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random32: fix off-by-one in seeding requirement 

For properly initialising the Tausworthe generator [1], we have
a strict seeding requirement, that is, s1 > 1, s2 > 7, s3 > 15.

Commit 697f8d0348 ("random32: seeding improvement") introduced
a __seed() function that imposes boundary checks proposed by the
errata paper [2] to properly ensure above conditions.

However, we're off by one, as the function is implemented as:
"return (x < m) ? x + m : x;", and called with __seed(X, 1),
__seed(X, 7), __seed(X, 15). Thus, an unwanted seed of 1, 7, 15
would be possible, whereas the lower boundary should actually
be of at least 2, 8, 16, just as GSL does. Fix this, as otherwise
an initialization with an unwanted seed could have the effect
that Tausworthe's PRNG properties cannot not be ensured.

Note that this PRNG is *not* used for cryptography in the kernel.

 [1] http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme.ps
 [2] http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme2.ps

Joint work with Hannes Frederic Sowa.

Fixes: 697f8d0348 ("random32: seeding improvement")
Cc: Stephen Hemminger <stephen@networkplumber.org>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Daniel Borkmann 2013-11-11 12:20:32 +01:00 committed by David S. Miller
parent 129596674c
commit 51c37a70aa
2 changed files with 10 additions and 10 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
include/linux/random.h
View File

@ -50,9 +50,9 @@ static inline void prandom_seed_state(struct rnd_state *state, u64 seed)
{
u32 i = (seed >> 32) ^ (seed << 10) ^ seed;
state->s1 = __seed(i, 1);
state->s2 = __seed(i, 7);
state->s3 = __seed(i, 15);
state->s1 = __seed(i, 2);
state->s2 = __seed(i, 8);
state->s3 = __seed(i, 16);
}
#ifdef CONFIG_ARCH_RANDOM

14
lib/random32.c
View File

@ -141,7 +141,7 @@ void prandom_seed(u32 entropy)
*/
for_each_possible_cpu (i) {
struct rnd_state *state = &per_cpu(net_rand_state, i);
state->s1 = __seed(state->s1 ^ entropy, 1);
state->s1 = __seed(state->s1 ^ entropy, 2);
}
}
EXPORT_SYMBOL(prandom_seed);
@ -158,9 +158,9 @@ static int __init prandom_init(void)
struct rnd_state *state = &per_cpu(net_rand_state,i);
#define LCG(x) ((x) * 69069) /* super-duper LCG */
state->s1 = __seed(LCG(i + jiffies), 1);
state->s2 = __seed(LCG(state->s1), 7);
state->s3 = __seed(LCG(state->s2), 15);
state->s1 = __seed(LCG(i + jiffies), 2);
state->s2 = __seed(LCG(state->s1), 8);
state->s3 = __seed(LCG(state->s2), 16);
/* "warm it up" */
prandom_u32_state(state);
@ -187,9 +187,9 @@ static int __init prandom_reseed(void)
u32 seeds[3];
get_random_bytes(&seeds, sizeof(seeds));
state->s1 = __seed(seeds[0], 1);
state->s2 = __seed(seeds[1], 7);
state->s3 = __seed(seeds[2], 15);
state->s1 = __seed(seeds[0], 2);
state->s2 = __seed(seeds[1], 8);
state->s3 = __seed(seeds[2], 16);
/* mix it in */
prandom_u32_state(state);