On Mon, Aug 14, 2017 at 09:55:00AM -0700, stan wrote: > On Mon, 14 Aug 2017 08:26:34 -0400 > Neil Horman <nhorman@xxxxxxxxxx> wrote: > > > On Sat, Aug 12, 2017 at 06:32:37AM -0700, stan wrote: > > > Hi, > > > > > > I compile a custom kernel locally from the Fedora src.rpm. I notice > > > that the most recent 4.13 kernel I compiled, 4.13.0-0.rc4.git3.1, > > > has different behavior for the kernel entropy pool than it used to > > > have. > > > > > > The way it used to work was that when the kernel entropy pool was > > > kept full, or fuller, it would bleed entropy across to the > > > pseudo-random generator in order to reseed it. This makes the > > > output sequence of the PRNG indeterminate, and thus more random, > > > and harder to attack. It is no longer a pseudo random number > > > generator, except in intervals. > > > > > > I notice now that there is no bleed happening. When the kernel > > > entropy pool gets full, if there are no calls to /dev/random, it > > > stays full. > > > > > > I don't have a hardware RNG, but I harvest entropy from a sound > > > device, and from an rtl2832 receiver (atmospheric). They still > > > work, if I do cat /dev/random | rngtest -b 10 > > > they get fired up and run full out. > > > > > > But if I do > > > cat /dev/urandom | rngtest -b 20000 > > > the entropy pool just sits there, and they don't run. > > > > > > The entropy pool can be looked at, as root, with > > > cat /proc/sys/kernel/random/entropy_avail > > > I reset the write_wakeup_threshold there to 3840 from its default > > > 1792, and lower the urandom_min_reseed_secs to 5 (I think the > > > default is 60). > > > > > > Can you explain why this was changed, or point me to a discussion of > > > the rationale for the change? It seems like it has implications for > > > system security, and on the surface seems to decrease security of > > > the PRNG. > > > > > > Thanks. > > > > you might be looking at commit > > e297a783e41560b44e3c14f38e420cba518113b8. Normally urandom doesn't > > block, but you're description suggests it is now, and I think that > > commit introduced that. Neil > > > I'm not sure that's correct. That commit seems to be for something > else. > > https://github.com/torvalds/linux/commit/e297a783e41560b44e3c14f38e420cba518113b8 my bad, I thought when you said, the entropy pool just sits there, and they don't run. You meant your process was blocking (which would be bad for urandom), and thats the first thing that caught my eye. That said, what do you mean when you say "don't run". Do you mean to say the fips test exits on duplicate data, or that it runs, its just that the urandom crng doesn't seem to get refilled from the kernel entropy pool) If the latter is what you are concerned about I'd look at commit f5b98461cb8167ba362ad9f74c41d126b7becea7. I don't know what the last kernel you used was back when it worked the way you thought it should, but that commit converted the urandom generator such that it uses a pseudo entropy pool made up of a chacha20 cipher block to extract entropy from during reseeds instead of the kernel entropy pool it seems. Neil > > It's creating a function to ensure any call to get random bytes blocks > until there is enough entropy to ensure valid random data is returned. > But that isn't what I'm seeing. There is no call to the blocking > function, and both /dev/random and /dev/urandom yield results > immediately. This is about the behind the scenes transfer of entropy > from the kernel entropy pool to urandom in order to reseed the > generator when the kernel entropy pool is full or nearly so. It used to > do that, it no longer does. > > I don't see how that can even be a side effect of this change. That > transfer of entropy is a core relationship of the /dev/urandom PRNG > with the /dev/random entropy pool. > _______________________________________________ > kernel mailing list -- kernel@xxxxxxxxxxxxxxxxxxxxxxx > To unsubscribe send an email to kernel-leave@xxxxxxxxxxxxxxxxxxxxxxx _______________________________________________ kernel mailing list -- kernel@xxxxxxxxxxxxxxxxxxxxxxx To unsubscribe send an email to kernel-leave@xxxxxxxxxxxxxxxxxxxxxxx
