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fix(flatbuffers): guard GetBufferStartFromRootPointer against OOB read before buffer start #9158

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fix(flatbuffers): guard GetBufferStartFromRootPointer against OOB read before buffer start #9158

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8 changes: 7 additions & 1 deletion include/flatbuffers/flatbuffers.h
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Original file line number Diff line number Diff line change
Expand Up @@ -41,7 +41,11 @@ namespace flatbuffers {
/// it is the opposite transformation of GetRoot().
/// This may be useful if you want to pass on a root and have the recipient
/// delete the buffer afterwards.
inline const uint8_t* GetBufferStartFromRootPointer(const void* root) {
/// @param buf_start Optional pointer to the start of the buffer allocation.
/// When provided, the search will not walk before this address, preventing
/// out-of-bounds reads on malformed or truncated buffers.
inline const uint8_t* GetBufferStartFromRootPointer(
const void* root, const uint8_t* buf_start = nullptr) {
auto table = reinterpret_cast<const Table*>(root);
auto vtable = table->GetVTable();
// Either the vtable is before the root or after the root.
Expand All @@ -62,6 +66,8 @@ inline const uint8_t* GetBufferStartFromRootPointer(const void* root) {
"file_identifier is assumed to be the same size as uoffset_t");
for (auto possible_roots = FLATBUFFERS_MAX_ALIGNMENT / sizeof(uoffset_t) + 1;
possible_roots; possible_roots--) {
// Do not walk before the known start of the allocation.
if (buf_start != nullptr && start < buf_start + sizeof(uoffset_t)) break;
start -= sizeof(uoffset_t);
if (ReadScalar<uoffset_t>(start) + start ==
reinterpret_cast<const uint8_t*>(root))
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