[OpenJDK 2D-Dev] [11] RFR JDK-8191023: PngReader throws NegativeArraySizeException when keyword length exceeds chunk size

Prahalad Kumar Narayanan prahalad.kumar.narayanan at oracle.com
Thu Jan 25 05:45:05 UTC 2018

Hello Jay

Sergey has suggested a valid point- If the CRC was generated with incorrect chunk data, the approach I suggested will succeed and we would still end up in the exception within the parse_<chunk> methods. Hence going by if (...) checks is appropriate.

Few minor corrections to the webrev.01 are as follows:

. Correction to the subtraction value from -2 to -1 in the following lines
  The original code had -2 because, one extra Unsigned byte was getting read prior to the size calculation.
  The current changes advance the calculation by a few lines ahead of reading extra UnsignedByte.

      431         int compressedProfileLength = chunkLength - keyword.length() - 2;
      671         int textLength = chunkLength - keyword.length() - 2;    

. The result of these two operations are not the same due to the order of execution.
  If you had checked the logic and corrected the usage, it should be fine. 

      // Original code
      518         chunkLength -= metadata.sPLT_paletteName.length() + 1;
      // Code in the webrev
      526         int remainingChunkLength = chunkLength -
      527                 metadata.sPLT_paletteName.length() + 1;

. All the if conditions in the proposed fix check whether remSize < 0. I believe, we will need size "<=" 0 check. 
  Reason is that new <Type>[0] will succeed but subsequent read with stream.read* method will throw "ArrayIndexOutOfBounds: 0" exception.

Thank you
Have a good day

Prahalad N.

-----Original Message-----
From: Sergey Bylokhov 
Sent: Thursday, January 25, 2018 5:44 AM
To: Prahalad Kumar Narayanan; Jayathirth D V; 2d-dev
Subject: Re: [OpenJDK 2D-Dev] [11] RFR JDK-8191023: PngReader throws NegativeArraySizeException when keyword length exceeds chunk size

On 22/01/2018 23:17, Prahalad Kumar Narayanan wrote:
> My suggestion was to -
> . 'Generate' CRC from Chunk data and compare it with the retrieved value at Line 731 'before' proceeding to process any of the chunks.
> . In mal-formed chunks (corrupted chunk length /or chunk data), the CRC check will fail thus giving an effective way to identify a valid chunk.
> . Many of the if (...) conditions that 've been added to parse_<Chunk> methods can be avoided with CRC check done upfront.

Is it possible that CRC will be broken/malformed as well as a chunk data?(For example if it is generated on top of incorrect data?), if yes then we should check the data itself for correct/incorrect values.

Best regards, Sergey.

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