_read = rng_get_data(rng, rng_buffer, size, 1);
> >       mutex_unlock(&reading_mutex);
> >       if (bytes_read > 0)
> > -             add_device_randomness(rng_buffer, bytes_read);
> > +             /* Allow crng to become initialized, but do not add
> > +              * entropy to the pool.
> > +              */
> > +             add_hwgenerator_randomness(rng_buffer, bytes_read, 0);
> >  }
> >
> >  static inline void cleanup_rng(struct kref *kref)
>
> I found your patch by chance, searching for a solution for crng init delay on my
> headless machine. Unfortunately it hardly makes any difference for me. With the
> patch the system hangs for about 80s instead of 120s until the "crng init done"
> message.In contrast, doing a `cat /dev/hwrng >/dev/random` or running rngd
> initializes the crng instantly.
>
> Isn't that delay the problem this patch tries to fix? Any idea what is wrong
> here?
>
> Thanks!
>
> Best regards
> Michael
>
>

Yes that is the problem this is trying to address. My guess would be
rng_get_data() is not returning as much data as requested, so the
delay is reduced but not eliminated. Looking at implementation of
rng_get_data() it appears this could be caused by device support for
read() vs data_read(). I don't have a good feel for whether looping to
retrieve more data here would be acceptable, it is certainly a bigger
change than currently proposed.

Thanks,
Louis