Can you "allocate" stack space with VirtualAlloc?

Question

I'm messing around with VirtualAlloc and dynamic code generation, and I've become curious about something.

The first parameter of VirtualAlloc specifies the start of the address range to be allocated, or more accurately, the page containing that address specifies the start of the page range to be allocated. Right?

I started wondering. Could you just make a bunch of space on the stack and "allocate" that memory with VirtualAlloc? For instance, to change its permissions to PAGE_EXECUTE_READWRITE?

(As an extension of the above, I'm curious where exactly the stack is in a Windows process. How is it set up? What sets it up?)

tl;dr Can you "allocate" stack space with VirtualAlloc?

Answer 1

Stack space is allocated by VirtualAlloc and the MEM_RESERVE flag (or perhaps directly using the underlying syscall) when a thread is created. This causes a chuck of the process's address space to be reserved for that thread stack.

A guard page is used to cause an access-violation when the stack grows past the region which is actually committed. The OS handles this automatically, by committing additional memory (if there is enough reserved space) or generating EXCEPTION_STACK_OVERFLOW to the process if the edge of the reserved area is reached. In the first case, a new guard page is set up. In the second, recreating the guard page is an important step if you try to handle that exception and recover.

You could use VirtualAlloc and VirtualProtect to precommit your thread's stack. But they don't touch the stack pointer, so they can't be used for stack allocation (code using the stack pointer would happily reuse "your" allocation for automatic variables, function parameters, etc). To allocate space from the stack, you need to adjust the stack pointer. Most C and C++ compilers provide an _alloca() intrinsic for doing this.

If you're doing dynamic code generation, don't use the stack for that. Non-executable stack is a valuable protection against remote execution vulnerabilities. You certainly can use VirtualAlloc for dynamic allocation in specialized cases like this, instead of the general-purpose allocators HeapAlloc and malloc and new[]. The general-purpose allocators all ultimately get their memory from VirtualAlloc, but then parcel it out in chunks that don't line up with page boundaries.