Fix 32-bit host context APIs and ExternalReader bitness detection

- Add GetThreadContext/SetThreadContext overloads accepting Context32 so a
  32-bit process on a native 32-bit OS can capture x86 thread context.
- DllInjector.InjectWithThreadHijack now selects the context API based on
  both process bitness and OS bitness:
  * 64-bit process -> native 64-bit context
  * 32-bit process on 64-bit OS -> WOW64 context
  * 32-bit process on 32-bit OS -> native x86 context
- ExternalReader now validates that the caller supplied
  ProcessAccess.QueryInformation, and surfaces any IsWow64Process failure
  instead of silently falling back to host bitness.

Tests: 207 passing, 4 skipped.

WhiteMagic/ExternalReader.cs

@@ -37,6 +37,13 @@ public sealed class ExternalReader : MemoryBase
     public ExternalReader(System.Diagnostics.Process process, ProcessAccess desiredAccess = DefaultAccess)
     {
         _processId = process.Id;
+        if ((desiredAccess & ProcessAccess.QueryInformation) == 0)
+        {
+            throw new ArgumentException(
+                "ExternalReader requires ProcessAccess.QueryInformation to determine target bitness.",
+                nameof(desiredAccess));
+        }
+
         _handle = NativeMethods.OpenProcess(desiredAccess, false, _processId);
         if (_handle.IsInvalid)
         {
@@ -46,11 +53,12 @@ public sealed class ExternalReader : MemoryBase
         }
 
         // Derive target bitness. A 64-bit host sees a 32-bit target as WOW64.
-        // A 32-bit host can only open 32-bit targets. If the API fails, fall
-        // back to the current process bitness (self-open path).
+        // A 32-bit host can only open 32-bit targets.
         if (!NativeMethods.IsWow64Process(_handle, out bool wow64))
         {
-            wow64 = false;
+            int error = Marshal.GetLastPInvokeError();
+            throw new InvalidOperationException(
+                $"IsWow64Process failed for PID {_processId}: error {error}.");
         }
 
         _is64Bit = Environment.Is64BitProcess && !wow64;

WhiteMagic/Injection/DllInjector.cs

@@ -215,7 +215,7 @@ public sealed class DllInjector
             {
                 IntPtr result;
 
-                if (_currentIs64Bit)
+                if (Environment.Is64BitProcess)
                 {
                     var originalContext = new Context64 { ContextFlags = ContextFlags.Amd64Full };
                     if (!NativeMethods.GetThreadContext(thread, ref originalContext))
@@ -264,21 +264,32 @@ public sealed class DllInjector
                 }
                 else
                 {
+                    // 32-bit process on either a 32-bit or 64-bit (WOW64) host.
+                    bool useWow64 = Environment.Is64BitOperatingSystem;
+
                     var originalContext = new Context32 { ContextFlags = ContextFlags.X86Full };
-                    if (!NativeMethods.Wow64GetThreadContext(thread, ref originalContext))
+                    bool gotContext = useWow64
+                        ? NativeMethods.Wow64GetThreadContext(thread, ref originalContext)
+                        : NativeMethods.GetThreadContext(thread, ref originalContext);
+                    if (!gotContext)
                     {
                         int error = Marshal.GetLastPInvokeError();
-                        throw new InvalidOperationException($"Wow64GetThreadContext failed (error {error}).");
+                        string api = useWow64 ? "Wow64GetThreadContext" : "GetThreadContext";
+                        throw new InvalidOperationException($"{api} failed (error {error}).");
                     }
 
                     var redirectContext = originalContext;
                     redirectContext.Eip = (uint)(nint)remoteBase;
                     redirectContext.Esp = (uint)(nint)stackTop;
 
-                    if (!NativeMethods.Wow64SetThreadContext(thread, ref redirectContext))
+                    bool setContext = useWow64
+                        ? NativeMethods.Wow64SetThreadContext(thread, ref redirectContext)
+                        : NativeMethods.SetThreadContext(thread, ref redirectContext);
+                    if (!setContext)
                     {
                         int error = Marshal.GetLastPInvokeError();
-                        throw new InvalidOperationException($"Wow64SetThreadContext failed (error {error}).");
+                        string api = useWow64 ? "Wow64SetThreadContext" : "SetThreadContext";
+                        throw new InvalidOperationException($"{api} failed (error {error}).");
                     }
 
                     if (NativeMethods.ResumeThread(thread) == 0xFFFFFFFF)
@@ -295,10 +306,14 @@ public sealed class DllInjector
                         throw new InvalidOperationException($"SuspendThread failed while capturing result (error {error}).");
                     }
 
-                    if (!NativeMethods.Wow64SetThreadContext(thread, ref originalContext))
+                    bool restoredContext = useWow64
+                        ? NativeMethods.Wow64SetThreadContext(thread, ref originalContext)
+                        : NativeMethods.SetThreadContext(thread, ref originalContext);
+                    if (!restoredContext)
                     {
                         int error = Marshal.GetLastPInvokeError();
-                        throw new InvalidOperationException($"Wow64SetThreadContext restore failed (error {error}).");
+                        string api = useWow64 ? "Wow64SetThreadContext" : "SetThreadContext";
+                        throw new InvalidOperationException($"{api} restore failed (error {error}).");
                     }
 
                     if (NativeMethods.ResumeThread(thread) == 0xFFFFFFFF)

WhiteMagic/Native/NativeMethods.cs

@@ -138,6 +138,20 @@ internal static partial class NativeMethods
         SafeMemoryHandle thread,
         ref Context64 context);
 
+    /// <summary>Sets a 32-bit thread context (x86 or WOW64).</summary>
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool SetThreadContext(
+        SafeMemoryHandle thread,
+        ref Context32 context);
+
+    /// <summary>Gets a 32-bit thread context (x86 or WOW64).</summary>
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool GetThreadContext(
+        SafeMemoryHandle thread,
+        ref Context32 context);
+
     /// <summary>Sets a 32-bit (WOW64) thread context.</summary>
     [LibraryImport("kernel32.dll", SetLastError = true)]
     [return: MarshalAs(UnmanagedType.Bool)]

WhiteMagicTest/MemoryHardeningTests.cs

@@ -151,6 +151,15 @@ public class MemoryHardeningTests
         Assert.False(reader.Handle.IsInvalid);
     }
 
+    [Fact]
+    public void ExternalReader_throws_when_query_information_access_missing()
+    {
+        var ex = Assert.Throws<ArgumentException>(() =>
+            new ExternalReader(Process.GetCurrentProcess(), ProcessAccess.VmRead));
+
+        Assert.Equal("desiredAccess", ex.ParamName);
+    }
+
     /// <summary>
     /// A <see cref="MemoryBase"/> that serves bytes from an in-memory buffer and
     /// caps every read to <c>maxChunk</c> bytes, to exercise partial-read handling.