Graphite2 - TtfUtil::CmapSubtable4NextCodepoint Heap Overread

Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=755

The following crash due to a heap-based buffer overread can be observed in an ASAN build of the standard Graphite2 gr2FontTest utility (git trunk), triggered with the following command:

$ ./gr2fonttest /path/to/file -auto

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==19167==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x60e00000dff1 at pc 0x000000553c7d bp 0x7ffc6c2c7100 sp 0x7ffc6c2c70f8
READ of size 1 at 0x60e00000dff1 thread T0
    #0 0x553c7c in unsigned long be::_peek<1>(unsigned char const*) graphite/src/./inc/Endian.h:77:73
    #1 0x553be8 in unsigned long be::_peek<2>(unsigned char const*) graphite/src/./inc/Endian.h:50:43
    #2 0x56d7e3 in unsigned short be::peek<unsigned short>(void const*) graphite/src/./inc/Endian.h:55:18
    #3 0x5f2bad in graphite2::TtfUtil::CmapSubtable4NextCodepoint(void const*, unsigned int, int*) graphite/src/TtfUtil.cpp:1042:16
    #4 0x4fce35 in bool cache_subtable<&graphite2::TtfUtil::CmapSubtable4NextCodepoint, &graphite2::TtfUtil::CmapSubtable4Lookup>(unsigned short**, void const*, unsigned int) graphite/src/CmapCache.cpp:65:33
    #5 0x4fb097 in graphite2::CachedCmap::CachedCmap(graphite2::Face const&) graphite/src/CmapCache.cpp:107:14
    #6 0x54b6d2 in graphite2::Face::readGlyphs(unsigned int) graphite/src/Face.cpp:108:22
    #7 0x56f5d4 in (anonymous namespace)::load_face(graphite2::Face&, unsigned int) graphite/src/gr_face.cpp:54:14
    #8 0x56f0e4 in gr_make_face_with_ops graphite/src/gr_face.cpp:89:16
    #9 0x571420 in gr_make_file_face graphite/src/gr_face.cpp:242:23
    #10 0x4ed0b3 in Parameters::testFileFont() const (graphite/gr2fonttest/gr2fonttest+0x4ed0b3)
    #11 0x4f06c9 in main (graphite/gr2fonttest/gr2fonttest+0x4f06c9)

0x60e00000dff1 is located 0 bytes to the right of 145-byte region [0x60e00000df60,0x60e00000dff1)
allocated by thread T0 here:
    #0 0x4b85b8 in malloc llvm/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:40
    #1 0x55d42b in graphite2::FileFace::get_table_fn(void const*, unsigned int, unsigned long*) graphite/src/FileFace.cpp:94:11
    #2 0x54f0d1 in graphite2::Face::Table::Table(graphite2::Face const&, graphite2::TtfUtil::Tag, unsigned int) graphite/src/Face.cpp:281:36
    #3 0x4faad3 in graphite2::CachedCmap::CachedCmap(graphite2::Face const&) graphite/src/CmapCache.cpp:91:23
    #4 0x54b6d2 in graphite2::Face::readGlyphs(unsigned int) graphite/src/Face.cpp:108:22
    #5 0x56f5d4 in (anonymous namespace)::load_face(graphite2::Face&, unsigned int) graphite/src/gr_face.cpp:54:14
    #6 0x56f0e4 in gr_make_face_with_ops graphite/src/gr_face.cpp:89:16
    #7 0x571420 in gr_make_file_face graphite/src/gr_face.cpp:242:23
    #8 0x4ed0b3 in Parameters::testFileFont() const (graphite/gr2fonttest/gr2fonttest+0x4ed0b3)
    #9 0x4f06c9 in main (graphite/gr2fonttest/gr2fonttest+0x4f06c9)

SUMMARY: AddressSanitizer: heap-buffer-overflow graphite/src/./inc/Endian.h:77:73 in unsigned long be::_peek<1>(unsigned char const*)
Shadow bytes around the buggy address:
  0x0c1c7fff9ba0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9bb0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9bc0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9bd0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9be0: fa fa fa fa fa fa fa fa fa fa fa fa 00 00 00 00
=>0x0c1c7fff9bf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00[01]fa
  0x0c1c7fff9c00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9c10: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9c20: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9c30: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c1c7fff9c40: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
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
  Heap right redzone:      fb
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack partial redzone:   f4
  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
==19167==ABORTING
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The bug was reported at https://bugzilla.mozilla.org/show_bug.cgi?id=1254487. Attached are three font files which reproduce the crash.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39862.zip