Index: lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
===================================================================
--- lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
+++ lib/Target/RISCV/Disassembler/RISCVDisassembler.cpp
@@ -11,6 +11,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include "MCTargetDesc/RISCVBaseInfo.h"
 #include "MCTargetDesc/RISCVMCTargetDesc.h"
 #include "llvm/MC/MCContext.h"
 #include "llvm/MC/MCDisassembler/MCDisassembler.h"
@@ -243,6 +244,17 @@
   return MCDisassembler::Success;
 }
 
+static DecodeStatus decodeFRMArg(MCInst &Inst, uint64_t Imm,
+                                 int64_t Address,
+                                 const void *Decoder) {
+  assert(isUInt<3>(Imm) && "Invalid immediate");
+  if (!llvm::RISCVFPRndMode::isValidRoundingMode(Imm))
+    return MCDisassembler::Fail;
+
+  Inst.addOperand(MCOperand::createImm(Imm));
+  return MCDisassembler::Success;
+}
+
 #include "RISCVGenDisassemblerTables.inc"
 
 DecodeStatus RISCVDisassembler::getInstruction(MCInst &MI, uint64_t &Size,
Index: lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h
===================================================================
--- lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h
+++ lib/Target/RISCV/MCTargetDesc/RISCVBaseInfo.h
@@ -104,6 +104,22 @@
       .Case("dyn", RISCVFPRndMode::DYN)
       .Default(RISCVFPRndMode::Invalid);
 }
+
+inline static bool isValidRoundingMode(unsigned Mode) {
+  switch (Mode) {
+  default:
+    return false;
+  case RISCVFPRndMode::RNE:
+  case RISCVFPRndMode::RTZ:
+  case RISCVFPRndMode::RDN:
+  case RISCVFPRndMode::RUP:
+  case RISCVFPRndMode::RMM:
+  case RISCVFPRndMode::DYN:
+    return true;
+  }
+  return false;
+}
+
 } // namespace RISCVFPRndMode
 } // namespace llvm
 
Index: lib/Target/RISCV/RISCVInstrInfoF.td
===================================================================
--- lib/Target/RISCV/RISCVInstrInfoF.td
+++ lib/Target/RISCV/RISCVInstrInfoF.td
@@ -27,7 +27,7 @@
 def frmarg : Operand<XLenVT> {
   let ParserMatchClass = FRMArg;
   let PrintMethod = "printFRMArg";
-  let DecoderMethod = "decodeUImmOperand<3>";
+  let DecoderMethod = "decodeFRMArg";
 }
 
 //===----------------------------------------------------------------------===//
Index: test/MC/RISCV/fuzzer-invalid.txt
===================================================================
--- /dev/null
+++ test/MC/RISCV/fuzzer-invalid.txt
@@ -0,0 +1,9 @@
+# RUN: not llvm-mc -disassemble -triple=riscv32 -mattr=+f,+d < %s 2>&1 | FileCheck %s
+# RUN: not llvm-mc -disassemble -triple=riscv64 -mattr=+f,+d < %s 2>&1 | FileCheck %s
+#
+# Test generated by a LLVM MC Disassembler Protocol Buffer Fuzzer
+# for the RISC-V assembly language.
+
+# This decodes as fadd.s  ft0, ft0, ft0 with unknown floating point rounding mode
+[0x53 0x50 0x00 0x00]
+# CHECK: warning: invalid instruction encoding