On Tue, Mar 22, 2022 at 01:14:36PM -0600, Jason A. Donenfeld wrote: > So this patch fixes the issue by including /dev/urandom writes as part > of the "fast init", but not crediting it as part of the fast init > counter. This is more or less exactly what's already done for > kernel-sourced entropy whose quality we don't know, when we use > add_device_randomness(), which both contributes to the input pool and to > the fast init key. One of the big issues with /dev/urandom writes is that *anyone*, including malicious user space, can force specific bytes to be mixed in. That's the source of the reluctance to immediate use inputs from writes into /dev/[u]random until there is a chance for it to be mixed in with other entropy which is hopefully not under the control of malicious userspace. Now, I recognize that things are a bit special in early boot, and if we have a malicious script running in a systemd unit script, we might as well go home. But something to consider is whether we want to do soemthing special if the process writing to /dev/[u]random has CAP_SYS_ADMIN, or some such. > There is one caveat to consider, which is what happens if the user > additionally calls RNDADDTOENTCNT after having written to /dev/urandom, > expecting to credit that write. That might give way to this pathological > pattern: Yeah, no one should ever ver ever be using RNDADDTOENTCNT. It's an ioctl which requires root privilegs, and if it breaks, you get to keep both pieces. > The better interface for userspace is RNDADDENTROPY, which takes the > input buffer and the entropy credit all at once, so we can make the > right decision. For the RNDADDENTROPY, we do not take part in fast init > if entropy is being credited. > > And perhaps we might consider attempting to deprecate RNDADDTOENTCNT at > some point in the future. That would be a good idea. :-) - Ted
