libgui: Check slot received from IGBP in Surface

Checks that the slot number received from mGraphicBufferProducer in
Surface::dequeueBuffer is on the interval [0, NUM_BUFFER_SLOTS) to
protect against a malicious BnGraphicBufferProducer.

Bug: 36991414
Change-Id: I1a76fd1bcce1c558f1c0c30f03638278288ed4fa
(cherry picked from commit 90ce2a9)

Author: Dan Stoza

libs/gui/Surface.cpp

@@ -191,6 +191,13 @@ int Surface::dequeueBuffer(android_native_buffer_t** buffer, int* fenceFd) {
              result);
         return result;
     }
+
+    if (buf < 0 || buf >= NUM_BUFFER_SLOTS) {
+        ALOGE("dequeueBuffer: IGraphicBufferProducer returned invalid slot number %d", buf);
+        android_errorWriteLog(0x534e4554, "36991414"); // SafetyNet logging
+        return FAILED_TRANSACTION;
+    }
+
     sp<GraphicBuffer>& gbuf(mSlots[buf].buffer);
 
     // this should never happen