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

[Thorsten Blum]

On Sat, Apr 25, 2026 at 03:46:30PM +0200, Thorsten Blum 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>

[...]

> > -	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);

I just noticed that index 1 is already defined as RSP_DATA_IDX in
drivers/crypto/atmel-i2c.h.

> 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.

Thanks,
Thorsten