libxml2 - xmlDictAddString Heap Buffer Overread

Source: https://code.google.com/p/google-security-research/issues/detail?id=637

The following crash due to a heap-based out-of-bounds memory read can be observed in an ASAN build of latest stable libxml2 (2.9.3, released 4 days ago), by feeding a malformed file to xmllint ("$ ./xmllint --html /path/to/file"):

--- cut ---
==25920==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x631000010810 at pc 0x0000004a2f25 bp 0x7ffc81805ae0 sp 0x7ffc81805290
READ of size 73661 at 0x631000010810 thread T0
    #0 0x4a2f24 in __asan_memcpy llvm/projects/compiler-rt/lib/asan/asan_interceptors.cc:393
    #1 0xd026b2 in xmlDictAddString libxml2-2.9.3/dict.c:285:5
    #2 0xd009e8 in xmlDictLookup libxml2-2.9.3/dict.c:926:11
    #3 0x806e4d in htmlParseNameComplex libxml2-2.9.3/HTMLparser.c:2517:12
    #4 0x7cc29d in htmlParseName libxml2-2.9.3/HTMLparser.c:2483:12
    #5 0x7ca6f1 in htmlParseEntityRef libxml2-2.9.3/HTMLparser.c:2682:16
    #6 0x820a0d in htmlParseReference libxml2-2.9.3/HTMLparser.c:4044:8
    #7 0x7df716 in htmlParseContentInternal libxml2-2.9.3/HTMLparser.c:4619:3
    #8 0x7e2f0f in htmlParseDocument libxml2-2.9.3/HTMLparser.c:4769:5
    #9 0x802c55 in htmlDoRead libxml2-2.9.3/HTMLparser.c:6741:5
    #10 0x8030b6 in htmlReadFile libxml2-2.9.3/HTMLparser.c:6799:13
    #11 0x4f47a5 in parseAndPrintFile libxml2-2.9.3/xmllint.c:2248:8
    #12 0x4ebe8f in main libxml2-2.9.3/xmllint.c:3759:7

0x631000010810 is located 0 bytes to the right of 65552-byte region [0x631000000800,0x631000010810)
allocated by thread T0 here:
    #0 0x4b8ef0 in realloc llvm/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:61
    #1 0xa079a5 in xmlBufGrowInternal libxml2-2.9.3/buf.c:486:23
    #2 0xa06722 in xmlBufGrow libxml2-2.9.3/buf.c:515:11
    #3 0x72fef4 in xmlParserInputBufferGrow libxml2-2.9.3/xmlIO.c:3326:9
    #4 0x543b22 in xmlParserInputGrow libxml2-2.9.3/parserInternals.c:320:8
    #5 0x8067f4 in htmlParseNameComplex libxml2-2.9.3/HTMLparser.c:2511:6
    #6 0x7cc29d in htmlParseName libxml2-2.9.3/HTMLparser.c:2483:12
    #7 0x7ca6f1 in htmlParseEntityRef libxml2-2.9.3/HTMLparser.c:2682:16
    #8 0x820a0d in htmlParseReference libxml2-2.9.3/HTMLparser.c:4044:8
    #9 0x7df716 in htmlParseContentInternal libxml2-2.9.3/HTMLparser.c:4619:3
    #10 0x7e2f0f in htmlParseDocument libxml2-2.9.3/HTMLparser.c:4769:5
    #11 0x802c55 in htmlDoRead libxml2-2.9.3/HTMLparser.c:6741:5
    #12 0x8030b6 in htmlReadFile libxml2-2.9.3/HTMLparser.c:6799:13
    #13 0x4f47a5 in parseAndPrintFile libxml2-2.9.3/xmllint.c:2248:8
    #14 0x4ebe8f in main libxml2-2.9.3/xmllint.c:3759:7

SUMMARY: AddressSanitizer: heap-buffer-overflow llvm/projects/compiler-rt/lib/asan/asan_interceptors.cc:393 in __asan_memcpy
Shadow bytes around the buggy address:
  0x0c627fffa0b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c627fffa0c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c627fffa0d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c627fffa0e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c627fffa0f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c627fffa100: 00 00[fa]fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c627fffa110: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c627fffa120: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c627fffa130: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c627fffa140: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c627fffa150: 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
==25920==ABORTING
--- cut ---

The crash was reported at https://bugzilla.gnome.org/show_bug.cgi?id=758605. Attached is an XML file which triggers the crash.


Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39491.zip