Re: [PATCH] Allow hwrng to initialize crng.

[Michael Niewöhner]

On Sat, 2018-12-15 at 18:11 +0100, Michael NiewÃhner wrote:
> On Thu, 2018-12-13 at 12:50 +0800, Louis Collard wrote:
> > On Sun, Nov 18, 2018 at 4:15 AM Michael NiewÃhner <linux@xxxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxx>
> > > > Acked-by: Jarkko Sakkinen <jarkko.sakkinen@xxxxxxxxxxxxxxx>
> > > > ---
> > > >  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
> 
> Hi Louis,
> 
> that is what I thought first, too, but I was able to verify that 64 bytes are
> read as expected.
> 
> It seems this is exactly what David noticed in your discussion about the
> quality
> module parameter (https://patchwork.kernel.org/patch/10453893/#22130681):
> 
> > Interesting.
> > 
> > add_hwgenereator_randomness() will call crng_fast_load(), regardless
> > of entropy estimate/quality, if crng_init is 0.  So initializing
> > crng_init from the hwrng, regardless of quality, is already the
> > intent.
> > 
> > But hw_random only calls add_hwgenerator_randomness() if
> > current_quality > 0, via the hwrng_fillfn() kthread.
> > 
> > All that to say, I agree.  add_early_randomness() should (indirectly)
> > call crng_fast_load(), like add_hwgenerator_randomness() does.
> 
> When I set rng_quality=1024, the crng does get initialized more or less
> instantly.
> 
> 
> dmesg with default rng_quality=0:
> 
> [    0.003831] ACPI: TPM2 0x000000009E0B7F70 000034 (v03 LENOVO TC-
> S06   00001260 AMI  00000000)
> [    0.161803] random: get_random_bytes called from start_kernel+0x8f/0x50e
> with
> crng_init=0
> [    3.590433] tpm_tis MSFT0101:00: 2.0 TPM (device-id 0xFE, rev-id 2)
> [    3.644148] random: fast init done
> [   85.183006] random: crng init done
> 
> 
> dmesg with rng_quality patch:
> 
> [    0.003837] ACPI: TPM2 0x000000009E0B7F70 000034 (v03 LENOVO TC-
> S06   00001260 AMI  00000000)
> [    0.162136] random: get_random_bytes called from start_kernel+0x8f/0x50e
> with
> crng_init=0
> [    3.582675] tpm_tis MSFT0101:00: 2.0 TPM (device-id 0xFE, rev-id 2)
> [    3.636408] random: fast init done
> [    3.650355] random: crng init done
> 
> 
> Test patch:
> 
> diff --git a/drivers/char/tpm/tpm_tis.c b/drivers/char/tpm/tpm_tis.c
> index f08949a5f678..59e5a8753ba1 100644
> --- a/drivers/char/tpm/tpm_tis.c
> +++ b/drivers/char/tpm/tpm_tis.c
> @@ -214,6 +214,8 @@ static int tpm_tis_init(struct device *dev, struct
> tpm_info
> *tpm_info)
>         if (itpm || is_itpm(ACPI_COMPANION(dev)))
>                 phy->priv.flags |= TPM_TIS_ITPM_WORKAROUND;
>  
> +       priv->rng_quality = 1;
> +
>         return tpm_tis_core_init(dev, &phy->priv, irq, &tpm_tcg,
>                                  ACPI_HANDLE(dev));
>  }
> 
> 
> 
> Thanks
> Michael


Hi Louis,

I think I might know what is causing the delay of crng init.

Your patch is working as expected but only for fast init.
add_hwgenerator_randomness calls crng_fast_load which does stage 1
initialization of the crng ("fast init done" -> crng_init=1).

Currently no entropy comes from the TPM at all for 1) the second stage init /
reseed 2) later operation because tpm_tis sets no quality while the default in
tpm-chip is quality=0 and so khwrngd will not be started in hwrng_init.
This is why setting the quality to 1024 instantly leads to setting crng_init=2
and reseeding the crng ("crng init done").

That means the current default value of rng_quality=0 renders the tpm hwrng
completely useless.