AK+LibWeb: Make StringBase::bytes() lvalue-only

Disallow calling `StringBase::bytes()` on temporaries to avoid returning
`ReadonlyBytes` that outlive the underlying string.

With this change, we catch a real UAF:
`load_result.data = maybe_response.release_value().bytes();`
All other updated call sites were already safe, they just needed to use
an intermediate named variable to satisfy the new lvalue-only
requirement.

AK/StringBase.h

@@ -71,7 +71,8 @@ public:
 
     // Returns the underlying UTF-8 encoded bytes.
     // NOTE: There is no guarantee about null-termination.
-    [[nodiscard]] ReadonlyBytes bytes() const LIFETIME_BOUND;
+    [[nodiscard]] ReadonlyBytes bytes() const&& = delete;
+    [[nodiscard]] ReadonlyBytes bytes() const& LIFETIME_BOUND;
     [[nodiscard]] u32 hash() const;
     [[nodiscard]] size_t byte_count() const;
     [[nodiscard]] ALWAYS_INLINE size_t length_in_code_units() const { return byte_count(); }
@@ -204,7 +205,7 @@ inline size_t ShortString::byte_count() const
     return byte_count_and_short_string_flag >> StringBase::SHORT_STRING_BYTE_COUNT_SHIFT_COUNT;
 }
 
-inline ReadonlyBytes StringBase::bytes() const
+inline ReadonlyBytes StringBase::bytes() const&
 {
     if (is_short_string())
         return m_impl.short_string.bytes();

Libraries/LibWeb/Fetch/BodyInit.cpp

@@ -103,7 +103,8 @@ WebIDL::ExceptionOr<Infrastructure::BodyWithType> extract_body(JS::Realm& realm,
             source = serialized_form_data.serialized_data;
             // FIXME: Set length to unclear, see html/6424 for improving this.
             // Set type to `multipart/form-data; boundary=`, followed by the multipart/form-data boundary string generated by the multipart/form-data encoding algorithm.
-            type = MUST(ByteBuffer::copy(MUST(String::formatted("multipart/form-data; boundary={}", serialized_form_data.boundary)).bytes()));
+            auto type_string = MUST(String::formatted("multipart/form-data; boundary={}", serialized_form_data.boundary));
+            type = MUST(ByteBuffer::copy(type_string.bytes()));
             return {};
         },
         [&](GC::Root<DOMURL::URLSearchParams> const& url_search_params) -> WebIDL::ExceptionOr<void> {

Libraries/LibWeb/Fetch/Fetching/Fetching.cpp

@@ -1795,8 +1795,10 @@ GC::Ref<PendingResponse> http_network_or_cache_fetch(JS::Realm& realm, Infrastru
 
         // 8. If contentLength is non-null, then set contentLengthHeaderValue to contentLength, serialized and
         //    isomorphic encoded.
-        if (content_length.has_value())
-            content_length_header_value = MUST(ByteBuffer::copy(String::number(*content_length).bytes()));
+        if (content_length.has_value()) {
+            auto content_length_string = String::number(*content_length);
+            content_length_header_value = MUST(ByteBuffer::copy(content_length_string.bytes()));
+        }
 
         // 9. If contentLengthHeaderValue is non-null, then append (`Content-Length`, contentLengthHeaderValue) to
         //    httpRequest’s header list.

Libraries/LibWeb/Fetch/Infrastructure/HTTP/Requests.cpp

@@ -215,7 +215,8 @@ String Request::serialize_origin() const
 ByteBuffer Request::byte_serialize_origin() const
 {
     // Byte-serializing a request origin, given a request request, is to return the result of serializing a request origin with request, isomorphic encoded.
-    return MUST(ByteBuffer::copy(serialize_origin().bytes()));
+    auto serialized_origin = serialize_origin();
+    return MUST(ByteBuffer::copy(serialized_origin.bytes()));
 }
 
 // https://fetch.spec.whatwg.org/#concept-request-clone
@@ -285,14 +286,17 @@ void Request::add_range_header(u64 first, Optional<u64> const& last)
     auto range_value = MUST(ByteBuffer::copy("bytes"sv.bytes()));
 
     // 3. Serialize and isomorphic encode first, and append the result to rangeValue.
-    range_value.append(String::number(first).bytes());
+    auto serialized_first = String::number(first);
+    range_value.append(serialized_first.bytes());
 
     // 4. Append 0x2D (-) to rangeValue.
     range_value.append('-');
 
     // 5. If last is given, then serialize and isomorphic encode it, and append the result to rangeValue.
-    if (last.has_value())
-        range_value.append(String::number(*last).bytes());
+    if (last.has_value()) {
+        auto serialized_last = String::number(*last);
+        range_value.append(serialized_last.bytes());
+    }
 
     // 6. Append (`Range`, rangeValue) to request’s header list.
     auto header = Header {

Libraries/LibWeb/HTML/HTMLFormElement.cpp

@@ -861,7 +861,8 @@ ErrorOr<void> HTMLFormElement::submit_as_entity_body(URL::URL parsed_action, Vec
         auto pairs = TRY(convert_to_list_of_name_value_pairs(entry_list));
 
         // 2. Let body be the result of running the application/x-www-form-urlencoded serializer with pairs and encoding.
-        body = TRY(ByteBuffer::copy(url_encode(pairs, encoding).bytes()));
+        auto query = url_encode(pairs, encoding);
+        body = TRY(ByteBuffer::copy(query.bytes()));
 
         // 3. Set body to the result of encoding body.
         // NOTE: `encoding` refers to `UTF-8 encode`, which body already is encoded as because it uses AK::String.
@@ -888,7 +889,8 @@ ErrorOr<void> HTMLFormElement::submit_as_entity_body(URL::URL parsed_action, Vec
         auto pairs = TRY(convert_to_list_of_name_value_pairs(entry_list));
 
         // 2. Let body be the result of running the text/plain encoding algorithm with pairs.
-        body = TRY(ByteBuffer::copy(TRY(plain_text_encode(pairs)).bytes()));
+        auto serialized_body = TRY(plain_text_encode(pairs));
+        body = TRY(ByteBuffer::copy(serialized_body.bytes()));
 
         // FIXME: 3. Set body to the result of encoding body using encoding.
 

Libraries/LibWeb/Loader/ResourceLoader.cpp

@@ -326,7 +326,7 @@ void ResourceLoader::handle_resource_load_request(LoadRequest const& request, Re
         }
 
         FileLoadResult load_result;
-        load_result.data = maybe_response.release_value().bytes();
+        load_result.data = maybe_response.value().bytes();
         load_result.response_headers.set("Content-Type"sv, "text/html"sv);
         on_resource(load_result);
         return;

Libraries/LibWeb/ReferrerPolicy/AbstractOperations.cpp

@@ -108,8 +108,11 @@ Optional<URL::URL> determine_requests_referrer(Fetch::Infrastructure::Request co
 
     // 6. If the result of serializing referrerURL is a string whose length is greater than 4096, set referrerURL to
     //    referrerOrigin.
-    if (referrer_url.has_value() && referrer_url.value().serialize().bytes().size() > 4096)
+    if (referrer_url.has_value()) {
+        auto serialized_referrer_url = referrer_url.value().serialize();
+        if (serialized_referrer_url.bytes().size() > 4096)
             referrer_url = referrer_origin;
+    }
 
     // 7. The user agent MAY alter referrerURL or referrerOrigin at this point to enforce arbitrary policy
     //    considerations in the interests of minimizing data leakage. For example, the user agent could strip the URL

Libraries/LibWeb/XHR/XMLHttpRequest.cpp

@@ -573,7 +573,8 @@ WebIDL::ExceptionOr<void> XMLHttpRequest::send(Optional<DocumentOrXMLHttpRequest
         // 2. If body is a Document, then set this’s request body to body, serialized, converted, and UTF-8 encoded.
         if (body->has<GC::Root<DOM::Document>>()) {
             auto string_serialized_document = TRY(body->get<GC::Root<DOM::Document>>().cell()->serialize_fragment(HTML::RequireWellFormed::No));
-            m_request_body = Fetch::Infrastructure::byte_sequence_as_body(realm, string_serialized_document.to_utf8().bytes());
+            auto string_serialized_document_utf8 = string_serialized_document.to_utf8();
+            m_request_body = Fetch::Infrastructure::byte_sequence_as_body(realm, string_serialized_document_utf8.bytes());
         }
         // 3. Otherwise:
         else {
@@ -1014,7 +1015,8 @@ String XMLHttpRequest::get_all_response_headers() const
         output.append(0x3A); // ':'
         output.append(0x20); // ' '
         // FIXME: The spec does not mention isomorphic decode. Spec bug?
-        output.append(Infra::isomorphic_decode(header.value).bytes());
+        auto decoder_header = Infra::isomorphic_decode(header.value);
+        output.append(decoder_header.bytes());
         output.append(0x0D); // '\r'
         output.append(0x0A); // '\n'
     }