[lmm]

> UB is in fact not worse than a memory safety issue

The worst case of UB is worse than the worst case of most kinds of non-UB memory safety issues.

> NULL pointer dereferences are almost never exploitable

Disagree; we've seen enough cases where they become exploitable (usually due to the impact of optimisations) that we can't say "almost never". They may not be the lowest hanging fruit, but they're still too dangerous to be acceptable.

[tptacek]

What is the worst case of UB that you're thinking of that is worse than the worst memory safety issue?

[lmm]

Essentially Descartes' evil demon, since there are no limits at all on what UB can do.

[tptacek]

Can I ask you to be specific here? The worse memory corruption vulnerabilities enable trivial remote code execution and full and surreptitious reliable takeovers of victim machines. What's a non-memory-corruption UB that has a worse impact? Thanks!

I know we've talked about this before! So I figure you have an answer here.

[lmm]

> Can I ask you to be specific here? The worse memory corruption vulnerabilities enable trivial remote code execution and full and surreptitious reliable takeovers of victim machines. What's a non-memory-corruption UB that has a worse impact?

I guess just the same kind of vulnerability, but plus the fact that there are no possible countermeasures even in theory. I'm not sure I have a full picture of what kind of non-UB memory-corruption cases lead to trivial remote code execution, but I imagine them as being things like overwriting a single segment of memory. It's at least conceivable that someone could, with copious machine assistance, write a program that was safe against any single segment overwrite at any point during its execution. Even if you don't go that far, you can reason about what kinds of corruption can occur and do things to reduce their likelihood or impact. Whereas UB offers no guarantees like that, so there's no way to even begin to mitigate its impact (and this does matter in practice - we've seen people write things like defensive null checks that were intended to protect their programs against "impossible" conditions, but were optimised out because the check could only ever fail on a codepath that had been reached via undefined behaviour).

[tptacek]

I'm sorry, I'm worried I've cost us some time by being unclear. It would be easy for me to cite some worst-case memory corruption vulnerabilities with real world consequences. Can you do that with your worst-case UB? I'm looking for, like, a CVE.

[lmm]

> It would be easy for me to cite some worst-case memory corruption vulnerabilities with real world consequences.

Could you do that for a couple of non-UB ones then? That'll make things a lot more concrete. As far as I can remember most big-name memory safety vulnerabilities (e.g. the zlib double free or, IDK, any random buffer overflow like CVE-2020-17541) have been UB.