[PATCH v3 2/3] crypto: atmel-sha204a - fix truncated 32-byte blocking read

[Lothar Rubusch]

Hi Thorsten!

On Sat, Apr 25, 2026 at 3:46 PM Thorsten Blum <thorsten.blum at linux.dev> wrote:
>
> Hi Lothar,
>
> On Wed, Apr 22, 2026 at 09:09:35PM +0000, Lothar Rubusch wrote:
> > The ATSHA204A returns a 35-byte packet consisting of a 1-byte count,
> > 32 bytes of entropy, and a 2-byte CRC. The current blocking read
> > implementation was incorrectly copying data starting from the
> > count byte, leading to offset data and truncated entropy.
> >
> > Additionally, the chip requires significant execution time to
> > generate random numbers, going by the datasheet. Reading the I2C bus
> > too early results in the chip NACK-ing or returning a partial buffer
> > followed by zeros.
> >
> > Verification:
> > Tests before showed repeadetly reading only 8 bytes of entropy:
> > $ head -c 32 /dev/hwrng | hexdump -C
> > 00000000  02 28 85 b3 47 40 f2 ee  00 00 00 00 00 00 00 00  |.(..G at ..........|
> > 00000010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
> > 00000020
> >
> > After this patch applied, the result will be as follows:
> > $ head -c 32 /dev/hwrng | hexdump -C
> > 00000000  5a fc 3f 13 14 68 fe 06  68 0a bd 04 83 6e 09 69  |Z.?..h..h....n.i|
> > 00000010  75 ff cf 87 10 84 3b c9  c1 df ae eb 45 53 4c c3  |u.....;.....ESL.|
> > 00000020
> >
> > Fix these issues by:
> > Increase cmd.msecs to 30ms to provide sufficient execution time. Then
> > set cmd.rxsize to RANDOM_RSP_SIZE (35 bytes) to capture the entire
> > hardware response. Eventually, correct the memcpy() offset to index 1 of
> > the data buffer to skip the count byte and retrieve exactly 32 bytes of
> > entropy.
> >
> > Fixes: da001fb651b0 ("crypto: atmel-i2c - add support for SHA204A random number generator")
> > Signed-off-by: Lothar Rubusch <l.rubusch at gmail.com>
>
> Thank you for your patches. I tested patch 2/3 on real hardware and it
> fixes rngtest for me. However, I have a few comments below.
>
> > ---
> >  drivers/crypto/atmel-sha204a.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/crypto/atmel-sha204a.c b/drivers/crypto/atmel-sha204a.c
> > index 19720bdd446d..f7dc00d0f4cd 100644
> > --- a/drivers/crypto/atmel-sha204a.c
> > +++ b/drivers/crypto/atmel-sha204a.c
> > @@ -19,6 +19,9 @@
> >  #include <linux/workqueue.h>
> >  #include "atmel-i2c.h"
> >
> > +#define ATMEL_RNG_BLOCK_SIZE 32
> > +#define ATMEL_RNG_EXEC_TIME 30
> > +
> >  static void atmel_sha204a_rng_done(struct atmel_i2c_work_data *work_data,
> >                                  void *areq, int status)
> >  {
> > @@ -91,13 +94,15 @@ static int atmel_sha204a_rng_read(struct hwrng *rng, void *data, size_t max,
> >       i2c_priv = container_of(rng, struct atmel_i2c_client_priv, hwrng);
> >
> >       atmel_i2c_init_random_cmd(&cmd);
> > +     cmd.msecs = ATMEL_RNG_EXEC_TIME;
> > +     cmd.rxsize = RANDOM_RSP_SIZE;
>
> atmel_i2c_init_random_cmd() already sets cmd.rxsize to RANDOM_RSP_SIZE.
>
> Changing cmd.msecs does not appear to be strictly necessary for the fix.
> The only difference I observe is that rngtest runs faster with the new
> value.
>
> Here are my test results without changing cmd.msecs:
>
> $ sudo head -c 300000 /dev/hwrng | rngtest -c 100
> [...]
> rngtest: starting FIPS tests...
> rngtest: bits received from input: 2000032
> rngtest: FIPS 140-2 successes: 100
> rngtest: FIPS 140-2 failures: 0
> [...]
> rngtest: input channel speed: (min=1.118; avg=3.746; max=6510416.667)Kibits/s
> rngtest: FIPS tests speed: (min=26.272; avg=27.471; max=32.829)Mibits/s
> rngtest: Program run time: 538942640 microseconds
>
> and with 'cmd.msecs = ATMEL_RNG_EXEC_TIME':
>
> $ sudo head -c 300000 /dev/hwrng | rngtest -c 100
> [...]
> rngtest: starting FIPS tests...
> rngtest: bits received from input: 2000032
> rngtest: FIPS 140-2 successes: 100
> rngtest: FIPS 140-2 failures: 0
> [...]
> rngtest: input channel speed: (min=1.584; avg=5.295; max=6510416.667)Kibits/s
> rngtest: FIPS tests speed: (min=26.602; avg=27.321; max=28.257)Mibits/s
> rngtest: Program run time: 381309284 microseconds
>
> >       ret = atmel_i2c_send_receive(i2c_priv->client, &cmd);
> >       if (ret)
> >               return ret;
> >
> > -     max = min(sizeof(cmd.data), max);
> > -     memcpy(data, cmd.data, max);
> > +     max = min_t(size_t, ATMEL_RNG_BLOCK_SIZE, max);
> > +     memcpy(data, &cmd.data[1], max);
>
> This works and fixes rngtest for me on real hardware.
>
> min_t() is not strictly necessary here, since the types are compatible
> and min() is sufficient.
>
> I agree with Ard that patch 2/3 should be a separate patch for easier
> stable backporting.
>

I appreciate your answers and will take the review into account. Looks
like we had similar plans for
this weekend. So, since I answered to Ard quite in parallel to your
review, I'll restructure as I said in my
last mail on this list. I'll go through your comments and think about it.

Best,
L




> Thanks,
> Thorsten