[nine_k]

My takeaway from the article is that Fuchsia exposes a capability-based interface externally, but uses the old kind of privilege-checking inside the kernel. Once a single sloppy check was found, the game was over: a privilege escalation and planting of arbitrary code into the kernel followed.

Did I miss anything?

[DannyBee]

Yes - you can't run untrusted native code in the first place outside of the emulator ;)

That's why the bug says: "The overall impact of this bug is pretty minimal in our current set of supported products, since none support running untrusted native code, and if you can run your own code on the system, then (at present) you can also use other existing supported workflows to obtain kernel logs, but it does seem to be a useful stepping stone towards privilege escalation if you have already obtained code-exec in some process through another exploit."

So while not awesome, also not possible on a real device right now without a code-exec exploit.

[londons_explore]

Don't forget that the use-after-free used was also artificial - ie. OP didn't discover one, he added a UAF bug to go exploit.

The fact he got KASAN working and talks about fuzzing suggests he looked for one, but couldn't find one, which is a good sign.

[phlip9]

From the article, it looks like the syzkaller fuzzer integration was stale and not working, so there might still be some juice to squeeze if someone can get that running again : )

[fay59]

The intent of the post isn’t to claim that any Fuschia device currently sold is vulnerable. Unless Fuschia never graduates to running third-party code, that seemed like the right assessment to me.

[DannyBee]

Sure - but he also added the vulnerability he exploited in the first place?

[fay59]

That’s not really a “but” to the comment, which was that you need to find one bug and it’s game over. We’ve known for a long time that best practices aren’t enough to prevent memory corruption in large enough C++ code bases, so it’s likely a motivated attacker would eventually find something.

[DannyBee]

Sure - look i'm not gonna argue about C++ memory safety - i've funded and spent time trying to ensure we are funding figuring out how to get off of C++.

I just assume C++ code is unsafe, because it's really really hard to make it safe.

However, at the same time, the privilege escalation issues would have happened in any language - if you don't implement the check, you don't implement the check.

(and you could make it equally automatic in most languages)

[fay59]

What the missing access check protected was a stream of information that could defeat ASLR. If Zircon was written in a memory-safe language, that would have been the end of the issue. Logic bugs and missing access checks are still possible, but defeating them has fairly well definable consequences. Since Zircon isn’t written in a memory-safe language, the author was able to use that to fully compromise the kernel instead. I don’t mean that you can’t write bugs in memory-safe languages, but in the end the attacker still has to play by your rules. With a memory safety bug, attackers play by no one’s rules.

[ncmncm]

The code exploited was very, very far from any "best practice" in this decade, and probably the previous one.

[pjmlp]

Unfortunately still a common reality across many corporations.

[tyingq]

Not familiar with capability-based practices, but wouldn't there always be a "if (has_this_capability(WHATEVER_CAPABILITY))" at the very bottom...one that could be sloppy? Doesn't something, somewhere do a comparison?

[nine_k]

Not necessarily.

The core idea of capabilities is more like having a URL to a Web page. Using the URL (the capability), you can access the contents of the page. Inside the contents, you can possibly find other URLs (more privileges granted to you). But the URL happens to be something like an UUID, or a short link; looking at it, you cannot derive another URL (discover another "capability", not granted to you).

In other words, a capability is like a key in a hash table, and unlike an index in an array.

[why_only_15]

Interesting. Is this in practice implemented as just capabilities being large numbers so it's impractical to guess them, or does the kernel have a table with all of a process's capabilities and when a message is sent to a process with capabilities the kernel adds them to the table?

That is -- are capabilities just pieces of data in a message you can detect and try to use, or do they have to be added explicitly to a message to send them

[jamesr_]

The kernel maintains a table of which handle values each process owns and uses that to check the capabilities of the calling process when handling a syscall. Sending a capability in a message updates this table as the ownership changes.

We use (somewhat) large and non-dense numerical values for handle values to reduce the risk of accidental reuse of values.

[giyanani]

I’m not sure how fuschia does it, or how feature-based capabilities work, but Cheri[1] uses capabilities for memory management and isolation.

It uses a couple of techniques, like wide/tagged pointers, object ids, and a special hardware managed bit to track illegal modifications.

If I had to hazard a guess, those object ids are probably useful for general capability systems.

I think apple (maybe as just an arm feature?) can do encrypted pointers, with a per application key tracked by the kernel.

[1] https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/

[zahllos]

> I think apple (maybe as just an arm feature?) can do encrypted pointers, with a per application key tracked by the kernel.

ARM has added both pointer authentication codes and memory tagging extensions to their ISA, from (I think) ARMv8.5-A. Apple are the only ones to implement silicon that supports PAC that anyone can buy today but a) this will change fast and b) I might've missed something else.

I wouldn't say "encrypt". You can still read what the pointer is, you just can't forge it. It is more like a message authentication code, or keyed hash. Confusingly, the ARM ISA suggests you use the block cipher QARMA, a lightweight block cipher of their design. This is not a mistake - from a crypto academic's perspective the distinction between block cipher and hash is not so easy to draw as it tends to be communicated: you can get a block cipher out of the SHA family of hash functions (called SHACAL) and so on. There was a line of work on finding a suitable family of permutations (e.g. Gimli) that could be hardware-accelerated and appropriate constructions built from that.

tl;dr it is kinda a "signed pointer", and if you forge the pointer but get it wrong you don't get another chance because the CPU triggers a fault, so you can use lower security lightweight ciphers and still make exploitation very unreliable.

Now Fuchsia:

As to how Fuchsia does it, we can just look at the source:

1) https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/z... 2) https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/z... 3) https://fuchsia.googlesource.com/fuchsia/+/refs/heads/main/z...

For example. I don't think there's any magic here: the kernel maintains some structure to reference jobs, tasks or whatever we are calling our isolated privilege boundaries, in which the capabilities are described. On the face of it this "looks" a lot like the traditional permission-style system, but the key difference is that the root job will hand out capabilities to processes it has created. An example is probably the best way to proceed: on a linux system, you might decide your process needs to read `/system/sensitive_file`. To allow this to happen, you might grant access to the user using some other command: chown .../chmod ..., or you might set an selinux policy, semanage fcontext -a -t myprocess_t /system/sensitive_file. In a capability-based system, chmod/chown and even running the process as a user don't exist as concepts. You would need to use a process authorized to transfer a capability to your target process to access this file. In some ways, this is closer to selinux, in the sense that semanage fcontext updates policy, except that the policy is somewhat dynamic here: if your controlling process provides the capability, then the child process gets it.

In a microkernel-based architecture, the idea is that "servers" (jobs that are arguably part of the system, but don't need to be in kernel) are similarly userspace processes subject to said capabilities also.

Of course, some part of the code still needs to be privileged here, let's call it the "executive". This code is doing the capability enforcement, and if that code contains bugs, all bets are off, as always. You can't defend such code from itself, even with capabilities.

The real disappointment is that Zircon is written in C++. Granted it was probably started before Rust was a thing, but while Ada/SPARK might be considered a bit unfashionable, it has some formal verification available, and if you're going for a software solution this should help reduce exploitable bugs. Personally I also wonder what about seL4 made it unsuitable for this: granted, it is not complete as a system, but neither is Zircon alone.

[conioh]

> ARM has added both pointer authentication codes and memory tagging extensions to their ISA, from (I think) ARMv8.5-A. Apple are the only ones to implement silicon that supports PAC that anyone can buy today but a) this will change fast and b) I might've missed something else.

I'm not aware of other generally available chips with pointer authentication. The Qualcomm Snapdragon 8cx Gen 3 supposedly support PA. The Lenovo ThinkPad X13s supposedly contains it, but when will it actually be commercially available is anyone's guess.

Also, I think it's ARMv8.3.

[pjmlp]

Apple isn't the only one, Oracle did it first with SPARC ADI on Solaris.

[saagarjha]

PAC is ARMv8.3, and Apple implements their own custom algorithm rather than QARMA.

[formerly_proven]

You know a limited kind of capability - file descriptors (or kernel handles). Those are just a number that allow you to manipulate some object in a defined way. You can give this number to someone else, and they can't make use of it at all, you have to go ask the kernel (using e.g. a unix socket and ancillary messages) to pass the capability to another process.

[staticassertion]

That is sort of the opposite of a capability. In fact, files are capabilities exactly because you can hand off a file descriptor and, by virtue of that handle, you grant access.

File systems aren't actually capability based (generally, in practice) because you can 'ls' and 'cd ../'. Otherwise they could be.

Dropbox Paper is a good example of a capability based system. Anyone with a URL can perform actions on a page, but there is no way to derive a URL without already having access to it, you must be told what it is. This is because the urls are sufficiently random so as to be unguessable.

[trasz]

>File systems aren't actually capability based (generally, in practice) because you can 'ls' and 'cd ../'. Otherwise they could be.

That's precisely how it works in FreeBSD (https://www.freebsd.org/cgi/man.cgi?capsicum).

[throwaway894345]

Are capabilities rotated periodically? Or how do you deal with leaked values?

[throwawaymaths]

In theory you could do even better than that -- you could make capabilities cryptographically signed tokens, so that you don't need to ask the kernel to verify the validity of your request every time. If your chipset supports crypto intrinsics this will almost certainly be better than an interrupted syscall.

[staticassertion]

Yeah, reminds me of biscuits[0] - because they're based on asymmetric cryptography they can be delegated, but also attenuated. If you just use a uuid that's not really going to work as well, unless you're willing to go back to some authority to forge a new, lesser capability.

[jnwatson]

Not even theoretical. An early capability based OS called KeyKOS worked that way.

[throwawaymaths]

Oh man KeyKOS, that brings back memories. I wonder if crypto intrinsics might breathe life back into something like KeyKOS. More contemporary, I think Sel4 does as well.

In my comment I meant "in theory fuscia could have been architected/could be rearchitected that way", sorry should have been more clear.

[monocasa]

It's per process like the file descriptor tables under most kernels. Knowing that some other process has a certain socket open as fd '5' on their fd table doesn't give you enough information to attach the same socket to your fd table at fd 5 or otherwise.

[jackdaniel]

It is more like:

(caller_capabilities[WHATEVER_CAPABILITY])(...);

when the capability is not present, then you get no_method_found or something similar.

[Commodore63]

Capability based operating systems require that your program possess something, like a handle, in order to do something privileged. Don't have the handle, can't do the thing. It's not about consulting ACLs or bitmasks or whatever.