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fix(flexbuffers): guard Map::Keys() against OOB heap read before buffer start #9159

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fix(flexbuffers): guard Map::Keys() against OOB heap read before buffer start #9159

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32 changes: 24 additions & 8 deletions include/flatbuffers/flexbuffers.h
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Original file line number Diff line number Diff line change
Expand Up @@ -341,7 +341,9 @@ class FixedTypedVector : public Object {

class Map : public Vector {
public:
Map(const uint8_t* data, uint8_t byte_width) : Vector(data, byte_width) {}
Map(const uint8_t* data, uint8_t byte_width,
const uint8_t* buf_start = nullptr)
: Vector(data, byte_width), buf_start_(buf_start) {}

Reference operator[](const char* key) const;
Reference operator[](const std::string& key) const;
Expand All @@ -350,6 +352,12 @@ class Map : public Vector {

TypedVector Keys() const {
const size_t num_prefixed_fields = 3;
// Guard against reading before the start of the buffer allocation when a
// crafted FlexBuffer places the map data pointer too close to the start.
if (buf_start_ != nullptr &&
data_ < buf_start_ + byte_width_ * num_prefixed_fields) {
return TypedVector::EmptyTypedVector();
}
auto keys_offset = data_ - byte_width_ * num_prefixed_fields;
return TypedVector(Indirect(keys_offset, byte_width_),
static_cast<uint8_t>(
Expand All @@ -365,6 +373,9 @@ class Map : public Vector {
}

bool IsTheEmptyMap() const { return data_ == EmptyMap().data_; }

private:
const uint8_t* buf_start_;
};

inline void IndentString(std::string& s, int indent,
Expand Down Expand Up @@ -404,20 +415,24 @@ void AppendToString(std::string& s, T&& v, bool keys_quoted) {
class Reference {
public:
Reference()
: data_(nullptr), parent_width_(0), byte_width_(0), type_(FBT_NULL) {}
: data_(nullptr), parent_width_(0), byte_width_(0), type_(FBT_NULL),
buf_start_(nullptr) {}

Reference(const uint8_t* data, uint8_t parent_width, uint8_t byte_width,
Type type)
Type type, const uint8_t* buf_start = nullptr)
: data_(data),
parent_width_(parent_width),
byte_width_(byte_width),
type_(type) {}
type_(type),
buf_start_(buf_start) {}

Reference(const uint8_t* data, uint8_t parent_width, uint8_t packed_type)
Reference(const uint8_t* data, uint8_t parent_width, uint8_t packed_type,
const uint8_t* buf_start = nullptr)
: data_(data),
parent_width_(parent_width),
byte_width_(static_cast<uint8_t>(1 << (packed_type & 3))),
type_(static_cast<Type>(packed_type >> 2)) {}
type_(static_cast<Type>(packed_type >> 2)),
buf_start_(buf_start) {}

Type GetType() const { return type_; }

Expand Down Expand Up @@ -730,7 +745,7 @@ class Reference {

Map AsMap() const {
if (type_ == FBT_MAP) {
return Map(Indirect(), byte_width_);
return Map(Indirect(), byte_width_, buf_start_);
} else {
return Map::EmptyMap();
}
Expand Down Expand Up @@ -849,6 +864,7 @@ class Reference {
uint8_t parent_width_;
uint8_t byte_width_;
Type type_;
const uint8_t* buf_start_;
};

// Template specialization for As().
Expand Down Expand Up @@ -1011,7 +1027,7 @@ inline Reference GetRoot(const uint8_t* buffer, size_t size) {
auto byte_width = *--end;
auto packed_type = *--end;
end -= byte_width; // The root data item.
return Reference(end, byte_width, packed_type);
return Reference(end, byte_width, packed_type, buffer);
}

inline Reference GetRoot(const std::vector<uint8_t>& buffer) {
Expand Down