Re: [PATCH v4 1/1] crypto: atmel-sha204a - fix non-blocking read logic

Thorsten Blum
2026-04-26T16:43:09Z

On Sun, Apr 26, 2026 at 03:49:40PM +0000, Lothar Rubusch wrote:
> The blocking and non-blocking paths were failing to provide valid entropy
> due to improper buffer management. Read the buffer starting from bit 1,
> only fetch the 32 bytes of random data of the return message.
>
> Tested on a Atmel SHA204a device.
>
> Before (here for blocking) tests showed repeadetly reading reduced bytes of

s/repeadetly/repeatedly/

> 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@..........|
> 00000010 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
> 00000020
>
> After, the result will be similar to the following:
> $ 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
>
> Fixes: da001fb651b0 ("crypto: atmel-i2c - add support for SHA204A random number generator")
> Suggested-by: Ard Biesheuvel <ardb@kernel.org>
> Signed-off-by: Lothar Rubusch <l.rubusch@gmail.com>
> ---
> drivers/crypto/atmel-sha204a.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/crypto/atmel-sha204a.c b/drivers/crypto/atmel-sha204a.c
> index dbb39ed0cea1..39a229086a84 100644
> --- a/drivers/crypto/atmel-sha204a.c
> +++ b/drivers/crypto/atmel-sha204a.c
> @@ -48,8 +48,8 @@ static int atmel_sha204a_rng_read_nonblocking(struct hwrng *rng, void *data,
>
> if (rng->priv) {
> work_data = (struct atmel_i2c_work_data *)rng->priv;
> - max = min(sizeof(work_data->cmd.data), max);
> - memcpy(data, &work_data->cmd.data, max);
> + max = min(RANDOM_RSP_SIZE - CMD_OVERHEAD_SIZE, max);
> + memcpy(data, &work_data->cmd.data[1], max);

Please use RSP_DATA_IDX instead of 1 as the index.

> rng->priv = 0;
> } else {
> work_data = kmalloc_obj(*work_data, GFP_ATOMIC);
> @@ -87,8 +87,8 @@ static int atmel_sha204a_rng_read(struct hwrng *rng, void *data, size_t max,
> if (ret)
> return ret;
>
> - max = min(sizeof(cmd.data), max);
> - memcpy(data, cmd.data, max);
> + max = min(RANDOM_RSP_SIZE - CMD_OVERHEAD_SIZE, max);
> + memcpy(data, &cmd.data[1], max);

Same here.

Thanks,
Thorsten