decoder: fix heap-buffer-overflow in Intra_PCM decoding#3950
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Fix a heap-buffer-overflow vulnerability in `WelsActualDecodeMbCavlcISlice`, `WelsActualDecodeMbCavlcPSlice`, and `WelsActualDecodeMbCavlcBSlice` when handling `MB_TYPE_INTRA_PCM` macroblocks. The decoder previously performed an unconditional read of 384 bytes from the bitstream buffer without verifying the remaining buffer size or the `uiChromaFormatIdc` value. On truncated or malformed bitstreams, this resulted in reading past `pBs->pEndBuf`. Additionally, the code incorrectly attempted to process chroma data for monochrome (4:0:0) streams. Changes: - Added a boundary check against `pBs->pEndBuf` before reading PCM data. - Calculated required data size (384 bytes for 4:2:0, 256 bytes for 4:0:0) based on `uiChromaFormatIdc`. - Conditioned chroma data copying and bitstream pointer advancement on the chroma format. - Ensured `pBs->pCurBuf` is advanced by the correct byte count. This fix prevents out-of-bounds access when `pBs->pCurBuf - iIndex + size` exceeds the allocated bitstream buffer. Co-authored-by: CodeMender <codemender-patching@google.com> Fixes: https://issues.oss-fuzz.com/issues/479908882
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Fix a heap-buffer-overflow vulnerability in
WelsActualDecodeMbCavlcISlice,WelsActualDecodeMbCavlcPSlice, andWelsActualDecodeMbCavlcBSlicewhen handlingMB_TYPE_INTRA_PCMmacroblocks.The decoder previously performed an unconditional read of 384 bytes from the bitstream buffer without verifying the remaining buffer size or the
uiChromaFormatIdcvalue. On truncated or malformed bitstreams, this resulted in reading pastpBs->pEndBuf. Additionally, the code incorrectly attempted to process chroma data for monochrome (4:0:0) streams.Changes:
pBs->pEndBufbefore reading PCM data.uiChromaFormatIdc.pBs->pCurBufis advanced by the correct byte count.This fix prevents out-of-bounds access when
pBs->pCurBuf - iIndex + sizeexceeds the allocated bitstream buffer.Fixes: https://issues.oss-fuzz.com/issues/479908882