C99 doesn't need function bodies, or 'VLAs are Turing complete'

Re:C99 doesn't need function bodies, or 'VLAs are Turing com... by Someone on Friday August 05, 2022 @03:22AM (#32352813)

An attacker could trigger a large VLA allocation that jumps over the guard page, and a write to that allocation. That write would start _below_ the guard page, so damage would be done before the page fault occurs (ideally, that write wouldn’t touch the guard page and there wouldn’t be a page fault but that typically is harder to do; the VLA memory allocation typically is done to be fully used)

Triggering use of the injected code may require another call timed precisely to hit the changed code before the page fault occurs.

Of course, the compiler could and should check for stack allocations that may jump over guard pages and abort the program (or, if in a syscall, the OS) or grow the stack when needed. Also, VLAs aren’t needed for this. If the programmer creates a multi-megabyte local array, this happens, too (and that can happen accidentally, for example when increasing a #define and recompiling)

The lesson is, though, that guard pages alone don’t fully protect against such attacks. The compiler must check total stack space allocated by a function, and, if it can’t determine that that’s under the size of your guard page, insert code to do additional runtime checks.

I don’t see that as a reason to outright ban VLAs, though.

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Re:C99 doesn't need function bodies, or 'VLAs are Turing com... by bjourne on Friday August 05, 2022 @03:41PM (#32360973)

VLAs give the attacker an extra attack vector. The size of the VLA is runtime-determined and potentially controlled by user input. Thus, the only safe way to handle VLAs is to check that there is enough stack space for every VLA allocation. Which may be prohibitively expensive and even impossible on some embedded platforms. Stack overflows may happen for other reasons too, but letting programmers put dynamic allocations on the stack is just asking for trouble.

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Re:C99 doesn't need function bodies, or 'VLAs are Turing com... by Someone on Saturday August 06, 2022 @01:49AM (#32365257)

I don’t think “may be prohibitively expensive and even impossible on some embedded platforms” is a strong argument for not including it in C. There are many other features in C for which that holds, such as recursion, dynamic memory allocation, or even floating point.

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