Question

CreateProcess("something.exe", NULL, NULL, NULL, FALSE, CREATE_SUSPENDED, 
    NULL, NULL, &sInfo, &pInfo);

ReadProcessMemory(pInfo.hProcess, (LPCVOID) (contx.Ebx + 8),
  (LPVOID) &baseAddress, sizeof(baseAddress), NULL);

hProcess=OpenProcess(PROCESS_VM_READ|PROCESS_VM_WRITE|PROCESS_VM_OPERATION,
  FALSE, pInfo.dwProcessId);

VirtualProtectEx(hProcess, (LPVOID) baseAddress, sizeof(IMAGE_DOS_HEADER),
    PAGE_EXECUTE_READWRITE, NULL);

WriteProcessMemory(hProcess, (LPVOID) baseAddress, (LPCVOID) pidh,
    sizeof(IMAGE_DOS_HEADER), NULL);

Why VirtualProctecEx gives me ERROR_NOACCESS ?

Answer 1

The VirtualProtectEx docs say

lpflOldProtect [out] A pointer to a variable that receives the
previous access protection of the first page in the specified region
of pages. If this parameter is NULL or does not point to a valid
variable, the function fails.

about the last argument. You’re passing NULL so should expect it to fail.

Its possible there are also other problems. e.g. you don’t check the return value from ReadProcessMemory so baseAddress may be invalid.