[PATCH] hw_random: atmel-rng: fix race condition leading to repeated bits

[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@xxxxxxxxxxxxxxx
Signed-off-by: Peter Korsgaard <jacmet@xxxxxxxxxx>
Reported-by: George Pontis <GPontis@xxxxxx>
---
 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;
-- 
1.7.10

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