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Define minimum buffer size (#7440)

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* add check for zero sized buffers

* Define minimum flatbuffer size
This commit is contained in:
Derek Bailey
2022-08-14 12:40:57 -07:00
committed by GitHub
parent 3cc2daa78f
commit 44a7dc9995
3 changed files with 40 additions and 0 deletions
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7
include/flatbuffers/base.h
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@@ -331,6 +331,13 @@ typedef uintmax_t largest_scalar_t;
// In 32bits, this evaluates to 2GB - 1
#define FLATBUFFERS_MAX_BUFFER_SIZE ((1ULL << (sizeof(::flatbuffers::soffset_t) * 8 - 1)) - 1)
// The minimum size buffer that can be a valid flatbuffer.
// Includes the offset to the root table (uoffset_t), the offset to the vtable
// of the root table (soffset_t), the size of the vtable (uint16_t), and the
// size of the referring table (uint16_t).
#define FLATBUFFERS_MIN_BUFFER_SIZE sizeof(uoffset_t) + sizeof(soffset_t) + \
sizeof(uint16_t) + sizeof(uint16_t)
// We support aligning the contents of buffers up to this size.
#ifndef FLATBUFFERS_MAX_ALIGNMENT
#define FLATBUFFERS_MAX_ALIGNMENT 32

11
include/flatbuffers/verifier.h
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@@ -178,6 +178,12 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
template<typename T>
bool VerifyBufferFromStart(const char *const identifier, const size_t start) {
// Buffers have to be of some size to be valid. The reason it is a runtime
// check instead of static_assert, is that nested flatbuffers go through
// this call and their size is determined at runtime.
if (!Check(size_ >= FLATBUFFERS_MIN_BUFFER_SIZE)) return false;
// If an identifier is provided, check that we have a buffer
if (identifier && !Check((size_ >= 2 * sizeof(flatbuffers::uoffset_t) &&
BufferHasIdentifier(buf_ + start, identifier)))) {
return false;
@@ -198,7 +204,12 @@ class Verifier FLATBUFFERS_FINAL_CLASS {
template<typename T>
bool VerifyNestedFlatBuffer(const Vector<uint8_t> *const buf,
const char *const identifier) {
// An empty buffer is OK as it indicates not present.
if (!buf) return true;
// If there is a nested buffer, it must be greater than the min size.
if(!Check(buf->size() >= FLATBUFFERS_MIN_BUFFER_SIZE)) return false;
Verifier nested_verifier(buf->data(), buf->size());
return nested_verifier.VerifyBuffer<T>(identifier);
}

22
tests/test.cpp
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@@ -4340,6 +4340,28 @@ void NestedVerifierTest() {
builder.GetSize());
TEST_EQ(false, VerifyMonsterBuffer(verifier));
}
{
// Create the outer monster.
flatbuffers::FlatBufferBuilder builder;
// Purposely invalidate the nested flatbuffer setting its length to 0, an
// invalid length.
uint8_t *invalid_nested_buffer = nullptr;
auto nested_monster_bytes = builder.CreateVector(invalid_nested_buffer, 0);
auto name = builder.CreateString("OuterMonster");
MonsterBuilder mon_builder(builder);
mon_builder.add_name(name);
mon_builder.add_testnestedflatbuffer(nested_monster_bytes);
FinishMonsterBuffer(builder, mon_builder.Finish());
// Verify the root monster fails, since the included nested monster fails.
flatbuffers::Verifier verifier(builder.GetBufferPointer(),
builder.GetSize());
TEST_EQ(false, VerifyMonsterBuffer(verifier));
}
}
void ParseIncorrectMonsterJsonTest() {