Am Freitag, 10. Januar 2014, 12:37:26 schrieb Clemens Ladisch: Hi Clemens, >Stephan Mueller wrote: >> Am Freitag, 10. Januar 2014, 09:13:57 schrieb Clemens Ladisch: >>> Rafael Aquini wrote: >>>> This patch introduces changes to the random_write method so it can >>>> split the given seed and completely stir the output pools with >>>> different halves of it, when seed lenght allows us doing so. >>>> >>>> - ret = write_pool(&blocking_pool, buffer, count); >>>> + ret = write_pool(pool1, buffer, count1); >>>> >>>> if (ret) >>>> >>>> return ret; >>>> >>>> - ret = write_pool(&nonblocking_pool, buffer, count); >>>> + ret = write_pool(pool2, buffer + offset, count2); >>> >>> Doesn't this assume that both halves of the buffer contain some >>> (uncredited) entropy? In other words, wouldn't this result in worse >>> randomness for pool2 if the second half of the buffer contains just >>> zero padding? >> >> [...] >> Coming back to your concern: sure, the caller can pad any data >> injected into /dev/?random with zeros. > >Assume that the userspace of an embedded device wants to do the same >kind of initialization that a call to add_device_randomness() does, and >that it has some data like "char serial_number[256]". The padding >wouldn't be done intentionally, it's just a property of the data (and >it wouldn't have mattered before this patch). > >> But as writing to the character files is allowed to every user, this >> per definition must not matter (e.g. an attacker may simply write >> zeros or other known data into the character file). And the random.c >> driver handles that case appropriately by not increasing the entropy >> estimator when receiving data. > >The problem is not with the entropy estimate. > >> All the patch tries to achieve is to ensure that both pools are not >> always mixed with the same values. > >Before this patch, both pools got mixed with the same values. After >this patch, both pools indeed get mixed with different values, but now >one pool gets mixed with a known value if one half of the buffer >happens to be known. Do you imply in your example above that the serial number is unknown? Anything that unprivileged user space tries to inject into /dev/?random should be considered data with known value. Ciao Stephan -- To unsubscribe from this list: send the line "unsubscribe linux-crypto" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
