On Tue, Jul 26, 2022 at 01:28:10AM +0200, Jason A. Donenfeld wrote: > Rather than buffering 16 MiB of entropy in userspace (by way of > chacha20), simply call getrandom() every time. > > This approach is doubtlessly slower, for now, but trying to prematurely > optimize arc4random appears to be leading toward all sorts of nasty > properties and gotchas. Instead, this patch takes a much more > conservative approach. The interface is added as a basic loop wrapper > around getrandom(), and then later, the kernel and libc together can > work together on optimizing that. > > This prevents numerous issues in which userspace is unaware of when it > really must throw away its buffer, since we avoid buffering all > together. Future improvements may include userspace learning more from > the kernel about when to do that, which might make these sorts of > chacha20-based optimizations more possible. The current heuristic of 16 > MiB is meaningless garbage that doesn't correspond to anything the > kernel might know about. So for now, let's just do something > conservative that we know is correct and won't lead to cryptographic > issues for users of this function. > > This patch might be considered along the lines of, "optimization is the > root of all evil," in that the much more complex implementation it > replaces moves too fast without considering security implications, > whereas the incremental approach done here is a much safer way of going > about things. Once this lands, we can take our time in optimizing this > properly using new interplay between the kernel and userspace. > > getrandom(0) is used, since that's the one that ensures the bytes > returned are cryptographically secure. But on systems without it, we > fallback to using /dev/urandom. This is unfortunate because it means > opening a file descriptor, but there's not much of a choice. Secondly, > as part of the fallback, in order to get more or less the same > properties of getrandom(0), we poll on /dev/random, and if the poll > succeeds at least once, then we assume the RNG is initialized. This is a > rough approximation, as the ancient "non-blocking pool" initialized > after the "blocking pool", not before, but it's the best approximation > we can do. > > The motivation for including arc4random, in the first place, is to have > source-level compatibility with existing code. That means this patch > doesn't attempt to litigate the interface itself. It does, however, > choose a conservative approach for implementing it. > > Cc: Adhemerval Zanella Netto <adhemerval.zanella@xxxxxxxxxx> > Cc: Florian Weimer <fweimer@xxxxxxxxxx> > Cc: Cristian Rodríguez <crrodriguez@xxxxxxxxxxxx> > Cc: Paul Eggert <eggert@xxxxxxxxxxx> > Cc: linux-crypto@xxxxxxxxxxxxxxx > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> This looks good to me. There are still a few bits that need to be removed/updated. With a quick grep, I found: sysdeps/generic/tls-internal-struct.h: struct arc4random_state_t *rand_state; sysdeps/unix/sysv/linux/tls-internal.h:/* Reset the arc4random TCB state on fork. * NEWS: ... The functions use a pseudo-random number generator along with NEWS: entropy from the kernel. Also, the documentation in manual/math.texi should say that the randomness is cryptographically secure. - Eric
