The patch titled Bug in random32.c: all-zero outputs with probability 1/2^32, other seeding bugs has been added to the -mm tree. Its filename is bug-in-random32c-all-zero-outputs-with-probability-1-232-other-seeding-bugs.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/SubmitChecklist when testing your code *** See http://www.zip.com.au/~akpm/linux/patches/stuff/added-to-mm.txt to find out what to do about this The current -mm tree may be found at http://userweb.kernel.org/~akpm/mmotm/ ------------------------------------------------------ Subject: Bug in random32.c: all-zero outputs with probability 1/2^32, other seeding bugs From: Jack Lloyd <lloyd@xxxxxxxxxxxxx> There appears to be an error in how random seeding is done in the random32.c RNG. I am looking at 2.6.25.7. For background, that file contains this comment: """ ... the k_j most significant bits of z_j must be non- zero, for each j. (Note: this restriction also applies to the computer code given in [4], but was mistakenly not mentioned in that paper.) This affects the seeding procedure by imposing the requirement s1 > 1, s2 > 7, s3 > 15. """ The function random32_reseed takes an unsigned long's worth of entropy from the entropy pool (get_random_bytes) and passes it to __set_random32. That function (seemingly attempts) to impose the requirement mentioned above by first checking if the seed is == 0, and if so setting it to one, then setting s2 to s*69069 and s3 to s2*69069 However this will allow bad seeds. On 64-bit systems, if the low order 32 bits of the seed are zero, then the check for s == 0 will pass, but s1, s2, and s3 will all be initialized to zero due to truncation. This will, since there are no additive factors in the RNG sequence, cause the RNG to output nothing but zero values. Assuming get_random_bytes returns uniform random numbers, this will occur with probability around 1/2^32. An easy and straightforward fix for this that doesn't require changing any interfaces is to add s &= 0xFFFFFFFF; before the check in __set_random32, which ensures this condition will be caught by the check. Alternately, you could replace the check for s == 0 with some logic like: if((s & 0xFFFFFFFF) == 0) s += 1; since just chopping the seed to 32 bits does throw away some of your seed input (with sizeof(long) == 8, at least; doesn't make any difference for sizeof(long) == 4) There are also many other seeds which will cause some (but not all) of the seeding requirements to be violated. Which seeds will cause this problem depend on the size of unsigned long, since the multiplications by 69069 are done in that size. For instance on 32-bit systems, a seed of 0x4BC54E0A will generate the state: s1 = 0x4BC54E0A s2 = (s1 * 69069) % 2^32 = 2 s3 = 2*69069 = 138138 In general, there seem to be large numbers of seeds that cause at least one of the restrictions on s1, s2, s3 to be false. The paper referenced in the source is not clear exactly how badly the generator fails when only one of the restrictions is violated. Certainly it seems much less serious than the 64-bit specific bug described above; which is fortunate since it seems to occur much more often. A trivial patch for both problems is attached. Cc: Theodore Ts'o <tytso@xxxxxxx> Cc: Matt Mackall <mpm@xxxxxxxxxxx> Cc: Stephen Hemminger <shemminger@xxxxxxxx> Cc: David S. Miller <davem@xxxxxxxxxxxxx> Cc: Andi Kleen <ak@xxxxxxx> Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> --- lib/random32.c | 4 ++++ 1 file changed, 4 insertions(+) diff -puN lib/random32.c~bug-in-random32c-all-zero-outputs-with-probability-1-232-other-seeding-bugs lib/random32.c --- a/lib/random32.c~bug-in-random32c-all-zero-outputs-with-probability-1-232-other-seeding-bugs +++ a/lib/random32.c @@ -58,13 +58,17 @@ static u32 __random32(struct rnd_state * static void __set_random32(struct rnd_state *state, unsigned long s) { + s &= 0xFFFFFFFF; if (s == 0) s = 1; /* default seed is 1 */ #define LCG(n) (69069 * n) state->s1 = LCG(s); + if(state->s1 < 2) state->s1 += 2; state->s2 = LCG(state->s1); + if(state->s2 < 8) state->s2 += 8; state->s3 = LCG(state->s2); + if(state->s3 < 16) state->s3 += 16; /* "warm it up" */ __random32(state); _ Patches currently in -mm which might be from lloyd@xxxxxxxxxxxxx are bug-in-random32c-all-zero-outputs-with-probability-1-232-other-seeding-bugs.patch -- To unsubscribe from this list: send the line "unsubscribe mm-commits" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html