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fix(SgpdAtom): guard bytes_available against underflow to stop giant SetDataSize #1097

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fix(SgpdAtom): guard bytes_available against underflow to stop giant SetDataSize #1097

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7 changes: 7 additions & 0 deletions Source/C++/Core/Ap4SgpdAtom.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -78,15 +78,22 @@ AP4_SgpdAtom::AP4_SgpdAtom(AP4_UI32 size,
m_DefaultLength(0)
{
AP4_Size bytes_available = size-AP4_FULL_ATOM_HEADER_SIZE;
// bytes_available is an unsigned counter of what's left in this atom;
// guard every read-and-decrement so a truncated sgpd cannot wrap it to
// 0xFFFFFFFC and let the entry loop below hand a giant length to
// SetDataSize (issue #1042).
if (bytes_available < 4) return;
stream.ReadUI32(m_GroupingType);
bytes_available -= 4;
if (version >= 1) {
if (bytes_available < 4) return;
stream.ReadUI32(m_DefaultLength);
bytes_available -= 4;
}

// read the number of entries
AP4_UI32 entry_count = 0;
if (bytes_available < 4) return;
AP4_Result result = stream.ReadUI32(entry_count);
if (AP4_FAILED(result)) return;
bytes_available -= 4;
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