create a sample to demonstrate a timing attack

This sample attempts to re-create an image's alpha channel by drawing it
one pixel at a time and timing how long each pixel takes to draw.

The "abc" text should appear twice normally, and the third and fourth
versions are reconstructed from timing, one by timing 1:1 pixel draws,
the other by timing 1x1:1024x1024 upscale into an offscreen.  It's not
meant to be an exact reconstruction, but you can easily see the shapes,
particularly at -O0, -O1, and -Os.  Auto-vectorization from -O2/-O3 do
a good amount to cover up the problem.

The legacy CPU backend is the main place to look.  I haven't been able
to reconstruct any images using SkRasterPipelineBlitter or SkVMBlitter,
and while on the GPU I do see non-random patterns in the timing, it
appears to be the same single pattern across devices, OSes, GPUs, GPU
APIs and content... I assume it's something like our resource caching
policy.

This can't really be a GM, given how it draws non-deterministically.

Bug: chromium:1088224
Change-Id: I2ec79c8dd407ecb104fd9bf0c8039cb6dd1fe436
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/313466
Commit-Queue: Mike Klein <mtklein@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Reed <reed@google.com>

gn/samples.gni

@@ -89,6 +89,7 @@ samples_sources = [
   "$_samplecode/SampleTextEffects.cpp",
   "$_samplecode/SampleTextureUpload.cpp",
   "$_samplecode/SampleThinAA.cpp",
+  "$_samplecode/SampleTiming.cpp",
   "$_samplecode/SampleUnpremul.cpp",
   "$_samplecode/SampleVertices.cpp",
   "$_samplecode/SampleWritePixels.cpp",

samplecode/SampleTiming.cpp

@@ -0,0 +1,92 @@
+/*
+ * Copyright 2020 Google Inc.
+ *
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
+#include "include/core/SkCanvas.h"
+#include "include/core/SkFont.h"
+#include "include/core/SkSurface.h"
+#include "samplecode/Sample.h"
+#include <chrono>
+
+struct TimingSample : public Sample {
+    static constexpr int W = 24,
+                         H = 16;
+    sk_sp<SkImage> fImg;
+
+    SkString name() override { return SkString("Timing"); }
+
+    void onOnceBeforeDraw() override {
+        sk_sp<SkSurface> surf = SkSurface::MakeRasterN32Premul(W,H);
+        surf->getCanvas()->drawString("abc", 2,H-4, SkFont{}, SkPaint{});
+        fImg = surf->makeImageSnapshot();
+    }
+
+    void onDrawContent(SkCanvas* canvas) override {
+        canvas->scale(8,8);
+
+        // Draw normally.
+        canvas->drawImage(fImg, 0,0);
+
+        canvas->translate(0,H);
+
+        // Draw one pixel at a time with drawImageRect(),
+        // timing how long each drawImageRect() call takes.
+        double cost[H][W];
+        double min = +INFINITY,
+               max = -INFINITY;
+        for (int y = 0; y < H; y++)
+        for (int x = 0; x < W; x++) {
+            auto start = std::chrono::steady_clock::now();
+            canvas->drawImageRect(fImg, SkRect::MakeXYWH(x,y,1,1)
+                                      , SkRect::MakeXYWH(x,y,1,1)
+                                      , /*paint=*/nullptr);
+            auto elapsed = std::chrono::steady_clock::now() - start;
+
+            cost[y][x] = elapsed.count();
+            min = std::min(min, cost[y][x]);
+            max = std::max(max, cost[y][x]);
+        }
+
+        canvas->translate(0,H);
+
+        // Draw using those per-pixel timings,
+        // with the slowest pixel scaled to alpha=1, the fastest to alpha=0.
+        for (int y = 0; y < H; y++)
+        for (int x = 0; x < W; x++) {
+            SkPaint p;
+            p.setAlphaf( (cost[y][x] - min) / (max - min) );
+            canvas->drawRect(SkRect::MakeXYWH(x,y,1,1), p);
+        }
+
+        canvas->translate(0,H);
+
+        // Draw each pixel into offscreen, timing each draw.
+        SkImageInfo info = canvas->imageInfo().makeWH(1024,1024);
+        if (sk_sp<SkSurface> offscreen = canvas->makeSurface(info)) {
+            min = +INFINITY;
+            max = -INFINITY;
+            for (int y = 0; y < H; y++)
+            for (int x = 0; x < W; x++) {
+                auto start = std::chrono::steady_clock::now();
+                offscreen->getCanvas()->drawImageRect(fImg, SkRect::MakeXYWH(x,y,1,1)
+                                                          , SkRect::MakeXYWH(0,0,1024,1024)
+                                                          , /*paint=*/nullptr);
+                auto elapsed = std::chrono::steady_clock::now() - start;
+
+                cost[y][x] = elapsed.count();
+                min = std::min(min, cost[y][x]);
+                max = std::max(max, cost[y][x]);
+            }
+            for (int y = 0; y < H; y++)
+            for (int x = 0; x < W; x++) {
+                SkPaint p;
+                p.setAlphaf( (cost[y][x] - min) / (max - min) );
+                canvas->drawRect(SkRect::MakeXYWH(x,y,1,1), p);
+            }
+        }
+    }
+};
+DEF_SAMPLE( return new TimingSample; )