On Fri, 29 Jul 2016 15:12:30 +0200 Stephan Mueller <smueller@xxxxxxxxxx> wrote as excerpted: > Am Freitag, 29. Juli 2016, 09:03:45 CEST schrieb Alex Xu: > > In my opinion, assuming I am not doing something terribly wrong, > > this constitutes a bug in the kernel's handling of getrandom calls > > at boot, possibly only when the primary source of entropy is > > virtio. > > Nope, I do not think that this is true: > > - /dev/random returns one byte for one byte of entropy received, but > it has a lower limit of 64 bits > > - getrandom behaves like /dev/urandom (i.e. nonblocking) except > during boot where it waits until the RNG has collected 128 bits > before operating like a DRNG that is seeded once in a while when > entropy comes in. > > > Ciao > Stephan I don't follow. Assuming you are correct and this is the issue, then reading 128 bits (16 bytes) from /dev/random should "exhaust the supply" and then both reads from /dev/random and calling getrandom should block. That, however, is not the behavior I observed, which is that reading any amount from /dev/random will never block (since it is fed from /dev/urandom on the host side) whereas calling getrandom will always block unless /dev/random is read from first. Moreover, as long as virtio-rng is available (and fed from /dev/urandom), /proc/sys/kernel/random/entropy_avail is always 961 immediately after booting, which is more than enough to satisfy a one-byte read. After reading 1 byte, the estimate decreases to 896 or 897, but after reading 29 more bytes it increases to 1106. Again, these observations are consistent with the conjecture that the issue arises since virtio-rng is a "pull" source of entropy whereas most other methods (e.g. interrupt timing) are "push" sources. I suspect that a similar issue occurs if RDRAND is the only source of entropy. I also tried running rngd in the guest which resolved the issue but seems entirely stupid to me, even moreso since http://rhelblog.redhat.com/2015/03/09/red-hat-enterprise-linux-virtual-machines-access-to-random-numbers-made-easy/ says that "The use of rngd is now not required and the guest kernel itself fetches entropy from the host when the available entropy falls below a specific threshold.". -- 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
