[BitcodeReader] Validate OpNum, before accessing Record array.

Currently invalid bitcode files can cause a crash, when OpNum exceeds
the number of elements in Record, like in the attached bitcode file.

The test case was generated by clusterfuzz: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=15698

Reviewers: t.p.northover, thegameg, jfb

Reviewed By: jfb

Differential Revision: https://reviews.llvm.org/D64507

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@365750 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -4165,6 +4165,10 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
           popValue(Record, OpNum, NextValueNo, LHS->getType(), RHS))
         return error("Invalid record");
 
+      if (OpNum >= Record.size())
+        return error(
+            "Invalid record: operand number exceeded available operands");
+
       unsigned PredVal = Record[OpNum];
       bool IsFP = LHS->getType()->isFPOrFPVectorTy();
       FastMathFlags FMF;

test/Bitcode/invalid.test

@@ -235,3 +235,8 @@ RUN: not llvm-dis -disable-output %p/Inputs/invalid-nonpointer-atomicrmw.bc 2>&1
 RUN:   FileCheck --check-prefix=NONPOINTER-ATOMICRMW %s
 
 NONPOINTER-ATOMICRMW: Invalid record
+
+RUN: not llvm-dis -disable-output %p/Inputs/invalid-fcmp-opnum.bc 2>&1 | \
+RUN:   FileCheck --check-prefix=INVALID-FCMP-OPNUM %s
+
+INVALID-FCMP-OPNUM: Invalid record: operand number exceeded available operands