Rust's semantics are to abort on a stack overflow. A language like C or C++ have no such semantics, they may abort or they may continue running and producing jibberish.
I guarantee I could exploit this on a system that does not have virtual memory, or a runtime that does not have unmapped addresses at the end of the stack, to, say, manipulate the contents of another thread’s stack. Therefore, this behavior is undefined.
Now perhaps this means that there are real rust deployments that are "wrong", but that shouldn't include regular sane standard systems, and embedded users should know the tradeoffs.
https://godbolt.org/z/Y75KTT87M:
.LBB3_1:
sub rsp, 4096
mov qword ptr [rsp], 0
cmp rsp, r11
jne .LBB3_1
Stack overflows are checked in C on macOS not because of guard pages but because the compiler emits stack checks (with cookies). Probably the same is true here.
> I guarantee I could exploit this on a system that does not have virtual memory, or a runtime that does not have unmapped addresses at the end of the stack, to, say, manipulate the contents of another thread’s stack. Therefore, this behavior is undefined.
That's implementation-defined, not undefined.
Compiler-emitted stack checking is optional and not the default, and definitely not what is causing the crash here.
> That's implementation-defined, not undefined.
How could an implementation reasonably define the behavior for a stack overflow that silently corrupts another variable?
It is the default on macOS for clang.
> How could an implementation reasonably define the behavior for a stack overflow that silently corrupts another variable?
Mandate stack checking.
Mandating guard pages/MPU protection would rule out targeting embedded platforms which lack sufficient hardware support.
If you can't have hardware support, it's trivial for the compiler to do it in software - just an "if (stack_curr - stack_end < desired_size) abort();". I can't imagine a platform where there you cannot reasonably get a lower bound for the range of stack available. Worst-case, you ditch the architectural stack pointer and manage your own stack on the heap, if that's what you need to ensure correct Rust behavior on your funky platform (or accept the non-compliant compromise of no stack checking).
If your thread overflows the stack, it could start writing into memory for which it does not hold a reference. If the thread is preempted before the stack checker can run (see below*) and detect the overflow, and another thread runs which accesses the now-corrupted memory, then you're hosed.
> just an "if (stack_curr - stack_end < desired_size) abort();"
That's not how the compiler-emitted stack checking works AFAIK (*I believe it uses canaries on the stack which are checked at certain points in code). But, I could see this solving the problem. Basically, for every instruction that manipulates the stack pointer (function calls, alloca's, and on some arch's interrupts use the current stack), the resulting address would need to be checked. That would be costly and require OS awareness, but I think it would be safe. Is this an option that the compiler provides? It would save me a lot of time debugging.*
In my sibling comment showing the assembly that your Rust program generates, it is writing a "0" every 4096 bytes of the stack range that is intended to be later used as the buffer (this "0" is independent from the "0" in your "[0; N]"; it's just an arbitrary value to ensure that the page is writable). It does this, once, at the very start of the function, before everything else (i.e. before the variable "var" even exists, much less is accessible by anything or even initialized). This is effectively exactly the same as my "if (stack_curr - stack_end < desired_size) abort();", just implemented via guaranteed page faults. You can enable this on clang & gcc with -fstack-clash-protection where supported.
Indeed, stack checking can have overhead (so do other requirements Rust makes!), but in general it's not that large. If you don't have stack-allocated VLAs, it's a constant amount of machine code at the start of every function, checking that all possible stack usage the function may do is accessible. And on systems with guard pages (i.e. all of non-embedded) the overhead is trivially none for functions with frame size below 4096 bytes (or however big the guard range is; and for larger frame sizes the overhead of this check will be miniscule compared to whatever actually uses the massive amount of stack).
Yes it does, unless you're violating the memory model. Or are you thinking of Unix signals? Those do seem a bit harder to implement perfectly.
> Mandating guard pages/MPU protection would rule out targeting embedded platforms which lack sufficient hardware support.
Such systems are not secure if they don't have IOMMUs. But can always emulate everything in software and you must do so here.
Overflowing the stack violates the memory model.
> Such systems are not secure if they don't have IOMMUs.
Secure in what sense? I was under the impression that Rust could run on embedded devices like the ARM Cortex-M3, but maybe I'm wrong.
EDIT: Going to take it back. I'm unable to create a situation where I create a large stack array that doesn't result in an immediate stack overflow. I even tried nightly MaybeUninit::uninit_array but that crashed explicitly with a "fatal runtime error: stack overflow" so it seems like the standard library has improved reporting instead of the old SEGFAULT. So no UB.
An out of bounds access in Rust will result in a panic but a stack overflow is an abort.
The stack guards would normally be setup by the system runtime (e.g. kernel in the case of the main thread stack, libc for thread stacks), not Rust's runtime. Likewise, stack probes that ensure stack operations don't skip guard pages are usually (always?) emitted by the compiler backend (e.g. GCC, LLVM), not Rust's instrumentation, per se.
In this sense Rust isn't doing anything different than any other typical C or C++ binary, except that automagically hijacking SIGSEGV (or any other signal) from non-application code as Rust does is normally frowned upon, especially when it's merely for aesthetics--i.e. printing a pretty message in-process before dying. Also, attempting to introspect current thread metadata from a signal handler gives me pause. I'm not familiar enough with Rust to track down the underlying implementation code. I presume it's using some POSIX threads interfaces, but POSIX threads interfaces aren't async-signal safe, and though SIGSEGV would normally be sent synchronously (sometimes permitting greater assumptions about the state of the thread), that doesn't mean the Rust runtime isn't technically relying on undefined behavior.
EDIT: To get the guard page range it's using pthread_self, pthread_getattr_np, pthread_attr_getstack, and friends, of which only pthread_self is async-signal safe. See https://github.com/rust-lang/rust/blob/411f34b/library/std/s... I have no concrete evidence to believe the reliance isn't safe in practice on the targeted platforms (OTOH, I could imagine the opposite), but it's a little ironic that it's depending on undefined behavior.
“ //! Finally it's worth noting that at the time of this writing LLVM only has //! support for stack probes on x86 and x86_64. There's no support for stack //! probes on any other architecture like ARM or PowerPC64. LLVM I'm sure would //! be more than welcome to accept such a change! ”
https://github.com/rust-lang/compiler-builtins/blob/master/s...
AFAICT those methods are called from `guard::current`. In turn, `guard::current` is used to initialize TLS data when a thread is spawned before a signal is generated (& right after the signal handler is installed): https://github.com/rust-lang/rust/blob/26907374b9478d84d766a...
It doesn't look like there's any UB behavior being relied upon but I could very easily be misreading. If I missed it, please give me some more pointers cause this should be a github issue if it's the case - calling non async-safe methods from a signal handler typically can result in a deadlock which is no bueno.