LibCompress: Make the Zlib decompressor fail gracefuly

This commit adds a verify-less try_create method to the Zlib
decompressor to allow for graceful failures of parsing the
Zlib headers.

Author: IdanHo

Userland/Libraries/LibCompress/Zlib.cpp

@@ -24,7 +24,6 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <AK/Assertions.h>
 #include <AK/Span.h>
 #include <AK/Types.h>
 #include <AK/Vector.h>
@@ -33,26 +32,38 @@
 
 namespace Compress {
 
-Zlib::Zlib(ReadonlyBytes data)
+Optional<Zlib> Zlib::try_create(ReadonlyBytes data)
 {
-    m_input_data = data;
+    if (data.size() < 6)
+        return {}; // header + footer size is 6
+
+    Zlib zlib { data };
 
     u8 compression_info = data.at(0);
     u8 flags = data.at(1);
 
-    m_compression_method = compression_info & 0xF;
-    m_compression_info = (compression_info >> 4) & 0xF;
-    m_check_bits = flags & 0xF;
-    m_has_dictionary = (flags >> 5) & 0x1;
-    m_compression_level = (flags >> 6) & 0x3;
-    m_checksum = 0;
+    zlib.m_compression_method = compression_info & 0xF;
+    zlib.m_compression_info = (compression_info >> 4) & 0xF;
+    zlib.m_check_bits = flags & 0xF;
+    zlib.m_has_dictionary = (flags >> 5) & 0x1;
+    zlib.m_compression_level = (flags >> 6) & 0x3;
+
+    if (zlib.m_compression_method != 8 || zlib.m_compression_info > 7)
+        return {}; // non-deflate compression
+
+    if (zlib.m_has_dictionary)
+        return {}; // we dont support pre-defined dictionaries
 
-    VERIFY(m_compression_method == 8);
-    VERIFY(m_compression_info == 7);
-    VERIFY(!m_has_dictionary);
-    VERIFY((compression_info * 256 + flags) % 31 == 0);
+    if ((compression_info * 256 + flags) % 31 != 0)
+        return {}; // error correction code doesnt match
 
-    m_data_bytes = data.slice(2, data.size() - 2 - 4);
+    zlib.m_data_bytes = data.slice(2, data.size() - 2 - 4);
+    return zlib;
+}
+
+Zlib::Zlib(const ReadonlyBytes& data)
+    : m_input_data(data)
+{
 }
 
 Optional<ByteBuffer> Zlib::decompress()
@@ -62,8 +73,10 @@ Optional<ByteBuffer> Zlib::decompress()
 
 Optional<ByteBuffer> Zlib::decompress_all(ReadonlyBytes bytes)
 {
-    Zlib zlib { bytes };
-    return zlib.decompress();
+    auto zlib = try_create(bytes);
+    if (!zlib.has_value())
+        return {};
+    return zlib->decompress();
 }
 
 u32 Zlib::checksum()

Userland/Libraries/LibCompress/Zlib.h

@@ -34,21 +34,22 @@ namespace Compress {
 
 class Zlib {
 public:
-    Zlib(ReadonlyBytes data);
-
     Optional<ByteBuffer> decompress();
     u32 checksum();
 
+    static Optional<Zlib> try_create(ReadonlyBytes data);
     static Optional<ByteBuffer> decompress_all(ReadonlyBytes);
 
 private:
+    Zlib(const ReadonlyBytes& data);
+
     u8 m_compression_method;
     u8 m_compression_info;
     u8 m_check_bits;
     u8 m_has_dictionary;
     u8 m_compression_level;
 
-    u32 m_checksum;
+    u32 m_checksum { 0 };
     ReadonlyBytes m_input_data;
     ReadonlyBytes m_data_bytes;
 };