Re: [PATCH] Allow hwrng to initialize crng.

["Michael Niewöhner" <linux@mniewoehner.de>]

On Thu, 2018-12-13 at 12:50 +0800, Louis Collard wrote:
> 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

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