mimmutable() for OpenBSD
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lwn.net
What is the point of making something immutable if you can just create a new write-exec region? Surely if you have enough control over the process to change permissions on an existing memory segment you have enough to mmap a new region with the permissions you want?
You aren't the only thing using the memory. If you create a new region only you will be the one using it.
Of course, msyscall itself is a weak mitigation, because an attacker can (as you mentioned) allocate another page with arbitrary code in it save for the syscall instruction, construct a call state with it, and then jump to any syscall instruction in libc. So this doesn't protect against much, anyways.
mimmutable itself is not really a dud, it's just not that great if you try to layer it on top of something already useless. It's great if you want to make sure a page (of data, usually) really stays immutable. 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 mimmutable by itself doesn't give you.
(FWIW, macOS already has something similar to this called VM_FLAGS_PERMANENT, which prevents a mapping from being removed. This, together with the maximum protection on a mapping, can be used to prevent a region from ever gaining a certain permission. I believe you can reduce it to PROT_NONE though.)
You mean like pledge(2)? The vast majority of the base system is pledged. So programs without the prot_exec promise cannot make new PROT_EXEC mappings, or add it to any existing pages.
https://man.openbsd.org/pledge#prot_exec
Perhaps you should spend more than 5 minutes reading about OpenBSD's layers of mitigations.
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?
...ignoring other mitigations.
> 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.
Sounds like you need to spend 5 more minutes reading about retguard.
Anyways, the critical word in the thing you've quoted is "strong"–retguard falls apart if the attacker has the ability to leak the value being used to protect the stack. In almost all cases an attacker in the position to make a controlled call into libc has the ability to also leak or forge any value of their choosing in the address space. A strong implementation would typically move this value completely out of the reach of such an attacker, for example by implementing signing in hardware (e.g. PAC) or or at a higher privilege level. I don't think these things are really feasible yet for OpenBSD so I don't blame them for not doing it, but without strong CFI it's hard to actually prevent this kind of thing from happening. (And, as I mentioned, this was only a tangent anyways; even with good CFI the more fundamental issue remains.)
This is, unfortunately, a continuation of OpenBSD's long tradition of security "mitigations" that are entirely detached from any actual understanding of exploitation [1]. The above suggestion that attackers can no longer make syscalls from their shellcode, while ostensibly true, is moot because you can simply JOP to one of the authorized syscall instructions and entirely defeat the mitigation. Even if mimutable outright prevented any new code from being mapped, this still wouldn't really stop anyone; just look at iOS where attackers have thrived despite there only being one process on the entire OS which is able to map arbitrary code.
> Whenever a process enters the kernel, its stack pointer is checked to see whether it is, indeed, pointing into a stack region; if not, the process is killed.
This too is trivially defeated. You simply need to add another stage to your payload in which you copy your ROP payload onto the legitimate stack, pivot, and continue executing. If you managed to pivot off the original stack, you already have a pivot gadget and so you only then need to trigger a call to memcpy. This mitigation isn't stopping any real attacker.
While I never hope to discourage kernel developers from thinking about security mitigations, I think it is really important that people working on these mitigations actually converse with people who write exploits. Other vendors have seen considerable success with their mitigations [2] in that they specifically target things that attackers need and want to do, such as heap spray.
[1] See: ROP gadget elimination, a feature which does not actually stop ROP but merely hopes that "a substantial reduction of gadgets is powerful." <https://marc.info/?l=openbsd-tech&m=150317547021396&w=2>. Speaking from experience and having written plenty of actual exploits on attacker hostile platforms, trying to block code execution against an attacker who already has kernel read/write is an utterly ridiculous idea because even if you manage to stamp out every single ROP gadget, they can just go and stomp on your kernel page tables to map new kernel code. Even baring that, kernel R/W is still a complete compromise even if kernel code execution is somehow impossible because you can still just use R/W to bypass any control the kernel could ever hope to enforce.
[2] Life and death of an iOS attacker by Luca Todesco <https://www.youtube.com/watch?v=8mQAYeozl5I>, a discussion of exploit mitigations on iOS and the exponential cost of defeating mitigations on iOS. Luca's company writes exploits for iOS devices as a service and he is extremely knowledgable in this area.
iOS does this. Kernel code is immutable (obviously), page tables are locked at reset and cannot be modified except by PPL code. ROP and JOP are mitigated against using hardware CFI (PAC). As you mentioned, when vendors actually understand what exploits look like and what attackers need to do, it enables them to do a good job. OpenBSD remains a system with a few good ideas and a model of what exploits look like that appears to be informed by dreams and idle musings, rather than actual research.
The PPL is a strong mitigation, I agree, but it is definitely one that is really only supported through their ability to ship custom hardware. Pulling off a PPL-like thing using MPK or a hypervisor is technically possible but in practice no real platform other than Apple is actually able to enforce memory mappings on all devices on an SoC and so anything you dream up to protect page tables is as durable as a wet paper bag since there's usually a half dozen other chips that will hapilly DMA any and all parts of physical memory for you. I dream of a day where we have an SMMU in front of every single device on a regular smartphone SoC (or, hell, even a PC platform), but that day has not yet come and I'm not really holding my breath either. As such, nobody else can actually meaningfully guarantees kernel code integrity under kernel R/W and so even if they had a perfect PAC implementation (which, as I'm sure you know, is incredibly difficult), you could just corrupt the backing code pages and sidestep it.
But now you need to know where the libc is mapped in order to find those syscalls instructions yeah? Which increases the complexity of the exploit, because instead of making a syscall you have to find where the libc is mapped, and the entire point of ASLR is making that difficult, no?
I don't know how it works on OpenBSD, but on Linux there's a section of memory called the PLT that contains pointers for all the functions from dynamic libraries, including those from libc. Also, there's typically libc pointers all over the stack.
Have you, or someone else as knowledgeable as you, been able to provide a succesful POC doing this? I admit that I haven't gone about looking for results on this topic and method of attack but I would be very interested in reading about it and see a practical example achieving it, since it's (in your own words) trivially defeated and simply needs just this or that thing.
it's always the same thing whenever openbsd is mentioned.
"these mitigations don't work."
- "okay, please show us that they don't work"
"well, i don't use openbsd"
rinse. repeat.
it would also be nice to see some patches/fixes/suggestions/etc submitted after you've bypassed/defeated/whatever these things sent to the mailing lists. i don't suppose you'd agree to that?
Also, I don't think it is quite right of you to be miffed that people aren't writing exploit PoCs to prove these mitigations are moot. These mitigations are trivially wrong to anyone who has any experience with exploit development, and being told to "show proof then" is baffling. It's like trying to explain to your uncle that no, vaccines will not make your child autistic, and him demanding proof. Obviously, that's neither how autism or vaccines work and trying to demonstrate that takes significantly more effort than most anyone cares to put in to a random (internet) argument. But, I am a fool who has too much time, so I'll bite.
It seems many of you are missing the forest for the trees.
OpenBSD will make the above approach impossible, because with mimmutable(), libc will not be unmappable.
I would ask why you would want to do this.
I'd argue that things like msyscall and mstack don't at all because they cost attackers only a couple of minutes of time once to develop a bypass technique (ie move the stack pointer before a syscall, reuse the authorized syscall instruction) that they can apply everywhere. This greatly contrasts with mitigations like ASLR where each time an attacker wants to bypass the mitigation they are forced to develop a novel, program dependent strategy to leak some information. This is a huge pain and has definitely killed some otherwise exploited bugs because no such leak could be devised.
If you read up on library order randomization/re-link and retguard, you may find your technique won't be so reusable, even once you do manage to locate a syscall stub in libc.
Live patching un-stoppable systems may be a challenge unless you code to passing live IO and state across process boundaries