ARM Pointer Authentication
lwn.net
lwn.net
How about we use 24 bits of data pointers to keep the array size, or 1 bit to indicate "this is a pointer with a size" and 23 bits for the size, and then our load/store with index instructions, as well as freshly added pointer arithmetic instructions, trap when the index exceeds the size? Instead of using bits in instruction pointers to not let one of many kinds of buffer overflow create valid instruction pointers? No good?
How about we use 24 bits of data pointers to keep the array size, or 1 bit to indicate "this is a pointer with a size" and 23 bits for the size
then our load/store with index instructions, as well as freshly added pointer arithmetic instructions, trap when the index exceeds the size?
http://www.software-architect.net/blog/article/date/2015/03/...
How about we only use languages that (as you propose) work with memory slices not naked pointers and where the concept of a null pointer does not exist
How about we only operate on memory slices after checking boundaries
You can think yourself superiour and turn up your nose, but the fact remains that many people have perfectly good reasons to write and maintain C. Your haughty commentary has no impact on that.
You may think that bounds checking is the answer to all situations, but if you're writing a realtime system, there's often no point in running the program if it can fail from an out of bounds read or write anyway. In a flight control system, or an ECU, there is often nothing productive about crashing. You need to verify your pointer logic, instead of hoping your program will crash.
I love how most of the comments are, in essence "you're too dumb to use C". Just check the pointers right? Too bad people who think they are too smart do get bitten by those issues
I guess that's why people won't use other technologies beyond C in embedded systems (they do use)
How that would benefit stability? I'd argue that having an equivalent of NaN for integer types would benefit array index computations more.
Any time where you have an important distinction between the address of a valid object, and a non-address (next address at the end of a linked list, leaf node of a tree, failed initialization of a pointer). Zero is also used to terminate strings, for similar convenience/efficiency reasons.
C compilers and static analyzers together tend to catch possible null dereference bugs with near certainty these days, so people don't tend to ship them these days, if they make any effort at all. I have not encountered a null pointer dereference which wasn't typo-related in... I don't remember the last time it happened.
If you really want to be certain downstream users of your API won't struggle with it, put the null check in your sample code with a fat comment which says "This is NULL 0.001% of the time, and it really hurts when you don't handle that".
ALGOL, of course, is the sort of language which doesn't have pointer arithmetic. I would agree that a language without pointer arithmetic should not have NULL pointers, if only because it doesn't make any sense for there to be an abstract reference to an object which can not be used with functions designed for it.
In a language with integer pointers, like C, you check for null at allocation time. I've also seen people consider functions which could return NULL pointers to return something like an option type, where null is considered None, and everything else considered Some.
Some people should really be using bounds checks and option types more often, and I often use bounded functions for handling strings in fixed-size buffers. Some people write bugs into their programs for a lack of understanding or care given to these aspects of the language; but many people also make wonderful and unique things out of them.
I just don't think that the baby should be thrown out because somebody overfilled the bathwater. There is a time and a place for zero tests, null pointers, and pointer/index arithmetic.
Yes. In MMUless microprocessors with kbytes of memory or less
I would argue than in at least 80% of the cases you don't want your values to be null, and that's in those situations mistakes are made (because you don't expect the value to be null !)
There's really only one replacement for C right now and it's Rust. If you don't like or can't use Rust (for whatever reason) you're gonna stick with C, and when you consider how much work has gone into making Rust a viable C alternative, it's clear we're a long way away from having a healthy ecosystem of C replacements.
Rust, Go, Oberon, Common Lisp, Scheme, D, Java
- Lack of library and platform support
- Lack of tooling
- Lack of standardization
Go:
- Lack of tooling
- Lack of standardization
- GC
- Slower
- Higher memory usage
Oberon (Ada, RTS, etc.):
- Lack of library and platform support
- Lack of tooling
- Lack of standardization
- Obscure
Common Lisp/Scheme:
- Lack of library and platform support
- Lack of tooling
- Lack of standardization
- GC
- Slower
- Higher memory usage
- Obscure
- Can't access arbitrary memory locations (?)
Java:
- GC
- Slower
- Higher memory usage
- Can't access arbitrary memory locations
I'm not really a C apologist, but I am pretty irritated with the near-constant calls for C deprecation. It's a lot easier to say, "C sucks!" than it is to do something about it, and I think we should at least internalize how difficult replacing C will be before we go around castigating people for continuing to use it.
C:
- error prone
- Lack of modern typing (generics vs void pointer)
- Lack of dependency management
- no high level constructs
- unintuitive semantics (undefined behaviors)
And that's just the "this is impossible" level. Sure you can build a database in Python, but it'll be slow and a memory hog, so if your requirements are "database, fast, low memory profile", then you can't use Python. Importantly, if you think those ever will be your requirements, you can't use Python.
I say this a lot but, engineering is about tradeoffs. There are still plenty of valid reasons to use C/C++. I'm tired of the knee-jerk "BOOOOO C" on HN these days, and while I certainly think we need to dispel the myth that you can write a meaningfully large, memory-safe program in C, I don't think we need to go as far as "you should never use C ever again". In fact, I think we need to be honest about the current state of the art in order to fully replace C -- which I wholeheartedly support.
Well, if you're thinking in term of available resources, not really : most (if not every) chip on payment cards or SIM cards run Java[1] despite being incredibly limited in term of resources.
There are other (niche) example of Java running directly on bare metal, see Jazelle[2] for instance.
> or many platforms Rust just doesn't run on
Right now, absolutely but there is no technical limitation whatsoever that prevents Rust from running on these platforms. It might come, in the next decade or so if Rust gets enough traction, only time can tell.
I totally agree with the rest of your comment though.
And yeah most of the platforms Rust doesn't support are either legacy or very niche. It's an interesting topic though; platform developers and manufacturers seem to have no problem shipping tweaked C compilers (usually some awful old GCC fork), but I've yet to see them use LLVM. I think Rust is hamstrung a little by having only the one compiler, but it's an entirely unfair expectation of such a young and ambitious project. Plus, it's hard to outdo LLVM.
We'll see how it goes. Maybe we'll see a lot less platform proliferation as mindshare moves away from C, but it's also possible that LLVM will just grow its platform support.
I also wonder if we'll see industry become a little more relaxed in its requirements. Like requiring multiple implementations, language standardization, or security/development standardization and verification. Most of this stuff grew out of C's instability, but with a more stable language maybe it doesn't matter? Or there are parallels in the web world too, like multiple browser vendors have to be on board with a feature for it to eventually become a standard, whereas with Rust it's pretty much whatever the Rust community decides and LLVM supports. Do we still care about standards and multiple implementations? Are the roadblocks worth it? I feel like on one hand I think they are, but also that if we accept them then we're kind of implicitly accepting C forever.
You don't need any special instruction support to do bound checked memory access. Write in Rust or Swift or whatever, and you're already making buffer overflows "impossible". The buffer overflows are already out there, in billions of lines of C and C++ code, and since we can't rewrite all the code, we should mitigate it as best we can.
It'd require somewhat more ISA & compiler changes but it'd solve more problems that just the one problem they solve, and I think the security of this would be easier to demonstrate, too.
If you store the bounds separately (full runtime bounds checking) you lose efficiency on code which inherently can not overflow the bounds, and code where you have a large number of small objects (let's say you have 400GiB of 64-byte objects) with a known size. If you switch to a new language, great! But you obviously lose access to your existing code, which is a non-starter.
Pointer auth and control flow integrity techniques cover most (all?) memory corruption flaws, including memory lifecycle errors (which are probably the most common modern source of vulnerabilities). Built-in buffer bounds checks do not.
Don't burn the trees down, just because you don't want anyone else to have wood. Some of us will craft furniture, not smog.
All normal pointers and dereferencing continue to work the same. It only affects code that explicitly decides to use these special instructions to create/validate signed pointers. Only certain code (like the code that saves a return address to the stack) would choose to do this, for cases where an attacker is especially likely to try corrupting the pointer.
Hmm wouldn't a trap be better?
Or, the designers didn't want to add an interlock between the memory subsystems that handle traps, and the crypto subsystem that does the decryption.
By my reading, this allows not a whitelist of pages, but a whitelist of arbitrary addresses. Different granularities entirely. Can anyone else bring a light to bear on this?
The authentication code is a combination of key and context: there are 5 total keys in the system, and then an unlimited number of contexts. Contexts are I think most useable on the "return" edges, because you can add the current stack pointer value to the context when you push the boxed return address onto the stack, then re-derive that context when you're at the return site.
That exact scheme doesn't work that well on the forward edge, because the stack pointers will be different when calling function pointer F in function A vs function B. What you can probably do is encode something about the type of F into the context. However, as they outline in the white paper, this isn't enough on its own because the type signature of gets and system are really similar and if your type-to-context encoding scheme maps them to the same context, an attacker could take a call to gets-via-function-pointer and replace that value with the value of system as it appears elsewhere in your program.
Authenticated pointers can assume this threat model. The attacker can read and write arbitrary memory, but it doesn't do anything for their ability to hijack the control flow of the application because all values stored in memory that relate to control flow are signed and encrypted. The attacker can't create a new code-pointer value and write it in to memory without knowledge of the secret keys, which are not in memory. The attacker could cause the program to crash or exit early, but oh well.
You'll need entire new processor micro-architectures to use more bits from your 64-bit pointer. It's your hardware address bus that only supports 40- or 48-bits of address space, not your application or operating system. That's already 256 TB as well. I don't think you'll hit that limit any time soon.
I, for one, would love it if we had hardware support for "fat" pointers with a portion dedicated purely for addressing (possibly with the ability to use the n low bits as tag bits in n-bit aligned access) and another portion dedicated for auxiliary information. Furthermore, there'd need to be some agreement on how these bits are allocated between different actors (user, compiler/jit, OS?).
Right now, I would very much like to utilize some of the 64 bits we have. But I'm afraid that 10 years later, my software would be "old and cranky and does crazy shit with pointers so it's not compatible with modern systems and you'd have to rewrite parts of it to get it to run.. good luck".
I guess everyone who thought that "signed integers" are cryptographically signed weren't THAT wrong after all :D
It's not the misses you worry about, it's the hits