[RISCV] Fix AddressSanitizer heap-buffer-overflow in disassembling

[apazos]

Summary

RISCVDisassembler should check number of bytes available before reading them.
Crash noticed when enabling -DLLVM_USE_SANITIZER=Address.

This bug was uncovered by a LLVM MC Disassembler Protocol Buffer Fuzzer for the RISC-V assembly language.

[asb]

Looks good to me, thanks.

[apazos]

Moved test to MC/Disassembler/RISCV

llvm/trunk/lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp

DecodeStatus RISCVDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
                                                raw_ostream &CS) const {
   // TODO: This will need modification when supporting instruction set
   // extensions with instructions > 32-bits (up to 176 bits wide).
   uint32_t Insn;
   DecodeStatus Result;
 
   // It's a 32 bit instruction if bit 0 and 1 are 1.
   if ((Bytes[0] & 0x3) == 0x3) {
+    if (Bytes.size() < 4) {
+      Size = 0;
+      return MCDisassembler::Fail;
+    }
     Insn = support::endian::read32le(Bytes.data());
     LLVM_DEBUG(dbgs() << "Trying RISCV32 table :\n");
     Result = decodeInstruction(DecoderTable32, MI, Insn, Address, this, STI);
     Size = 4;
   } else {
+    if (Bytes.size() < 2) {
+      Size = 0;
+      return MCDisassembler::Fail;
+    }
     Insn = support::endian::read16le(Bytes.data());
 
     if (!STI.getFeatureBits()[RISCV::Feature64Bit]) {
       LLVM_DEBUG(
           dbgs() << "Trying RISCV32Only_16 table (16-bit Instruction):\n");
       // Calling the auto-generated decoder function.
       Result = decodeInstruction(DecoderTableRISCV32Only_16, MI, Insn, Address,
                                  this, STI);

llvm/trunk/test/MC/Disassembler/RISCV/fuzzer-invalid.txt

+# RUN: not llvm-mc -disassemble -triple=riscv32 < %s 2>&1 | FileCheck %s
+# RUN: not llvm-mc -disassemble -triple=riscv64 < %s 2>&1 | FileCheck %s
+#
+# Test generated by a LLVM MC Disassembler Protocol Buffer Fuzzer
+# for the RISC-V assembly language.
+
+[0xf9 0x95 0xab 0x99]
+# CHECK: warning: invalid instruction encoding

llvm/trunk/test/MC/Disassembler/RISCV/lit.local.cfg

+if not 'RISCV' in config.root.targets:
+    config.unsupported = True
+