[Bitstream] Check for unterminated VBR

This avoid shift larger than bitwidth UB.

Author: nikic

llvm/include/llvm/Bitstream/BitstreamReader.h

@@ -247,6 +247,10 @@ class SimpleBitstreamCursor {
         return Result;
 
       NextBit += NumBits-1;
+      if (NextBit >= 32)
+        return createStringError(std::errc::illegal_byte_sequence,
+                                 "Unterminated VBR");
+
       MaybeRead = Read(NumBits);
       if (!MaybeRead)
         return MaybeRead;
@@ -274,6 +278,10 @@ class SimpleBitstreamCursor {
         return Result;
 
       NextBit += NumBits-1;
+      if (NextBit >= 64)
+        return createStringError(std::errc::illegal_byte_sequence,
+                                 "Unterminated VBR");
+
       MaybeRead = Read(NumBits);
       if (!MaybeRead)
         return MaybeRead;

llvm/test/Bitcode/Inputs/invalid-value-symbol-table-2.bc

@@ -266,3 +266,8 @@ RUN: not llvm-dis -disable-output %p/Inputs/invalid-value-symbol-table.bc 2>&1 |
 RUN:   FileCheck --check-prefix=INVALID-VALUE-SYMBOL-TABLE %s
 
 INVALID-VALUE-SYMBOL-TABLE: Invalid value reference in symbol table
+
+RUN: not llvm-dis -disable-output %p/Inputs/unterminated-vbr.bc 2>&1 | \
+RUN:   FileCheck --check-prefix=UNTERMINATED-VBR %s
+
+UNTERMINATED-VBR: Unterminated VBR