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Thu, 23 Apr 2026 03:56:10 -0400 (EDT) X-Mailer: MessagingEngine.com Webmail Interface MIME-Version: 1.0 Date: Thu, 23 Apr 2026 09:55:50 +0200 From: "Ard Biesheuvel" To: "Lothar Rubusch" , "Herbert Xu" , "Thorsten Blum" , davem@davemloft.net, nicolas.ferre@microchip.com, alexandre.belloni@bootlin.com, claudiu.beznea@tuxon.dev, "Linus Walleij" Cc: linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Message-Id: <0da8b7c6-2fb9-4a27-8f2b-a916c85185a8@app.fastmail.com> In-Reply-To: <20260422210936.20095-3-l.rubusch@gmail.com> References: <20260422210936.20095-1-l.rubusch@gmail.com> <20260422210936.20095-3-l.rubusch@gmail.com> Subject: Re: [PATCH v3 2/3] crypto: atmel-sha204a - fix truncated 32-byte blocking read Content-Type: text/plain Content-Transfer-Encoding: 7bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260423_005612_743166_CD410776 X-CRM114-Status: GOOD ( 24.81 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, 22 Apr 2026, at 23:09, 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 > --- > 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; > Please fix atmel_i2c_init_random_cmd() instead if it doesn't set the right values for these fields. But afaict, you are decreasing the execution time here, so I struggle to see how this could explain the improved behavior. > 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 looks correct - better to put this in a separate patch. > return max; > } > -- > 2.53.0