Bug 1718414 - Stop canvas from initializing D3D in locked-down content process when GPU process fails r=sotaro,bobowen

Before Win32k Lockdown, Canvas would ensure that it would get the fastest possible implementation by initializing
devices in content process before allocating persistent buffers for its backing.

However, with Win32k Lockdown it's no longer possible, as initializing Direct3D and Direct2D make Win32k calls that
crash the locked-down content process.

This issue is generally solved by Remote Canvas; however, Remote Canvas is disabled if the GPU process is disabled.
If that happens, the current behavior is to attempt to initialize hardware acceleration again, causing a crash when
Win32k Lockdown is in effect.

This patch changes the behavior so that the devices will not initialize if they are in a locked-down content process,
even if Remote Canvas is disabled. The effect is that Canvas will fall back to using Skia for everything.

Differential Revision: https://phabricator.services.mozilla.com/D126761

gfx/layers/wr/WebRenderLayerManager.cpp

@@ -27,6 +27,7 @@
 
 #ifdef XP_WIN
 #  include "gfxDWriteFonts.h"
+#  include "mozilla/WindowsProcessMitigations.h"
 #endif
 
 namespace mozilla {
@@ -737,10 +738,18 @@ void WebRenderLayerManager::ScheduleComposite() {
 already_AddRefed<PersistentBufferProvider>
 WebRenderLayerManager::CreatePersistentBufferProvider(
     const gfx::IntSize& aSize, gfx::SurfaceFormat aFormat) {
-  // Ensure devices initialization for canvas 2d if not remote. The devices are
-  // lazily initialized with WebRender to reduce memory usage.
   if (!gfxPlatform::UseRemoteCanvas()) {
+#ifdef XP_WIN
+    // Any kind of hardware acceleration is incompatible with Win32k Lockdown
+    // We don't initialize devices here so that PersistentBufferProviderShared
+    // will fall back to using a piece of shared memory as a backing for the
+    // canvas
+    if (!IsWin32kLockedDown()) {
+      gfxPlatform::GetPlatform()->EnsureDevicesInitialized();
+    }
+#else
     gfxPlatform::GetPlatform()->EnsureDevicesInitialized();
+#endif
   }
 
   RefPtr<PersistentBufferProvider> provider =

gfx/thebes/gfxWindowsPlatform.cpp

@@ -1227,6 +1227,8 @@ void gfxWindowsPlatform::RecordStartupTelemetry() {
 // Supports lazy device initialization on Windows, so that WebRender can avoid
 // initializing GPU state and allocating swap chains for most non-GPU processes.
 void gfxWindowsPlatform::EnsureDevicesInitialized() {
+  MOZ_DIAGNOSTIC_ASSERT(!IsWin32kLockedDown());
+
   if (!mInitializedDevices) {
     mInitializedDevices = true;
     InitializeDevices();