On Wed, Sep 07, 2011 at 10:05:30PM +0300, Sasha Levin wrote: > On Wed, 2011-09-07 at 14:26 -0400, Jarod Wilson wrote: > > Sasha Levin wrote: > > > On Wed, 2011-09-07 at 13:38 -0400, Jarod Wilson wrote: > > >> Certain security-related certifications and their respective review > > >> bodies have said that they find use of /dev/urandom for certain > > >> functions, such as setting up ssh connections, is acceptable, but if and > > >> only if /dev/urandom can block after a certain threshold of bytes have > > >> been read from it with the entropy pool exhausted. Initially, we were > > >> investigating increasing entropy pool contributions, so that we could > > >> simply use /dev/random, but since that hasn't (yet) panned out, and > > >> upwards of five minutes to establsh an ssh connection using an > > >> entropy-starved /dev/random is unacceptable, we started looking at the > > >> blocking urandom approach. > > > > > > Can't you accomplish this in userspace by trying to read as much as you > > > can out of /dev/random without blocking, then reading out > > > of /dev/urandom the minimum between allowed threshold and remaining > > > bytes, and then blocking on /dev/random? > > > > > > For example, lets say you need 100 bytes of randomness, and your > > > threshold is 30 bytes. You try reading out of /dev/random and get 50 > > > bytes, at that point you'll read another 30 (=threshold) bytes > > > out /dev/urandom and then you'll need to block on /dev/random until you > > > get the remaining 20 bytes. > > > > We're looking for a generic solution here that doesn't require > > re-educating every single piece of userspace. [...] > > A flip-side here is that you're going to break every piece of userspace > which assumed (correctly) that /dev/urandom never blocks. Since this is > a sysctl you can't fine tune which processes/threads/file-handles will > block on /dev/urandom and which ones won't. > I'm also squeamish about exactly this - theres no way to allow for applications that expect a never-block /dev/urandom to co-exist with applications that want an 'eventually' block /dev/urandom That said, what about changing the model to make /dev/urandom configurable on a per-file-descriptor basis, Add an ioctl to the urandom_fops ioctl handler to specify on a per file struct basis the maximum number of bytes that can be read past a zero entropy point for that file. The kernel could then track the number of bytes read per fd after a zero entropy point (clearing it on a read when the pool has non-zero entropy), and blocking/delaying any call until entropy had been added back. That way apps the require a non-blocking urandom can co-exist with apps that have this new requirement, and only the new apps have to be taught about the additional urandom configuration. Neil -- 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
