Add limit parameter to decompressZlib

This can prevent untrusted data, such as sent over the network,
from consuming all memory with a specially crafted payload.

src/serialization.cpp

@@ -99,7 +99,7 @@ void compressZlib(const std::string &data, std::ostream &os, int level)
 	compressZlib((u8*)data.c_str(), data.size(), os, level);
 }
 
-void decompressZlib(std::istream &is, std::ostream &os)
+void decompressZlib(std::istream &is, std::ostream &os, size_t limit)
 {
 	z_stream z;
 	const s32 bufsize = 16384;
@@ -108,6 +108,7 @@ void decompressZlib(std::istream &is, std::ostream &os)
 	int status = 0;
 	int ret;
 	int bytes_read = 0;
+	int bytes_written = 0;
 	int input_buffer_len = 0;
 
 	z.zalloc = Z_NULL;
@@ -124,8 +125,20 @@ void decompressZlib(std::istream &is, std::ostream &os)
 
 	for(;;)
 	{
+		int output_size = bufsize;
 		z.next_out = (Bytef*)output_buffer;
-		z.avail_out = bufsize;
+		z.avail_out = output_size;
+
+		if (limit) {
+			int limit_remaining = limit - bytes_written;
+			if (limit_remaining <= 0) {
+				// we're aborting ahead of time - throw an error?
+				break;
+			}
+			if (limit_remaining < output_size) {
+				z.avail_out = output_size = limit_remaining;
+			}
+		}
 
 		if(z.avail_in == 0)
 		{
@@ -153,10 +166,11 @@ void decompressZlib(std::istream &is, std::ostream &os)
 			zerr(status);
 			throw SerializationError("decompressZlib: inflate failed");
 		}
-		int count = bufsize - z.avail_out;
+		int count = output_size - z.avail_out;
 		//dstream<<"count="<<count<<std::endl;
 		if(count)
 			os.write(output_buffer, count);
+		bytes_written += count;
 		if(status == Z_STREAM_END)
 		{
 			//dstream<<"Z_STREAM_END"<<std::endl;

src/serialization.h

@@ -87,7 +87,7 @@ inline bool ser_ver_supported(s32 v) {
 
 void compressZlib(const u8 *data, size_t data_size, std::ostream &os, int level = -1);
 void compressZlib(const std::string &data, std::ostream &os, int level = -1);
-void decompressZlib(std::istream &is, std::ostream &os);
+void decompressZlib(std::istream &is, std::ostream &os, size_t limit = 0);
 
 // These choose between zlib and a self-made one according to version
 void compress(const SharedBuffer<u8> &data, std::ostream &os, u8 version);

src/unittest/test_compression.cpp

@@ -37,6 +37,8 @@ public:
 	void testRLECompression();
 	void testZlibCompression();
 	void testZlibLargeData();
+	void testZlibLimit();
+	void _testZlibLimit(u32 size, u32 limit);
 };
 
 static TestCompression g_test_instance;
@@ -46,6 +48,7 @@ void TestCompression::runTests(IGameDef *gamedef)
 	TEST(testRLECompression);
 	TEST(testZlibCompression);
 	TEST(testZlibLargeData);
+	TEST(testZlibLimit);
 }
 
 ////////////////////////////////////////////////////////////////////////////////
@@ -170,3 +173,63 @@ void TestCompression::testZlibLargeData()
 				i, str_decompressed[i], i, data_in[i]);
 	}
 }
+
+void TestCompression::testZlibLimit()
+{
+	// edge cases
+	_testZlibLimit(1024, 1023);
+	_testZlibLimit(1024, 1024);
+	_testZlibLimit(1024, 1025);
+
+	// test around buffer borders
+	u32 bufsize = 16384; // as in implementation
+	for (int s = -1; s <= 1; s++)
+	{
+		for (int l = -1; l <= 1; l++)
+		{
+			_testZlibLimit(bufsize + s, bufsize + l);
+		}
+	}
+	// span multiple buffers
+	_testZlibLimit(35000, 22000);
+	_testZlibLimit(22000, 35000);
+}
+
+void TestCompression::_testZlibLimit(u32 size, u32 limit)
+{
+	infostream << "Test: Testing zlib wrappers with a decompression "
+		"memory limit of " << limit << std::endl;
+
+	infostream << "Test: Input size of compressZlib for limit is "
+		<< size << std::endl;
+
+	// how much data we expect to get
+	u32 expected = size < limit ? size : limit;
+
+	// create recognizable data
+	std::string data_in;
+	data_in.resize(size);
+	for (u32 i = 0; i < size; i++)
+		data_in[i] = (u8)(i % 256);
+
+	std::ostringstream os_compressed(std::ios::binary);
+	compressZlib(data_in, os_compressed);
+	infostream << "Test: Output size of compressZlib for limit is "
+		<< os_compressed.str().size()<<std::endl;
+
+	std::istringstream is_compressed(os_compressed.str(), std::ios::binary);
+	std::ostringstream os_decompressed(std::ios::binary);
+	decompressZlib(is_compressed, os_decompressed, limit);
+	infostream << "Test: Output size of decompressZlib with limit is "
+		<< os_decompressed.str().size() << std::endl;
+
+	std::string str_decompressed = os_decompressed.str();
+	UASSERTEQ(size_t, str_decompressed.size(), expected);
+
+	for (u32 i = 0; i < size && i < str_decompressed.size(); i++) {
+		UTEST(str_decompressed[i] == data_in[i],
+				"index out[%i]=%i differs from in[%i]=%i",
+				i, str_decompressed[i], i, data_in[i]);
+	}
+}
+