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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-crypto@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: X-Migadu-Flow: FLOW_OUT On Sat, Apr 25, 2026 at 03:46:30PM +0200, Thorsten Blum wrote: > 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 [...] > > - 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); I just noticed that index 1 is already defined as RSP_DATA_IDX in drivers/crypto/atmel-i2c.h. > 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