Does there actually exist any practical way to ensure user input does not cause mischief when authoring C/C++ programs at scale? Are memory-safe languages the only answer?
Much worse than that, even memory-safe languages like (safe) Rust, and the inevitable suggestion of AUTOSAR and so on aren't the answer. To properly answer your demand for a "practical way to ensure user input does not cause mischief" you want a drastically less capable language which cannot even in principle express the programs that should not exist, that's exactly what WUFFS is for.
This sort of bug can't happen in WUFFS because you can't express the idea "corrupt the heap memory" even if you desperately wanted to. The tell-tale sign of such languages is that they are not general purpose languages, because those are able to express a wide variety of stupid things you don't want to do.
Sure, for example a Rust program is allowed to open, read and write files. On Linux one of the files it's allowed to open /proc/self/mem points directly into its own heap, another is a list of what's in that heap and where - it can use these to scrawl on the heap and cause havoc.
That's the price of being a General Purpose programming language. We don't know if you might want to scribble on your own heap, or delete all the files labelled "Important, DO NOT DELETE" or mail a copy of the password database to a throwaway account, and so you can do all those things. Those Linux files pointing into process address space aren't a mistake, I wrote code that needs them (and then I ported it to safe Rust months ago) but with great power comes great box office potential or something like that.
Now you might say, "I'm sure I won't get something so obvious wrong", but the trouble is that's what the people who wrote this GitHub code apparently thought too. Hence I say we should use specialised languages with a deliberately narrower scope where this category of mistake is impossible.
WUFFS as it stands would be pretty exhausting to write a Markdown parser in because WUFFS doesn't believe in strings, at all. But it's already a better fit for this problem than C++ because the worst case scenario can't happen.
Oh interesting, thanks a lot for elaborating. Question: why would a memory-safe Rust program open a sensitive file like /proc/self/mem? Wouldn't that require corrupting memory somehow, which presumably the Rust compiler would be proving impossible? Or are you suggesting the file name might come from user input, and the user input would somehow find an exploit to redirect it to that file? Would that be possible/likely in any way for something like parsing?
That's because opening and writing to arbitrary files is marked a safe operation in Rust, and sometimes Rust programs open files whose filename were supplied by untrusted, potentially malicious input. And as said in the other comment, this can lead to UB: https://github.com/rust-lang/rust/issues/32670 (it's Linux here that is at fault, really, for offering a hugely unsafe API through the filesystem), so, we could imagine a paranoid (but technically correct) version of Rust where the APIs for opening and/or writing to a file were marked as unsafe.
But, since the operations in actual Rust are marked as safe, the compiler doesn't provide any checks here: we can cause UB in code without any unsafe { }. Moreover, checking if the path starts with /proc isn't enough to make the UB go away: procfs can be mounted on any dir, there can be bind mounts further obscuring the file resolution, etc.
This means that if you really care about memory safety (and correctness in general), the precise way you setup your environment is also critical, down to the minimum details. It's like your Dockerfile had a metaphorical unsafe { } block around it: in a system that doesn't mount /proc you just closed a whole host of bugs, and a buggy system that mounts procfs in other dirs may cause arbitrary havok. (note that mounting procfs is a privileged operation)
There are low level languages that, unlike Rust, completely prevents memory safety errors, like ATS. In ATS you can deal with pointers and pointer arithmetic (like in C or Rust) but to follow a pointer you need to provide a mathematical proof that they are valid. This is enough if we consider the program in isolation, but programs are never run in isolation. A proper mathematical proof of memory safety needs to consider ALL software running in the system, globally: then everything is mathematically verified, and the build step can just reject an unsound system setup.
That way we could theoretically be more precise about our memory safety guarantees: opening and writing to a file is safe, but only if procfs isn't mounted. If procfs is mounted anywhere, then this may go wrong: we need to prove we aren't doing something bad. This means that in a system where sysadmins can just log in and mount random stuff, writing to files must be unsafe!
Of course that's not very practical. It would be cumbersome to prove you're not doing /proc shenanigans every time you messed with files. And arguably, any program that open arbitrary filenames that came from untrusted input is buggy anyway. You should always do filename validations, specially to confine some input to some directory (when applicable), avoiding paths with ../ that escape it, for example. And, any setup that mounts procfs outside of /proc is irreparably broken. We don't have a tool to automatically check for such issues, but if those two things are followed, we won't have UB here.
How to do better than that? We need better system-level APIs, in which operations that are "obviously" safe can really be 100% memory safe all the time.
I get all this from a mathematical standpoint, but from a programming standpoint is this really that hard to solve in something like Rust? Like for this class of problems, can't you just (say) route every I/O syscall (like open()/openat()/etc.) through a custom layer that properly validates the path? Like making sure none of the directories cross into procfs or all that? At that point you just need to prove the correctness of the path validation (which should be pretty doable by hand) and let the Rust compiler take care of proving memory safety in the program itself... right?
Somebody call the Lockpicking Lawyer to shove a paperclip in these "security standards". They're flimsy attempts to excuse still doing something that's a bad idea (programming safety critical software in respectively C and C++) by promising to try harder to achieve the impossible standards needed by humans programming these languages.
And I do mean flimsy. Here's a fun example from a random copy of the AUTOSAR guidelines I found online labelled 17-03. AUTOSAR says if I have two 8-bit signed integers and I add them, that might overflow which is bad. So, what if I simply check that they're both less than 100, no more overflow? "Correct" says the AUTOSAR guide this is apparently OK.
Huh. Signed 8-bit integer. 99 + 99 = -58. This is probably not what the person who purchased your car thought the answer was, I hope whatever accident you just caused isn't fatal.
https://github.com/google/wuffs
This sort of bug can't happen in WUFFS because you can't express the idea "corrupt the heap memory" even if you desperately wanted to. The tell-tale sign of such languages is that they are not general purpose languages, because those are able to express a wide variety of stupid things you don't want to do.