[wasm] readSection: Avoid reading past eof (fixes oss-fuzz #3219)

A wasm file crafted with a bogus section size can trigger an ASan issue
in the DWARFObjInMemory constructor. Nip the problem in the bud when we
read the wasm section.

Found by OSS-Fuzz:
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=3219

Differential Revision: https://reviews.llvm.org/D38777

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316357 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Object/WasmObjectFile.cpp

@@ -178,14 +178,16 @@ static wasm::WasmTable readTable(const uint8_t *&Ptr) {
 }
 
 static Error readSection(WasmSection &Section, const uint8_t *&Ptr,
-                         const uint8_t *Start) {
-  // TODO(sbc): Avoid reading past EOF in the case of malformed files.
+                         const uint8_t *Start, const uint8_t *Eof) {
   Section.Offset = Ptr - Start;
   Section.Type = readVaruint7(Ptr);
   uint32_t Size = readVaruint32(Ptr);
   if (Size == 0)
     return make_error<StringError>("Zero length section",
                                    object_error::parse_failed);
+  if (Ptr + Size > Eof)
+    return make_error<StringError>("Section too large",
+                                   object_error::parse_failed);
   Section.Content = ArrayRef<uint8_t>(Ptr, Size);
   Ptr += Size;
   return Error::success();
@@ -221,7 +223,7 @@ WasmObjectFile::WasmObjectFile(MemoryBufferRef Buffer, Error &Err)
 
   WasmSection Sec;
   while (Ptr < Eof) {
-    if ((Err = readSection(Sec, Ptr, getPtr(0))))
+    if ((Err = readSection(Sec, Ptr, getPtr(0), Eof)))
       return;
     if ((Err = parseSection(Sec)))
       return;

test/tools/llvm-objdump/wasm-corrupt-section.test

@@ -0,0 +1,2 @@
+# RUN: not llvm-objdump -h %p/Inputs/corrupt-section.wasm 2>&1 | FileCheck %s
+# CHECK: '{{.*}}corrupt-section.wasm': Section too large