VirtualAllocEx hang in Windows 10

main.cpp

#include <Windows.h>
#include <stdio.h>
#include <type_traits>
#include <algorithm>


namespace pointer
{
	template <typename T>
	T AlignTop(const void* anyPointer, size_t alignment)
	{
		union
		{
			const void* as_void;
			uintptr_t as_uintptr_t;
			T as_T;
		};

		as_void = anyPointer;
		const size_t mask = alignment - 1u;
		as_uintptr_t += mask;
		as_uintptr_t &= ~mask;

		return as_T;
	}


	template <typename T, typename U>
	T Offset(void* anyPointer, U howManyBytes)
	{
		union
		{
			void* as_void;
			char* as_char;
			T as_T;
		};

		as_void = anyPointer;
		as_char += howManyBytes;

		return as_T;
	}
}


int main()
{
	// HOW TO REPRO:
	// 1) start notepad.exe
	// 2) get notepad.exe process ID from Task Manager
	// 3) replace PROCESSID with real ID
	// 4) compile and run

	// open notepad process
	HANDLE proc = ::OpenProcess(PROCESS_ALL_ACCESS, FALSE, /* PROCESSID */ 21508);

	const size_t alignment = 64u * 1024u;

	// chosen to make the loop "overflow", starting from 0x0 again.
	// this will reserve every possible memory region in the target process.
	// once everything has been reserved, the call to ::VirtualAllocEx will hang and never return.
	// this creates instability across the whole Windows system, making it impossible to kill this process, or sometimes even start new processes.
	// Rebooting no longer works.
	// Debugging this process doesn't work.
	// A full power cycle is required!
	const void* addressStart = (const void*)0x00007FFF7FF00000;
	const void* addressEnd = (const void*)0x000080007FF00000;
	for (const void* address = addressStart; address < addressEnd; /* nothing */)
	{
		// align address to be scanned
		address = pointer::AlignTop<const void*>(address, alignment);

		::MEMORY_BASIC_INFORMATION memoryInfo = {};
		const size_t bytesReturned = ::VirtualQueryEx(proc, address, &memoryInfo, sizeof(::MEMORY_BASIC_INFORMATION));

		// we are only interested in free pages
		if ((bytesReturned > 0u) && (memoryInfo.State == MEM_FREE))
		{
			const size_t bytesLeft = abs((intptr_t*)addressEnd - (intptr_t*)memoryInfo.BaseAddress);
			const size_t size = std::min<size_t>(memoryInfo.RegionSize, bytesLeft);

			printf("baseAddress: 0x%p, size: 0x%llX\n", memoryInfo.BaseAddress, size);
			void* baseAddress = ::VirtualAllocEx(proc, memoryInfo.BaseAddress, size, MEM_RESERVE, PAGE_NOACCESS);
			if (baseAddress)
			{
				printf("Reserving virtual memory region at 0x%p with size 0x%llX\n", baseAddress, size);
			}
		}

		// keep on searching
		address = pointer::Offset<const void*>(memoryInfo.BaseAddress, memoryInfo.RegionSize);
	}

	::CloseHandle(proc);

	return 0;
}

There is nothing to fix.
This was a bug in some versions of Windows 10 and 11 which could be exploited.
It's a good thing that the system no longer hangs, but that the process quits instead.