Earlier quoted context omitted.
No, you're not missing anything. The primary reason this thing was designed was so try to patch out some holes in another unusual "mitigation", where system calls are checked to make sure they come from OpenBSD's libc. In this case adding new pages with syscalls in them would not work, but it seems like Theo read a paper at some point where they mprotected libc and patched its code and this spooked him into realizing…
> But using it to prevent the introduction of new code is not all that effective, unless you are far more stringent about how you allow processes to allocate executable regions. For example, if you prevent a program from mapping in any new PROT_EXEC pages after it's initialized itself and then mimmutable all its code, then no new code can be introduced, which is a useful property. But you need that extra bit which mi…
pledge is a good mitigation by itself. It’s a great way to sandbox things, and solve the problem of “this process no longer has any need to do these operations, so let’s just remove its capabilities to do that”. If you use it correctly it’s one of the strongest ways to make your program secure.
mimmutable by itself can be used to force a mapping to remain non-writable. This can be valuable because if you make the code that reads from that data also immutable, you know it will always read the same data, regardless of what an attacker can do in your address space. That is also a useful thing to have in certain cases.
If you read the thread where mimmutable was introduced you can see that the primary usecase proposed by Theo is to shore up msyscall. The threat model he has in mind is an attacker who can mprotect libc to be writable, which if I remember correctly was some minor detail of a Linux writeup once. However, there are several problems here.
One is that mimmutable by itself doesn’t actually fix the problem here because of the concern brought up at the top of this thread. You will note that the thing you quoted practically screams “but what if there was a way to prevent new executable mappings?” I actually did this intentionally, with full knowledge of how you can use pledge to do this. I think it is important for people who are not security experts to be able to follow the thinking that goes into analyzing these things, and be able to synthesize the usecase I mentioned at the start of this comment for themselves.
You will still note that I called msyscall weak in my original comment, and it still remains weak even after mimmutable+pledge is implemented. As others have mentioned as well using ROP to jump to the syscall instructions in libc with your own arguments (it’s not special…) bypasses restrictions in the current design.
In fact I can extend it with something OpenBSD does not have a good implementation of yet, strong CFI, which would prevent jumping into the middle of a function to execute that syscall instruction. But there are more fundamental reasons why this doesn’t work.
Protecting certain “sensitive” operations is actually something you can do. PPL as I mentioned below does this. It’s built on top of strong CFI (PAC) and code integrity guarantees (KTRR), which we are assuming you can construct in userspace on OpenBSD. The difference is that PPL has a tiny surface, and OpenBSD’s libc has a large one. In practice this means that libc cannot actually tell whether a request coming from an application is legitimate or not. If you look at the threat model for mimmutable again, consider that it assumes an attacker has enough control to call mprotect with controlled arguments: this means they know the layout of the address space and they can subvert control flow. With this kind of control, it’s not that hard to just decide to do other system calls instead. And who needs a syscall for that? You can literally just call the libc function which is a wrapper around the syscall. How is it to know that my call to write is malicious and the seven thousand the program already made was not?