Comment by pizlonator

Comment by pizlonator 2 days ago

42 replies

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> Because thread 2 can observe a mismatch between a pointer and its capability, an attacker controlled index into P2 from thread 2 can access memory of an object other than the one to which P2 points.

Under Fil-C’s memory safety rules, „the object at which P points” is determined entirely by the capability and nothing else.

You got the capability for P1? You can access P1. That’s all there is to it. And the stores and loads of the capability itself never tear. They are atomic and monotonic (LLVM’s way of saying they follow something like the JMM).

This isn’t a violation of memory safety as most folks working in this space understand it. Memory safety is about preventing the weird execution that happens when an attacker can access all memory, not just the memory they happen to get a capability to.

> He claims Java has the same problem. It does not.

It does: in Java, what object you can access is entirely determined by what objects you got to load from memory, just like in Fil-C.

You’re trying to define „object” in terms of the untrusted intval, which for Fil-C’s execution model is just a glorified index.

Just because the nature of the guarantees doesn’t match your specific expectations does not mean that those guarantees are flawed. All type systems allow incorrect programs to do wrong things. Memory safety isn’t about 100% correctness - it’s about bounding the fallout of incorrect execution to a bounded set of memory.

> That's correct but irrelevant: thread 2 has P2 and it's paired with the wrong capability. Kaboom.

Yes, kaboom. The kaboom you get is a safety panic because a nonadversarial program would have had in bounds pointers and the tear that arises from the race causes an OOB pointer that panics on access. No memory safe language prevents adversarial programs from doing bad things (that’s what sandboxes are for, as TFA elucidates).

But that doesn’t matter. What matters is that someone attacking Fil-C cannot use a UAF or OOBA to access all memory. They can only use it to access whatever objects they happen to have visibility into based on local variables and whatever can be transitively loaded from them by the code being attacked.

That’s memory safety.

> He doesn't acknowledge corner cases like the one I've described.

You know about this case because it’s clearly documented in the Fil-C documentation. You’re just disagreeing with the notion that the pointer’s intval is untrusted and irrelevant to the threat model.

quotemstr 2 days ago

> The kaboom you get is a safety panic

You don't always get a panic. An attacker who can get a program to access an offset he controls relative to P2 can access P1 if P2 is torn such that it's still coupled, at the moment of adversarial access, with P1's capability. That's dangerous if a program has made a control decision based on the pointer bits being P2. IOW, an attacker controlled offset can transform P2 back into P1 and access memory using P1's capability even if program control flow has proceeded as though only P2 were accessible at the moment of adversarial access.

That can definitely enable a "weird execution" in the sense that it can let an attacker make the program follow an execution path that a plain reading of the source code suggests it can't.

Is it a corner case that'll seldom come up in practice? No. Is it a weakening of memory safety relative to what the JVM and Rust provide? Yes.

You are trying to define the problem away with sleigh-of-hand about the pointer "really" being its capability while ignoring that programs make decisions based on pointer identity independent of capability -- because they're C programs and can't even observe these capabilities. The JVM doesn't have this problem, because in the JVM, the pointer is the capability.

It's exactly this refusal to acknowledge limitations that spooks me about your whole system.

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  • pizlonator 2 days ago

    > An attacker who can get a program to access an offset he controls relative to P2 can access P1 if P2 is torn such that it's still coupled, at the moment of adversarial access, with P1's capability

    Only if the program was written in a way that allowed for legitimate access to P1. You’re articulating this as if P1 was out of thin air; it’s not. It’s the capability you loaded because the program was written in a way that let you have access to it. Like if you wrote a Java program in a way where a shared field F sometimes pointed to object P1. Of course that means loaders of F get to access P1.

    > That can definitely enable a "weird execution"

    Accessing a non-free object pointed by a pointer you loaded from the heap is not weird.

    I get the feeling that you’re not following me on what „weird execution” is. It’s when the attacker can use a bug in one part of the software to control the entire program’s behavior. Your example ain’t that.

    > Is it a corner case that'll seldom come up in practice? No. Is it a weakening of memory safety relative to what the JVM and Rust provide? Yes.

    I don’t care about whether it’s a corner case.

    My point is that there’s no capability model violation and no weird execution in your example.

    It’s exactly like what the JVM provides if you think of the intval as just a field selector.

    I’m not claiming it’s like what rust provides. Rust has stricter rules that are enforced less strictly (you can and do use the unsafe escape hatch in rust code to an extent that has no equal in Fil-C).

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    • lifis 2 days ago

      I think his argument is that you can have code this:

        user = s->user;
  if(user == bob)
    user->acls[s->idx]->has_all_privileges = true;
      And this happens: 1. s->user is initialized to alice 2. Thread 1 sets s->idx to ((alice - bob) / sizeof(...)) and s->user to Bob, but only the intval portion is executed and the capability still points to Alice 3. Thread 2 executes the if, which succeeds, and then gives all privileges to Alice unexpectedly since the bob intval plus the idx points to Alice, while the capability is still for Alice

      It does seem a real issue although perhaps not very likely to be present and exploitable.

      Seems perhaps fixable by making pointer equality require that capabilities are also equal.

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      • pizlonator 2 days ago

        I understand his argument.

        Here are the reasons why I don’t buy it:

        1. I’m not claiming that Fil-C fixes all security bugs. I’m only claiming that it’s memory safe and I am defining what that means with high precision. As with all definitions of memory safety, it doesn’t catch all things that all people consider to be bad.

        2. Your program would crash with a safety panic in the absence of a race. Security bugs are when the program runs fine normally, but is exploitable under adversarial use. Your program crashes normally, and is exploitable under adversarial use.

        So not only is it not likely to be present or exploitable, but if you wrote that code then you’d be crashing in Fil-C in whatever tests you ran at your desk or whenever a normal user tried to use your code.

        But perhaps point 1 is still the most important: of course you can write code with security bugs in Fil-C, Rust, or Java. Memory safety is just about making a local bug not result in control of arbitrary memory in the whole program. Fil-C achieves that key property here, hence its memory safe.

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      • quotemstr 2 days ago

        Exactly. I agree that this specific problem is hard to exploit.

        > Seems perhaps fixable by making pointer equality require that capabilities are also equal

        You'd need 128-bit atomics or something. You'd ruin performance. I think Fil-C is actually making the right engineering tradeoff here.

        My point is that the way Pizlo communicates about this issue and others makes me disinclined to trust his system.

        - His incorrect claims about the JVM worry me.

        - His schtick about how Fil-C is safer than Rust because the latter has the "unsafe" keyword and the former does not is more definitional shenanigans. Both Fil-C and Rust have unsafe code: it's just that in the Fil-C case, only Pizlo gets to write unsafe code and he calls it a runtime.

        What other caveats are hiding behind Pizlo's broadly confident but narrowly true assertions?

        I really want to like Fil-C. It's good technology and something like it can really improve the baseline level of information security in society. But Pizlo is either going to have to learn to be less grandiose and knock it off with the word games. If he doesn't, he'll be remembered not as the guy who finally fixed C security but merely as an inspiration for the guy who does.

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        • jstarks a day ago

          All I’m really hearing is that this guy rubs you the wrong way, so you’re not going to give him the benefit of the doubt that you’d give to others.

          I mean, maybe you’re right that his personality will turn everyone off and none of this stuff will ever make it upstream. But that kind of seems like a problem you’re actively trying to create via your discourse.

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    • quotemstr 2 days ago

      > Only if the program was written in a way that allowed for legitimate access to P1. You’re articulating this as if P1 was out of thin air; it’s not.

      My program:

        if (p == P2) return p[attacker_controlled_index];
      If the return statement can access P1, disjoint from P2, that's a weird execution for any useful definition of "weird". You can't just define the problem away.

      Your central claim is that you can take any old C program, compile it with Fil-C, and get a memory-safe C program. Turns out you get memory safety only if you write that C program with Fil-C's memory model and its limits in mind. If someone's going to do that, why not write instead with Rust's memory model in mind and not pay a 4x performance penalty?

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      • pizlonator 2 days ago

        > that's a weird execution for any useful definition of "weird".

        Weird execution is a term of art in the security biz. This is not that.

        Weird execution happens when the attacker can control all of memory, not just objects the victim program rightly loaded from the heap.

        > Your central claim is that you can take any old C program, compile it with Fil-C, and get a memory-safe C program.

        Yes. Your program is memory safe. You get to access P1 if p pointed at P1.

        You don’t get to define what memory safety means in Fil-C. I have defined it here: https://fil-c.org/gimso

        Not every memory safe language defines it the same way. Python and JavaScript have a weaker definition since they both have powerful reflection including eval and similar superpowers. Rust has a weaker definition if you consider that you can use `unsafe`. Go has a weaker definition if you consider that tearing in Go leads to actual weird execution (attacker gets to pop the entire Go type system). Java’s definition is most similar to Fil-C’s, but even there you could argue both ways (Java has more unsafe code in its implementation while Fil-C doesn’t have the strict aliasing of Java’s type system).

        You can always argue that someone else’s language isn’t memory safe if you allow yourself to define memory safety in a different way. That’s not a super useful line of argumentation, though it is amusing and fun

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      • dnr 2 days ago

        I'm not an expert here but I have to say this feels like a very weak objection.

        p points to P1. One thread reads through p. Another thread races with that and mutates p to point to P2. The result is the first thread reads from either P1 or P2 (but no other object).

        This seems totally fine and expected to me? If there's a data race on a pointer, you might read one or the other values, but not garbage and not out of bounds. I mean, if it could guarantee a panic that's nice, but that's a bonus, not required for safety.

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