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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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