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SubjectRe: [PATCH] hw_random: atmel-rng: fix race condition leading to repeated bits
On 05/25/2012 11:12 AM, Peter Korsgaard :
> Data valid gets cleared by reading the ISR (status register) and NOT from
> reading ODATA (data register). A new data word can become available between
> checking ISR and reading ODATA, causing us to reuse the same data word next
> time atmel_trng_read() gets called, if that happens before the following
> data word is ready.
>
> With this fixed, rngtest no longer complains of 'Continous run' errors.
> Before:
>
> rngtest -c 1000 < /dev/hwrng
> rngtest 3
> Copyright (c) 2004 by Henrique de Moraes Holschuh
> This is free software; see the source for copying conditions. There is NO warr.
>
> rngtest: starting FIPS tests...
> rngtest: bits received from input: 20000032
> rngtest: FIPS 140-2 successes: 923
> rngtest: FIPS 140-2 failures: 77
> rngtest: FIPS 140-2(2001-10-10) Monobit: 0
> rngtest: FIPS 140-2(2001-10-10) Poker: 0
> rngtest: FIPS 140-2(2001-10-10) Runs: 1
> rngtest: FIPS 140-2(2001-10-10) Long run: 0
> rngtest: FIPS 140-2(2001-10-10) Continuous run: 76
> rngtest: input channel speed: (min=721.402; avg=46003.510; max=49321.338)Kibitss
> rngtest: FIPS tests speed: (min=11.442; avg=12.714; max=12.801)Mibits/s
> rngtest: Program run time: 1931860 microseconds
>
> After:
>
> rngtest -c 1000 < /dev/hwrng
> rngtest 3
> Copyright (c) 2004 by Henrique de Moraes Holschuh
> This is free software; see the source for copying conditions. There is NO warr.
>
> rngtest: starting FIPS tests...
> rngtest: bits received from input: 20000032
> rngtest: FIPS 140-2 successes: 1000
> rngtest: FIPS 140-2 failures: 0
> rngtest: FIPS 140-2(2001-10-10) Monobit: 0
> rngtest: FIPS 140-2(2001-10-10) Poker: 0
> rngtest: FIPS 140-2(2001-10-10) Runs: 0
> rngtest: FIPS 140-2(2001-10-10) Long run: 0
> rngtest: FIPS 140-2(2001-10-10) Continuous run: 0
> rngtest: input channel speed: (min=777.518; avg=36988.482; max=43115.342)Kibitss
> rngtest: FIPS tests speed: (min=11.951; avg=12.715; max=12.887)Mibits/s
> rngtest: Program run time: 2035543 microseconds
>
> Cc: stable@vger.kernel.org
> Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
> Reported-by: George Pontis <GPontis@z9.com>

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>

Thanks!

> ---
> drivers/char/hw_random/atmel-rng.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/drivers/char/hw_random/atmel-rng.c b/drivers/char/hw_random/atmel-rng.c
> index d7ab920..731c904 100644
> --- a/drivers/char/hw_random/atmel-rng.c
> +++ b/drivers/char/hw_random/atmel-rng.c
> @@ -36,6 +36,13 @@ static int atmel_trng_read(struct hwrng *rng, void *buf, size_t max,
> /* data ready? */
> if (readl(trng->base + TRNG_ISR) & 1) {
> *data = readl(trng->base + TRNG_ODATA);
> + /*
> + ensure data ready is only set again AFTER the next data
> + word is ready in case it got set between checking ISR
> + and reading ODATA, so we don't risk re-reading the
> + same word
> + */
> + readl(trng->base + TRNG_ISR);
> return 4;
> } else
> return 0;


--
Nicolas Ferre


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