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

From: Thorsten Blum <[email-masked]>
Date: Sun, 26 Apr 2026 18:43:00 +0200

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 <[email-masked]>
> Signed-off-by: Lothar Rubusch <[email-masked]>
> ---
>  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