Because nobody is actually going to set CONFIG_MODULES=n as any kind of "security measure", first off. Or really ever do it at all. If CONFIG_MODULES=n is a really completely separate issue to security and it's about "don't use what you don't need" -- why did we ever bring it up in this thread at all? Clearly there is some connection - the idea less code is better, right? Don't allow flexibility where none is needed. Principle of least power. But if your point is not "overridden" by the existence of grsecurity -- a clearly security focused piece of kit -- why are we talking about it at all in this thread? We might think these are mostly unconnected because "They are obviously doing different things, so they must be unconnected, they're just kinda like peas-in-a-pod!" But they are very much intertwined, I think, about the way we think of these things. Dig on that.

My argument is that it seems like doing this helps. It obviously means less LOC, right? So clearly it's strictly less attack surface. That's clearly better, no questions asked, right? I mean, it's not anti-memory-corruption. But that's why they aren't the same! So they aren't connected in that sense, right? But it isn't attacking the real root problem. The root problem is extremely important in this case, because millions and millions of computers are impacted by it. You can't kick-the-can forever.

You're saying "Why not just disable it", but you need to ask yourself another question first: will anyone actually do that, anyway? I mean, aside from massive nerds like us - with free time. Another question is still: Can the problem be solved without this, without requiring more than is necessary while covering every perceivable use? The answer is, aside from people running on their laptops, nobody at scale -- unless they have very specific needs or resources -- are going to do this. This ship is very long gone and sailed. And you can definitely solve this without hacking your kernel config.

It also does not address the root issue. So we all turn off CONFIG_MODULES in every distro. What's next? The next major weak point in Linux's infrastructure? Then everyone sets CONFIG_FOO=n in order to avoid everything until Linux rolls in 10-million more lines of code, then there's CONFIG_BAZ=n to set? This is why it's whack-a-mole. BTW: remember the other millions of devices _for which you cannot turn off CONFIG_MODULES_? You won't be able to turn off CONFIG_BAZ in 5 years either, I'm afraid. Because you won't be able to on your shitty router.

Let me put it this way: if Linux had a vulnerability in its ethernet stack and it seemed to be a source of problems continuously -- is the answer turning off the ethernet stack for everyone? No. It is to find the root cause.

If browsers are attacked relentlessly through vulnerabilities -- do we stop using the web forever and everyone just deletes their browser? No. Do we tell every user they're doing it wrong by "trusting the browser" when they should "know" software has flaws? No- we instead engineered browsers to be as resilient as possible in the face of a very, very hostile internet. Chrome is an example that we have made real progress here. There is more to do. Why do this? Because this is simply the way most people use the internet: with a browser. With an ethernet stack. With CONFIG_MODULES=y.

Try using the "5 W's" here:

Why are so many vulnerabilities here? What trigger mechanisms do people use to attack it? Where are they introduced? When are they exploitable?

The reality is: a lot of the answers to these questions have very little to do with module autoloading.

Maybe there are lots of vulnerabilities because the code was not extensively hardened, or designed around the scenarios it is used now (user namespaces are a good example). Perhaps people very often use the same trigger mechanisms for a payload (for example, overwriting a function-ptr struct, such as the one that takes care of module callbacks, like read() syscalls, etc) It might have been they were introduced long ago. It might be actively and reliably exploitable right now, or maybe it is very difficult to exploit in any realistic scenario.

These are all very common attack scenarios for Linux exploits, historically, over the past few years. Global function pointers overwritten trivially (because of no `__ro_init` until 4.8+). Simple payloads because there wasn't even a way to block `commit_creds` for a long time. Exact same triggers, like refcounts, UAFs, double frees.

This is beyond autoloading. It is a process problem.

Again, "rebuild your kernel with a config option changed" does not logistically scale for the vast majority of people. It's basically a non-starter. People will go with their distro kernel instead (as they very likely should, to be honest).

Just to be clear: I am not saying CONFIG_MODULES=n is bad -- by all means, turn it off. If it makes you feel good, speeds up your kernel builds -- whatever. But it does not really address the root problem here, so its suggestion is a red herring; simply turning off this stuff is just a bandaid, it doesn't address actual attack vectors. If you're bring up CONFIG_MODULES=n not as a security measure but as a way to just "reduce attack surface via LOC" -- these are the same thing! It's just not an actually good security measure. It doesn't scale. Again, it is a herring if it is not actually a "security measure".

I suppose my point is there's a bigger gap here. We think very "simplistic" things like this are good, because they "strictly must improve everything and are justified by that" but often they treat symptoms and not the real disease. But they don't really, actually stop the attacks. They "shake the table" a little, to quote someone on Twitter recently. They can even increase complexity and make real defenses more brittle.

FWIW: Here's an example of that, where these kinds of "soft" mitigations actually backfire - that SLAB randomization code is only a small obstacle for a skilled attacker (and most kernel attackers _will_ be skilled): https://github.com/torvalds/linux/commit/c4e490cf148e85ead0d... -- so, given this: why exactly are we paying the cost of these regressions, of half-baked fixes that do not stop SLAB-based exploits and real attackers? If my kernel is going to have a shitty, bizarre regression, I might as well get actual security out of it.