random32: fix off-by-one in seeding requirement

[ Upstream commit 51c37a70aaa3f95773af560e6db3073520513912 ]

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: 697f8d0348a6 ("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>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/include/linux/random.h b/include/linux/random.h
index 7e77cee62bd9580bc8edeba13357b43aed984e56..f5e1311f491eb41ac7a427cf01b1053bb65e0744 100644 (file)
--- a/include/linux/random.h
+++ b/include/linux/random.h
@@ -89,9 +89,9 @@ static inline void prandom32_seed(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

diff --git a/lib/random32.c b/lib/random32.c
index fc3545a32771569dda97e8169d79151af8add933..1f44bdc6e8d34a2f55a5c57694c7e8d9ac124f6e 100644 (file)
--- a/lib/random32.c
+++ b/lib/random32.c
@@ -92,7 +92,7 @@ void srandom32(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(srandom32);
@@ -109,9 +109,9 @@ static int __init random32_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" */
                prandom32(state);
@@ -138,9 +138,9 @@ static int __init random32_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 */
                prandom32(state);