wasmparser: provide a better error if multiple modules/components concatenated (#2293)

Author: pchickey

crates/wasmparser/src/binary_reader.rs

@@ -736,6 +736,11 @@ impl<'a> BinaryReader<'a> {
         Ok(self.buffer[self.position])
     }
 
+    pub(crate) fn peek_bytes(&self, len: usize) -> Result<&[u8]> {
+        self.ensure_has_bytes(len)?;
+        Ok(&self.buffer[self.position..(self.position + len)])
+    }
+
     pub(crate) fn read_block_type(&mut self) -> Result<BlockType> {
         let b = self.peek()?;
 

crates/wasmparser/src/parser.rs

@@ -714,6 +714,22 @@ impl Parser {
                     return Ok(Payload::End(reader.original_position()));
                 }
 
+                // Corrupted binaries containing multiple modules or
+                // components will fail because a section can never start with
+                // the magic number: 0 is custom section, 'a' is section len
+                // of 97, `s` is section name string len of 115, at which
+                // point validation will fail because name string is bigger
+                // than section. Report a better error instead:
+                match reader.peek_bytes(4) {
+                    Ok(peek) if peek == WASM_MAGIC_NUMBER => {
+                        return Err(BinaryReaderError::new(
+                            "expected section, got wasm magic number",
+                            reader.original_position(),
+                        ));
+                    }
+                    _ => {}
+                }
+
                 let id_pos = reader.original_position();
                 let id = reader.read_u8()?;
                 if id & 0x80 != 0 {
@@ -738,7 +754,7 @@ impl Parser {
                 }
 
                 match (self.encoding, id) {
-                    // Sections for both modules and components.
+                    // Custom sections for both modules and components.
                     (_, 0) => section(reader, len, CustomSectionReader::new, CustomSection),
 
                     // Module sections

tests/cli/invalid/concat.wast

@@ -0,0 +1,12 @@
+;; RUN: wast --assert default --snapshot tests/snapshots %
+
+;; corrupt binary: multiple modules concatenated
+(assert_malformed
+  (module binary
+    "\00asm" "\01\00\00\00" ;; magic header
+    "\01\04"    ;; type section
+    "\01"       ;; 1 count
+    "\60\00\00" ;; no params or results
+    "\00asm" "\01\00\00\00" ;; magic header
+  )
+  "expected section, got wasm magic number")

tests/snapshots/cli/invalid/concat.wast.json

@@ -0,0 +1,12 @@
+{
+  "source_filename": "tests/cli/invalid/concat.wast",
+  "commands": [
+    {
+      "type": "assert_malformed",
+      "line": 5,
+      "filename": "concat.0.wasm",
+      "module_type": "binary",
+      "text": "expected section, got wasm magic number"
+    }
+  ]
+}