Oracle Java Runtime Environment - Heap Out-of-Bounds Read During TTF Font Rendering in ExtractBitMap_blocClass









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A heap-based out-of-bounds read was observed in Oracle Java Runtime Environment version 8u202 (latest at the time of this writing) while fuzz-testing the processing of TrueType fonts. It manifests itself in the form of the following (or similar) crash:

--- cut ---
  Iteration (0,0)
  Iteration (0,1)
  # A fatal error has been detected by the Java Runtime Environment:
  #  SIGSEGV (0xb) at pc=0x00007f857116fde3, pid=31542, tid=0x00007f85a5a70700
  # JRE version: Java(TM) SE Runtime Environment (8.0_202-b08) (build 1.8.0_202-b08)
  # Java VM: Java HotSpot(TM) 64-Bit Server VM (25.202-b08 mixed mode linux-amd64 compressed oops)
  # Problematic frame:
  # C  []  ExtractBitMap_blocClass+0xcc3
  # Failed to write core dump. Core dumps have been disabled. To enable core dumping, try "ulimit -c unlimited" before starting Java again
  # An error report file with more information is saved as:
  # jre/8u202/hs_err_pid31542.log
  # If you would like to submit a bug report, please visit:
  # The crash happened outside the Java Virtual Machine in native code.
  # See problematic frame for where to report the bug.
--- cut ---

Under gdb, we can find out that the ExtractBitMap_blocClass function attempts to access an invalid memory region:

--- cut ---
  gdb$ c
  Iteration (0,0)
  Iteration (0,1)

  Thread 2 "java" received signal SIGSEGV, Segmentation fault.
  RAX: 0x96e40 ('@n\t')
  RBX: 0x100c4
  RCX: 0x12dc8
  RDX: 0x757969c4
  RSI: 0x7ffff02f7238 --> 0x7840201008
     0x7fffbf616ddb <ExtractBitMap_blocClass+3259>:       mov    ecx,eax
     0x7fffbf616ddd <ExtractBitMap_blocClass+3261>:       sar    ecx,0x3
     0x7fffbf616de0 <ExtractBitMap_blocClass+3264>:       movsxd rcx,ecx
  => 0x7fffbf616de3 <ExtractBitMap_blocClass+3267>:       movzx  edi,BYTE PTR [rsi+rcx*1]
     0x7fffbf616de7 <ExtractBitMap_blocClass+3271>:       cmp    edx,r8d
     0x7fffbf616dea <ExtractBitMap_blocClass+3274>:       jl     0x7fffbf616dd0 <ExtractBitMap_blocClass+3248>
     0x7fffbf616dec <ExtractBitMap_blocClass+3276>:       mov    ecx,r11d
     0x7fffbf616def <ExtractBitMap_blocClass+3279>:       mov    r13d,0x80
  Legend: code, data, rodata, value
  Stopped reason: SIGSEGV
  0x00007fffbf616de3 in ExtractBitMap_blocClass () from jre/8u202/lib/amd64/
--- cut ---

The crash reproduces on both Windows and Linux platforms. On Windows, the crash manifests in the following way:

--- cut ---
  (5218.154c): Access violation - code c0000005 (first chance)
  First chance exceptions are reported before any exception handling.
  This exception may be expected and handled.
  00007ffa`0b8eb651 468a1c20        mov     r11b,byte ptr [rax+r12] ds:00000000`64c09000=??
  0:004> ? rax
  Evaluate expression: 84720 = 00000000`00014af0
  0:004> ? r12
  Evaluate expression: 1690256656 = 00000000`64bf4510
  0:004> k
   # Child-SP          RetAddr           Call Site
  00 00000000`04a8e9d0 00007ffa`0b8ebf92 t2k!Java_sun_font_T2KFontScaler_getGlyphVectorOutlineNative+0x2155
  01 00000000`04a8eaa0 00007ffa`0b8e8e05 t2k!Java_sun_font_T2KFontScaler_getGlyphVectorOutlineNative+0x2a96
  02 00000000`04a8eb80 00007ffa`0b8e9011 t2k!Java_sun_font_T2KFontScaler_getGlyphCodeNative+0xd4fd
  03 00000000`04a8ebf0 00007ffa`0b8daeab t2k!Java_sun_font_T2KFontScaler_getGlyphCodeNative+0xd709
  04 00000000`04a8ec90 00000000`04f18d27 t2k!Java_sun_font_T2KFontScaler_getGlyphImageNative+0x163
  05 00000000`04a8ed60 00000000`02a3e1f8 0x4f18d27
  06 00000000`04a8ed68 00000000`04a8ee48 0x2a3e1f8
  07 00000000`04a8ed70 00000000`5c36dff8 0x4a8ee48
  08 00000000`04a8ed78 00000000`00000000 0x5c36dff8
--- cut ---

Attached with this report are three mutated testcases, and a simple Java program used to reproduce the vulnerability by loading TrueType fonts specified through a command-line parameter.

Proof of Concept: