risks of writing to /dev/random, crediting entropy, RNDADDENTROPY related to untrusted root

[madaidan]

@Patrick

I don’t know if it requires any capabilities.

It requires CAP_SYS_ADMIN. We can’t remove that capability though since it’s widely used.

Very little things are actually restricted to uid 0 anymore in Linux, everything is split up into capabilities.

@Patrick

First, we need to understand better what are the legitimate use cases of these syscalls are and what currently legitimate programs are that make use of these. Likely users are haveged / jitterentropy-rng (user space daemon and kernel module) / virio-rng among perhaps kernel internal code. Such patches need to be carefully reviewed. Potential side effects:

• slow (or even broken) boot because RNDADDTOENTCNT is slow/broken
• /dev/random entropy starvation after system started
• low/zero entropy quality of /dev/urandom or even /dev/random
• broken haveged / jitterentropy-rng (user space daemon and kernel module) / virio-rng among perhaps kernel internal code

Userspace daemons like haveged do use RNDADDENTROPY AFAIK. We don’t have to patch that ioctl out entirely, but we can restrict it to only the things that need it.

Internal kernel code (including the jitterentropy and virtio-rng modules) don’t use these ioctls. I grepped the entire source tree and nothing uses it.

@HulaHoop

OK I hadn’t thought of this before, but they would still need to modify the package sig keychain to fool it into accepting the modified code. We don’t trust server-client connections for package authenticity anyhow. If it’s broken we are screwed.

Signature verification has been bypassed before. We don’t want to rely on it completely. Unencrypted connections are still dangerous.

Even ignoring that, sabotaging the entropy still opens up tons of new surveillance opportunities (e.g. the attacker can eavesdrop on browsing/messaging/etc.) and will weaken system security as a whole.

Root is isolated from other users. Even init doesn’t have access to the user’s home directory for example. More ways to further enhance the isolation was discussed here AppArmor for Complete System - Including init, PID1, Systemd, Everything! - Full System MAC policy - #251 by madaidan