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bug #43661: FreeType 2.5.3 CFF hintmap building stack-based arbitrary out-of-bounds write

Submitted by:  Mateusz Jurczyk <j00ru>
Submitted on:  Fri 21 Nov 2014 12:45:11 PM UTC  
 
Severity:  3 - Normal Item Group:  Crash
Status:  Fixed Privacy:  Public
Assigned to:  Dave Arnold <darnold> Open/Closed:  Closed
Planned Release:  2.5.4

Wed 26 Nov 2014 07:02:21 PM UTC, comment #3: 

The problem is that two objects, hintMask and hStemHintArray, are getting out of sync. Once the hintMask is constructed (marked valid) no additional hints should be added to hStemHintArray. The test fonts have illegal charstrings that try to do this. There is a test for this in the interpreter, but the recovery path is incorrect and allows the arrays to be extended. Since this affects interpreter logic, I want to do more testing to ensure valid fonts are not affected by my fix.

The check that Mateusz proposes would be a "defense in depth" measure that can be added, but should never execute.

Thanksgiving holiday is this week (in the US), but I can finish the testing by next week.

Dave Arnold <darnold>
Project MemberIn charge of this item.
Wed 26 Nov 2014 06:23:47 PM UTC, comment #2: 

Dave, will be you be able to cook something up within a week?  I would like to do a new FreeType release in the end of November or beginning of December...

Werner LEMBERG <wl>
Project Administrator
Fri 21 Nov 2014 01:57:41 PM UTC, comment #1: 

Dave, please have a look.

Werner LEMBERG <wl>
Project Administrator
Fri 21 Nov 2014 12:45:11 PM UTC, original submission:  

There is a vulnerability in the CFF CharString interpreter used by FreeType 2.5.3 (the Adobe CFF rasterizer contributed to the project in June 2013, see [1]), which makes it possible for a specially crafted OpenType file to perform arbitrary out-of-bounds writes on the interpreter function stack. This in turn makes it possible to create a font file which takes over the program control flow and reliably executes arbitrary code included in the input file.

Interestingly, this is not a new bug, but a variant of CVE-2014-2240 which I reported on February 24th, 2014 here: https://savannah.nongnu.org/bugs/?41697. As it turns out, the fix submitted in commit 0eae6eb0645264c98812f0095e0f5df4541830e6 [2] is not sufficient to entirely close the loophole. Attached are several testcases which reach the vulnerable "cf2_hintmap_build" function through a different code path ("cf2_glyphpath_moveTo" instead of directly from "cf2_interpT2CharString"). This makes it possible to bypass the original fix, because the if() statement in cf2hints.c:778 never executes, thus leading to the processing a "hStemHintArray" object longer than the maximum 96 allowed entries.

The issue has been reproduced with the current version of freetype2 from master git branch, with a 64-bit build of the ftbench utility compiled with AddressSanitizer:

$ ftbench <file>

I believe the immediate reason why the problem still exists today is that the following TODO comment in cf2hints.c:797 has still not been resolved, while in my opinion it should have been:

796:    /* use the hStem hints only, which are first in the mask */
797:    /* TODO: compare this to cffhintmaskGetBitCount */
798:    bitCount = cf2_arrstack_size( hStemHintArray );

If it was, the fix would be effective and couldn't be bypassed to cause a stack-based non-continuous buffer overflow anymore.

Below is an example AddressSanitizer report illustrating the problem:

=================================================================
==15055==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7fff2dc05b30 at pc 0x71134e bp 0x7fff2dc055b0 sp 0x7fff2dc055a8
READ of size 1 at 0x7fff2dc05b30 thread T0
    #0 0x71134d in cf2_hintmap_build freetype2/src/cff/cf2hints.c:822
    #1 0x7048e1 in cf2_glyphpath_moveTo freetype2/src/cff/cf2hints.c:1606
    #2 0x6f5259 in cf2_interpT2CharString freetype2/src/cff/cf2intrp.c:1243
    #3 0x6e8570 in cf2_getGlyphOutline freetype2/src/cff/cf2font.c:469
    #4 0x6e4c5e in cf2_decoder_parse_charstrings freetype2/src/cff/cf2ft.c:367
    #5 0x6da446 in cff_slot_load freetype2/src/cff/cffgload.c:2840
    #6 0x69dbcc in cff_glyph_load freetype2/src/cff/cffdrivr.c:185
    #7 0x4a427e in FT_Load_Glyph freetype2/src/base/ftobjs.c:726
    #8 0x491d69 in test_load ft2demos-2.5.3/src/ftbench.c:249
    #9 0x492b51 in benchmark ft2demos-2.5.3/src/ftbench.c:216
    #10 0x48e962 in main ft2demos-2.5.3/src/ftbench.c:1020

Address 0x7fff2dc05b30 is located in stack of thread T0 at offset 304 in frame
    #0 0x7100cf in cf2_hintmap_build freetype2/src/cff/cf2hints.c:754

  This frame has 20 object(s):
    [32, 40) ''
    [64, 72) ''
    [96, 104) ''
    [128, 136) ''
    [160, 164) ''
    [176, 177) ''
    [192, 200) 'maskPtr'
    [224, 232) 'font'
    [256, 304) 'tempHintMask' <== Memory access at offset 304 overflows this variable
    [336, 344) 'bitCount'
    [368, 376) 'i'
    [400, 401) 'maskByte'
    [416, 448) 'dummy'
    [480, 512) 'bottomHintEdge'
    [544, 576) 'topHintEdge'
    [608, 640) 'edge'
    [672, 704) 'invalid'
    [736, 768) 'bottomHintEdge1'
    [800, 832) 'topHintEdge2'
    [864, 872) 'stemhint'
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 freetype2/src/cff/cf2hints.c:822 cf2_hintmap_build
Shadow bytes around the buggy address:
  0x100065b78b10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100065b78b20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100065b78b30: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x100065b78b40: f1 f1 f1 f1 00 f2 f2 f2 00 f2 f2 f2 00 f2 f2 f2
  0x100065b78b50: 00 f2 f2 f2 04 f2 01 f2 00 f2 f2 f2 00 f2 f2 f2
=>0x100065b78b60: 00 00 00 00 00 00[f2]f2 f2 f2 00 f2 f2 f2 00 f2
  0x100065b78b70: f2 f2 01 f2 00 00 00 00 f2 f2 f2 f2 00 00 00 00
  0x100065b78b80: f2 f2 f2 f2 00 00 00 00 f2 f2 f2 f2 00 00 00 00
  0x100065b78b90: f2 f2 f2 f2 00 00 00 00 f2 f2 f2 f2 00 00 00 00
  0x100065b78ba0: f2 f2 f2 f2 00 00 00 00 f2 f2 f2 f2 00 f3 f3 f3
  0x100065b78bb0: 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
  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
  Contiguous container OOB:fc
  ASan internal:           fe
==15055==ABORTING

References:
[1] http://blog.typekit.com/2013/06/19/adobe-cff-font-rasterizer-accepted-by-freetype/
[2] http://git.savannah.gnu.org/cgit/freetype/freetype2.git/commit/?id=0eae6eb0645264c98812f0095e0f5df4541830e6

Mateusz Jurczyk <j00ru>
Project Member

 

Attached Files

 

Depends on the following items: None found

Items that depend on this one: None found

 

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    Follow 8 latest changes.

    Date Changed by Updated Field Previous Value => Replaced by
    2014-12-04 wl StatusNone => Fixed
        PrivacyPrivate => Public
        Open/ClosedOpen => Closed
        Planned ReleaseNone => 2.5.4
    2014-11-21 wl Assigned toNone => darnold
    2014-11-21 j00ru Attached File- => Added asan_stack-oob_703c16_2728_cov_367593004_aspartam.otf, #32509
        Attached File- => Added asan_stack-oob_703c16_5479_cov_4290077649_elsewher.otf, #32510
        Attached File- => Added asan_stack-oob_703c16_3507_cov_3211953920_ccapshad.otf, #32511

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