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integer overflow in size-prefixed verifier bounds check #9043

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integer overflow in size-prefixed verifier bounds check #9043

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11 changes: 6 additions & 5 deletions include/flatbuffers/verifier.h
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Original file line number Diff line number Diff line change
Expand Up @@ -254,11 +254,12 @@ class VerifierTemplate FLATBUFFERS_FINAL_CLASS {

template <typename T, typename SizeT = uoffset_t>
bool VerifySizePrefixedBuffer(const char* const identifier) {
return Verify<SizeT>(0U) &&
// Ensure the prefixed size is within the bounds of the provided
// length.
Check(ReadScalar<SizeT>(buf_) + sizeof(SizeT) <= size_) &&
VerifyBufferFromStart<T>(identifier, sizeof(SizeT));
if (!Verify<SizeT>(0U)) return false;
// Ensure the prefixed size is within the bounds of the provided length.
const auto prefixed_size = static_cast<uint64_t>(ReadScalar<SizeT>(buf_));
const auto max_payload_size = static_cast<uint64_t>(size_ - sizeof(SizeT));
if (!Check(prefixed_size <= max_payload_size)) return false;
return VerifyBufferFromStart<T>(identifier, sizeof(SizeT));
}

template <typename OffsetT = uoffset_t, typename SOffsetT = soffset_t>
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11 changes: 11 additions & 0 deletions tests/64bit/offset64_test.cpp
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Expand Up @@ -374,6 +374,17 @@ void Offset64SizePrefix() {

// Verify the fields.
TEST_EQ_STR(root_table->near_string()->c_str(), "some near string");

std::vector<uint8_t> tampered(builder.GetBufferPointer(),
builder.GetBufferPointer() + builder.GetSize());
WriteScalar<uoffset64_t>(tampered.data(),
(std::numeric_limits<uoffset64_t>::max)());

Verifier::Options tampered_options = options;
tampered_options.assert = false;
Verifier tampered_verifier(tampered.data(), tampered.size(),
tampered_options);
TEST_EQ(VerifySizePrefixedRootTableBuffer(tampered_verifier), false);
}

void Offset64ManyVectors() {
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