How to Change Access Permissions of a Running Task (c) - c++

Question

I want to change the permissions of a memory segment at run time (the memory section contains a function that I want to change its code at run time).
My first thought was to change the permissions to write - read, make the changes, return the permissions to execute only.
The thing is that I need to get the base address of my page, not the address of my function and only then I can tweak the permissions.
Here is the code, where I tried to execute it:
void encrypt_function()
{
PBYTE start_addr = (PBYTE)&function_to_hide1; // get location of function (can get some time jmp stub instead of actual function - need to find better approach).
DWORD size = (UINT_PTR)&empty_staub - (UINT_PTR)&function_to_hide1;
// try to get the base address of my function page - failed
_MEMORY_BASIC_INFORMATION lpBuffer;
VirtualQuery(start_addr, &lpBuffer, size);
DWORD old_flags = 0;
if (!VirtualProtect(start_addr, size, PAGE_EXECUTE_READWRITE, &old_flags)) // ERROR_INVALID_ADDRESS - it's not the base address
{
printf("went rung %d", GetLastError());
}// make function page read write execute permission
// can't be done - i dont have permission
for (DWORD i = 0; i < size; i++)
{
start_addr[i] ^= 0x55;
}
VirtualProtect(start_addr, size, old_flags, NULL);
}
For some reason, I cannot get the base address (the linker will not compile this code for some reason).
Does anyone know a better way to do this (modify permissions and perform function modification)?

Answer 1

To get the base address of the page in which your function resides you can use VirtualQuery(), then you pass mbi.BaseAddress to the VirtualProtect() call.
MEMORY_BASIC_INFORMATION mbi;
VirtualQuery(start_addr, &mbi, sizeof(mbi));
DWORD oldprotect;
VirtualProtect(mbi.BaseAddress, mbi.RegionSize, PAGE_EXECUTE_READWRITE, &oldprotect);
As was discussed in the comments you should use VirtualAlloc() to get a new memory space and decrypt your function there,and do a simple detour to jump to it rather than doing the decryption in place.