Am Sonntag, 22. November 2015, 02:15:12 schrieb Nick Kossifidis: Hi Nick, >Hello all, > >I've been doing some reading on hw_random core and the drivers that >use it and it seems that three of them use the quality parameter on >the hwrng struct. When a driver sets this value, it ends up directly >on credit_entropy_bits in random through add_hwgenerator_randomness >and there is no way of disabling this behavior. IIRC, you can disable or alter the quality value them using the kernel command line. > >Even if the user sets hw_random's default_quality to 0 (the >default/safe value), the value on the hwrng struct overrides it: > >hw_random/core.c: >current_quality = rng->quality ? : default_quality; Looking at that code, I would think that this is wrong as this overwrites the kernel command line option. > >The three drivers that set rng->quality are: > >virtio-rng: sets it to 1000 >chaoskey: sets it to 1024 + 1023 (!) which gets re-set to 1024 in hwrng_init >zcrypt: sets it to 900 by default but provides a module option to >disable this behavior and set it to 0 (so that's an exception) > >I suggest to use the quality parameter of hwrng as "minimum entropy" >or "guaranteed entropy" of the hwrng and if default_quality (the value >set by the user) is larger than this, use it as an upper boundary >(because the device can't "guarantee" more entropy), else If >default_quality is less than this or zero, use default_quality >instead. Do you agree with this, should I send a patch for it ? > >Also there is another issue when drivers call >add_hwgenerator_randomness directly, ath9k does this -on >wireless-testing- to provide random data from its ADC and use the >wireless card as an entropy source. Other drivers might also do it in >the future (I couldn't find more users other than hw_random/core.c and >ath9k for now). Again there is no way to check or override the quality >parameter passed there and it goes directly to random's >credit_entropy_bits. Is this function meant to be used by anyone or is >it an API between hw_random and random only ? The documentation on >hw_random (it's declared on hw_random.h) hasn't been updated to >reflect how this function should be used or what restrictions apply to >the quality parameter when used (the commit log said that developers >should be careful but nothing specific). > >We could have a parameter like "max_hwgenerator_quality" on random >that makes it possible to filter the quality parameter passed on >add_hwgenerator_randomness, no matter who uses it. > >Finally I suggest that we have an ioctl on random to completely >disable add_hwgenerator_randomness and that's because if someone wants Again, I thought you can set that with the kernel command line, provided the aforementioned code is changed. >to use a userspace daemon to seed random through /dev/hwrng by doing >further statistical analysis/monitoring (e.g. rngd or something >similar), he/she will be mixing the same data on the pool twice (one >through hw_random -> add_hwgenerator_randomness -even with zero >quality- and one trough sysctl). I 'd also like to send a patch for >this if you agree. > >Thanks a lot for your time, have a nice weekend. 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
