On Mon, Nov 07, 2022 at 01:24:55PM +0100, Jason A. Donenfeld wrote: > Most hw_random devices return entropy which is assumed to be of full > quality, but driver authors don't bother setting the quality knob. Some > hw_random devices return less than full quality entropy, and then driver > authors set the quality knob. Therefore, the entropy crediting should be > opt-out rather than opt-in per-driver, to reflect the actual reality on > the ground. > > For example, the two Raspberry Pi RNG drivers produce full entropy > randomness, and both EDK2 and U-Boot's drivers for these treat them as > such. The result is that EFI then uses these numbers and passes the to > Linux, and Linux credits them as boot, thereby initializing the RNG. > Yet, in Linux, the quality knob was never set to anything, and so on the > chance that Linux is booted without EFI, nothing is ever credited. > That's annoying. > > The same pattern appears to repeat itself throughout various drivers. In > fact, very very few drivers have bothered setting quality=1024. > > Looking at the git history of existing drivers and corresponding mailing > list discussion, this conclusion tracks. There's been a decent amount of > discussion about drivers that set quality < 1024 -- somebody read and > interepreted a datasheet, or made some back of the envelope calculation > somehow. But there's been very little, if any, discussion about most > drivers where the quality is just set to 1024 or unset (or set to 1000 > when the authors misunderstood the API and assumed it was base-10 rather > than base-2); in both cases the intent was fairly clear of, "this is a > hardware random device; it's fine." > > So let's invert this logic. A hw_random struct's quality knob now > controls the maximum quality a driver can produce, or 0 to specify 1024. > Then, the module-wide switch called "default_quality" is changed to > represent the maximum quality of any driver. By default it's 1024, and > the quality of any particular driver is then given by: > > min(default_quality, rng->quality ?: 1024); > > This way, the user can still turn this off for weird reasons (and we can > replace whatever driver-specific disabling hacks existed in the past), > yet we get proper crediting for relevant RNGs. > > Cc: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> > Cc: Ard Biesheuvel <ardb@xxxxxxxxxx> > Cc: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx> > Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx> Thanks for the additional explanation! Reviewed-by: Dominik Brodowski <linux@xxxxxxxxxxxxxxxxxxxx> Thanks, Dominik
