commit f5bb97baaa23a2813b4f3379f5e3fa8ce3defd79
author chaviw <chaviw@google.com> Wed Apr 28 15:35:37 2021 -0500
committer chaviw <chaviw@google.com> Thu May 13 11:30:19 2021 -0500
tree 1d6829859c3f862d2f38bb775b1456c38e49ff2a
parent 6e2e0effb3d4727b321942a0e7d015c4a1a27d71

Add CAPTURE_BLACKOUT_CONTENT permission check for screenshots

Previously, only graphics and system were allowed to take screenshots of
layers that are normally blacked out. This change adds an additional check so
processes that have the permission CAPTURE_BLACKOUT_CONTENT can also take
screenshots of that content. Only system applications can have that
permission

Test: Builds, screenshots still work as before
Bug: 173746627
Change-Id: I53a1738be8e4787507a76f8b0ac703f3e413b30c

services/surfaceflinger/SurfaceFlinger.cpp

diff --git a/services/surfaceflinger/SurfaceFlinger.cpp b/services/surfaceflinger/SurfaceFlinger.cpp
index 3edbe1d..a1b151b 100644
--- a/services/surfaceflinger/SurfaceFlinger.cpp
+++ b/services/surfaceflinger/SurfaceFlinger.cpp
@@ -287,6 +287,8 @@
 const String16 sReadFramebuffer("android.permission.READ_FRAME_BUFFER");
 const String16 sControlDisplayBrightness("android.permission.CONTROL_DISPLAY_BRIGHTNESS");
 const String16 sDump("android.permission.DUMP");
+const String16 sCaptureBlackoutContent("android.permission.CAPTURE_BLACKOUT_CONTENT");
+
 const char* KERNEL_IDLE_TIMER_PROP = "graphics.display.kernel_idle_timer.enabled";
 
 // ---------------------------------------------------------------------------
@@ -5681,6 +5683,14 @@
     }
 }
 
+static bool hasCaptureBlackoutContentPermission() {
+    IPCThreadState* ipc = IPCThreadState::self();
+    const int pid = ipc->getCallingPid();
+    const int uid = ipc->getCallingUid();
+    return uid == AID_GRAPHICS || uid == AID_SYSTEM ||
+            PermissionCache::checkPermission(sCaptureBlackoutContent, pid, uid);
+}
+
 static status_t validateScreenshotPermissions(const CaptureArgs& captureArgs) {
     IPCThreadState* ipc = IPCThreadState::self();
     const int pid = ipc->getCallingPid();
@@ -5851,6 +5861,10 @@
     Rect layerStackSpaceRect;
     ui::Dataspace dataspace;
     bool captureSecureLayers;
+
+    // Call this before holding mStateLock to avoid any deadlocking.
+    bool canCaptureBlackoutContent = hasCaptureBlackoutContentPermission();
+
     {
         Mutex::Autolock lock(mStateLock);
 
@@ -5860,9 +5874,8 @@
             return NAME_NOT_FOUND;
         }
 
-        const int uid = IPCThreadState::self()->getCallingUid();
-        const bool forSystem = uid == AID_GRAPHICS || uid == AID_SYSTEM;
-        if (!forSystem && parent->getCurrentState().flags & layer_state_t::eLayerSecure) {
+        if (!canCaptureBlackoutContent &&
+            parent->getCurrentState().flags & layer_state_t::eLayerSecure) {
             ALOGW("Attempting to capture secure layer: PERMISSION_DENIED");
             return PERMISSION_DENIED;
         }
@@ -6012,8 +6025,7 @@
         return BAD_VALUE;
     }
 
-    const int uid = IPCThreadState::self()->getCallingUid();
-    const bool forSystem = uid == AID_GRAPHICS || uid == AID_SYSTEM;
+    bool canCaptureBlackoutContent = hasCaptureBlackoutContentPermission();
 
     static_cast<void>(schedule([=, renderAreaFuture = std::move(renderAreaFuture)]() mutable {
         if (mRefreshPending) {
@@ -6033,8 +6045,9 @@
 
         status_t result = NO_ERROR;
         renderArea->render([&] {
-            result = renderScreenImplLocked(*renderArea, traverseLayers, buffer, forSystem,
-                                            regionSampling, grayscale, captureResults);
+            result = renderScreenImplLocked(*renderArea, traverseLayers, buffer,
+                                            canCaptureBlackoutContent, regionSampling, grayscale,
+                                            captureResults);
         });
 
         captureResults.result = result;
@@ -6046,8 +6059,9 @@
 
 status_t SurfaceFlinger::renderScreenImplLocked(
         const RenderArea& renderArea, TraverseLayersFunction traverseLayers,
-        const std::shared_ptr<renderengine::ExternalTexture>& buffer, bool forSystem,
-        bool regionSampling, bool grayscale, ScreenCaptureResults& captureResults) {
+        const std::shared_ptr<renderengine::ExternalTexture>& buffer,
+        bool canCaptureBlackoutContent, bool regionSampling, bool grayscale,
+        ScreenCaptureResults& captureResults) {
     ATRACE_CALL();
 
     traverseLayers([&](Layer* layer) {
@@ -6060,7 +6074,7 @@
     // We allow the system server to take screenshots of secure layers for
     // use in situations like the Screen-rotation animation and place
     // the impetus on WindowManager to not persist them.
-    if (captureResults.capturedSecureLayers && !forSystem) {
+    if (captureResults.capturedSecureLayers && !canCaptureBlackoutContent) {
         ALOGW("FB is protected: PERMISSION_DENIED");
         return PERMISSION_DENIED;
     }