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From: Thorsten Blum To: Lothar Rubusch Cc: herbert@gondor.apana.org.au, davem@davemloft.net, nicolas.ferre@microchip.com, alexandre.belloni@bootlin.com, claudiu.beznea@tuxon.dev, ardb@kernel.org, linusw@kernel.org, linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 2/3] crypto: atmel-sha204a - fix truncated 32-byte blocking read Message-ID: References: <20260422210936.20095-1-l.rubusch@gmail.com> <20260422210936.20095-3-l.rubusch@gmail.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260422210936.20095-3-l.rubusch@gmail.com> X-Migadu-Flow: FLOW_OUT 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@..........| > 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 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 > #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. Thanks, Thorsten