[Bug 800585] CVE-2012-1129 freetype: heap off-by-one buffer underflow in Type42 parser t42_parse_sfnts() (#35602)

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https://bugzilla.redhat.com/show_bug.cgi?id=800585

Tomas Hoger <thoger at redhat.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |CLOSED
         Resolution|                            |NOTABUG
            Summary|CVE-2012-1129 freetype:     |CVE-2012-1129 freetype:
                   |Out-of heap-based buffer    |heap off-by-one buffer
                   |read when parsing certain   |underflow in Type42 parser
                   |SFNT strings by Type42 font |t42_parse_sfnts() (#35602)
                   |parser (FU#35602)           |
  Status Whiteboard|impact=low,public=20120223, |impact=low,public=20120223,
                   |reported=20120302,source=go |reported=20120302,source=se
                   |ogle,cvss2=4.3/AV:N/AC:M/Au |calert,cvss2=2.6/AV:N/AC:H/
                   |:N/C:N/I:N/A:P,rhel-5/freet |Au:N/C:N/I:N/A:P,rhel-5/fre
                   |ype=affected,rhel-6/freetyp |etype=affected,rhel-6/freet
                   |e=affected,fedora-all/freet |ype=affected,fedora-all/fre
                   |ype=affected                |etype=affected,fedora-all/m
                   |                            |ingw32-freetype=affected
        Last Closed|                            |2012-03-15 08:28:18

--- Comment #5 from Tomas Hoger <thoger at redhat.com> 2012-03-15 08:28:18 EDT ---
This bug is not a security problem.  Affected code is the following code from
src/type42/t42parse.c :

http://git.savannah.gnu.org/cgit/freetype/freetype2.git/tree/src/type42/t42parse.c?id=0b1c0c6b#n610

 610     /* A string can have a trailing zero byte for padding.  Ignore it. */
 611     if ( string_buf[string_size - 1] == 0 && ( string_size % 2 == 1 ) )
 612       string_size--;

Here problem occurs when string_size == 0.  In that case string_buf[string_size
- 1] reads one byte right before the beginning of the heap-based string_buf[]
buffer.  Such one byte buffer underflow can only lead to program crash when
string_buf[] starts at the memory page boundary and the previous page is not
mapped.  However, that can not happen with the current glibc malloc()
implementation.  A memory address returned to a program by malloc() function
has additional malloc header located right before it.  Hence
string_buf[string_size - 1] reads part of that header and can not access
unmapped memory.

This bug can be detected by memory usage checkers as valgrind or
AddressSanitizer, but does not lead to crash during the normal use.

The value read from the malloc header has no impact on whether the "if"
expression is evaluated as true or falls.  The second part of the "if"
expression is: ( string_size % 2 == 1 ) which evaluates to false when
string_size == 0.  Hence string_size is not decremented and remains 0.

The code immediately following is:

 614     if ( !string_size )
 615     {
 616       FT_ERROR(( "t42_parse_sfnts: invalid string\n" ));
 617       error = T42_Err_Invalid_File_Format;
 618       goto Fail;
 619     }

which causes the font file to be rejected as invalid.

Statement:

This bug is not a security issue. For detailed explanation, refer to:
https://bugzilla.redhat.com/show_bug.cgi?id=CVE-2012-1129#c5

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