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[84.74.0.139]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-48a4b329542sm352469085e9.3.2026.04.22.14.09.49 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 22 Apr 2026 14:09:49 -0700 (PDT) From: Lothar Rubusch To: herbert@gondor.apana.org.au, thorsten.blum@linux.dev, davem@davemloft.net, nicolas.ferre@microchip.com, alexandre.belloni@bootlin.com, claudiu.beznea@tuxon.dev, ardb@kernel.org, linusw@kernel.org Cc: linux-crypto@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, l.rubusch@gmail.com Subject: [PATCH v3 2/3] crypto: atmel-sha204a - fix truncated 32-byte blocking read Date: Wed, 22 Apr 2026 21:09:35 +0000 Message-Id: <20260422210936.20095-3-l.rubusch@gmail.com> X-Mailer: git-send-email 2.39.5 In-Reply-To: <20260422210936.20095-1-l.rubusch@gmail.com> References: <20260422210936.20095-1-l.rubusch@gmail.com> Precedence: bulk X-Mailing-List: linux-crypto@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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; 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); return max; } -- 2.53.0