[X86] Add Indirect Thunk Support to X86 to mitigate Load Value Injection (LVI) [3/3]

[sconstab]

Summary

This pass replaces each indirect call/jump with a direct call to a thunk that looks like:

lfence
jmpq *%r11

This ensures that if the value in register %r11 was loaded from memory, then
the value in %r11 is (architecturally) correct prior to the jump.
Also adds a new target feature to X86: +lvi-cfi
("cfi" meaning control-flow integrity)
The feature can be added via clang CLI using -mlvi-cfi.

This is an alternate implementation to https://reviews.llvm.org/D75934 That merges the thunk insertion functionality with the existing X86 retpoline code.

[sconstab]

Added a comment to the header of X86IndirectThunks.cpp to indicate support for LVI thunks.

[craig.topper]

llvm/lib/Target/X86/X86IndirectThunks.cpp

97

Why don't you need the code from retpoline that erases extra BBs?

[sconstab]

@craig.topper I think that removing spurious MBBs is not really necessary because the emitted machine code doesn't contain the spurious MBBs, from what I have observed. I added the check anyways, if only because others may look at this discrepancy and have the same question.

[craig.topper]

LGTM

clang/docs/ClangCommandLineReference.rst

 .. option:: -miphoneos-version-min=<arg>, -mios-version-min=<arg>
 
 .. option:: -mkernel
 
 .. option:: -mlong-calls, -mno-long-calls
 
 Generate branches with extended addressability, usually via indirect jumps.
 
+.. option:: -mlvi-cfi, -mno-lvi-cfi
+
+Enable only control-flow mitigations for Load Value Injection (LVI)
+
 .. option:: -mmacosx-version-min=<arg>, -mmacos-version-min=<arg>
 
 Set Mac OS X deployment target
 
 .. option:: -mmcu=<arg>
 
 .. option:: -mms-bitfields, -mno-ms-bitfields
 

clang/include/clang/Driver/Options.td

 def mno_stackrealign : Flag<["-"], "mno-stackrealign">, Group<m_Group>;
 
 def mretpoline : Flag<["-"], "mretpoline">, Group<m_Group>, Flags<[CoreOption,DriverOption]>;
 def mno_retpoline : Flag<["-"], "mno-retpoline">, Group<m_Group>, Flags<[CoreOption,DriverOption]>;
 def mspeculative_load_hardening : Flag<["-"], "mspeculative-load-hardening">,
   Group<m_Group>, Flags<[CoreOption,CC1Option]>;
 def mno_speculative_load_hardening : Flag<["-"], "mno-speculative-load-hardening">,
   Group<m_Group>, Flags<[CoreOption]>;
+def mlvi_cfi : Flag<["-"], "mlvi-cfi">, Group<m_Group>, Flags<[CoreOption,DriverOption]>,
+  HelpText<"Enable only control-flow mitigations for Load Value Injection (LVI)">;
+def mno_lvi_cfi : Flag<["-"], "mno-lvi-cfi">, Group<m_Group>, Flags<[CoreOption,DriverOption]>,
+  HelpText<"Disable control-flow mitigations for Load Value Injection (LVI)">;
 
 def mrelax : Flag<["-"], "mrelax">, Group<m_riscv_Features_Group>,
   HelpText<"Enable linker relaxation">;
 def mno_relax : Flag<["-"], "mno-relax">, Group<m_riscv_Features_Group>,
   HelpText<"Disable linker relaxation">;
 def msmall_data_limit_EQ : Joined<["-"], "msmall-data-limit=">, Group<m_Group>,
   Alias<G>,
   HelpText<"Put global and static data smaller than the limit into a special section">;

clang/lib/Driver/ToolChains/Arch/X86.cpp

void x86::getX86TargetFeatures(const Driver &D, const llvm::Triple &Triple,
   }
 
   // Translate the high level `-mretpoline` flag to the specific target feature
   // flags. We also detect if the user asked for retpoline external thunks but
   // failed to ask for retpolines themselves (through any of the different
   // flags). This is a bit hacky but keeps existing usages working. We should
   // consider deprecating this and instead warn if the user requests external
   // retpoline thunks and *doesn't* request some form of retpolines.
+  auto SpectreOpt = clang::driver::options::ID::OPT_INVALID;
   if (Args.hasArgNoClaim(options::OPT_mretpoline, options::OPT_mno_retpoline,
                          options::OPT_mspeculative_load_hardening,
                          options::OPT_mno_speculative_load_hardening)) {
     if (Args.hasFlag(options::OPT_mretpoline, options::OPT_mno_retpoline,
                      false)) {
       Features.push_back("+retpoline-indirect-calls");
       Features.push_back("+retpoline-indirect-branches");
+      SpectreOpt = options::OPT_mretpoline;
     } else if (Args.hasFlag(options::OPT_mspeculative_load_hardening,
                             options::OPT_mno_speculative_load_hardening,
                             false)) {
       // On x86, speculative load hardening relies on at least using retpolines
       // for indirect calls.
       Features.push_back("+retpoline-indirect-calls");
+      SpectreOpt = options::OPT_mspeculative_load_hardening;
     }
   } else if (Args.hasFlag(options::OPT_mretpoline_external_thunk,
                           options::OPT_mno_retpoline_external_thunk, false)) {
     // FIXME: Add a warning about failing to specify `-mretpoline` and
     // eventually switch to an error here.
     Features.push_back("+retpoline-indirect-calls");
     Features.push_back("+retpoline-indirect-branches");
+    SpectreOpt = options::OPT_mretpoline_external_thunk;
+  }
+
+  auto LVIOpt = clang::driver::options::ID::OPT_INVALID;
+  if (Args.hasFlag(options::OPT_mlvi_cfi, options::OPT_mno_lvi_cfi, false)) {
+    Features.push_back("+lvi-cfi");
+    LVIOpt = options::OPT_mlvi_cfi;
+  }
+
+  if (SpectreOpt != clang::driver::options::ID::OPT_INVALID &&
+      LVIOpt != clang::driver::options::ID::OPT_INVALID) {
+    D.Diag(diag::err_drv_argument_not_allowed_with)
+        << D.getOpts().getOptionName(SpectreOpt)
+        << D.getOpts().getOptionName(LVIOpt);
   }
 
   // Now add any that the user explicitly requested on the command line,
   // which may override the defaults.
   handleTargetFeaturesGroup(Args, Features, options::OPT_m_x86_Features_Group);
 }

clang/test/Driver/x86-target-features.c

 // RUN: %clang -target i386-linux-gnu -mretpoline -mspeculative-load-hardening %s -### -o %t.o 2>&1 | FileCheck -check-prefix=RETPOLINE %s
 // RUN: %clang -target i386-linux-gnu -mno-speculative-load-hardening %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-SLH %s
 // SLH-NOT: retpoline
 // SLH: "-target-feature" "+retpoline-indirect-calls"
 // SLH-NOT: retpoline
 // SLH: "-mspeculative-load-hardening"
 // NO-SLH-NOT: retpoline
 
+// RUN: %clang -target i386-linux-gnu -mlvi-cfi %s -### -o %t.o 2>&1 | FileCheck -check-prefix=LVICFI %s
+// RUN: %clang -target i386-linux-gnu -mno-lvi-cfi %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-LVICFI %s
+// LVICFI: "-target-feature" "+lvi-cfi"
+// NO-LVICFI-NOT: lvi-cfi
+
 // RUN: %clang -target i386-linux-gnu -mwaitpkg %s -### -o %t.o 2>&1 | FileCheck -check-prefix=WAITPKG %s
 // RUN: %clang -target i386-linux-gnu -mno-waitpkg %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-WAITPKG %s
 // WAITPKG: "-target-feature" "+waitpkg"
 // NO-WAITPKG: "-target-feature" "-waitpkg"
 
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mmovdiri %s -### -o %t.o 2>&1 | FileCheck -check-prefix=MOVDIRI %s
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-movdiri %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-MOVDIRI %s
 // MOVDIRI: "-target-feature" "+movdiri"

llvm/lib/Target/X86/X86.td

 def FeatureRetpolineExternalThunk
     : SubtargetFeature<
           "retpoline-external-thunk", "UseRetpolineExternalThunk", "true",
           "When lowering an indirect call or branch using a `retpoline`, rely "
           "on the specified user provided thunk rather than emitting one "
           "ourselves. Only has effect when combined with some other retpoline "
           "feature", [FeatureRetpolineIndirectCalls]>;
 
+// Mitigate LVI attacks against indirect calls/branches and call returns
+def FeatureLVIControlFlowIntegrity
+    : SubtargetFeature<
+          "lvi-cfi", "UseLVIControlFlowIntegrity", "true",
+          "Prevent indirect calls/branches from using a memory operand, and "
+          "precede all indirect calls/branches from a register with an "
+          "LFENCE instruction to serialize control flow. Also decompose RET "
+          "instructions into a POP+LFENCE+JMP sequence.">;
+
 // Direct Move instructions.
 def FeatureMOVDIRI  : SubtargetFeature<"movdiri", "HasMOVDIRI", "true",
                                        "Support movdiri instruction">;
 def FeatureMOVDIR64B : SubtargetFeature<"movdir64b", "HasMOVDIR64B", "true",
                                         "Support movdir64b instruction">;
 
 def FeatureFastBEXTR : SubtargetFeature<"fast-bextr", "HasFastBEXTR", "true",
           "Indicates that the BEXTR instruction is implemented as a single uop "

llvm/lib/Target/X86/X86ISelLowering.cpp

case X86::EDI:
       assert(!Subtarget.is64Bit() && "Should not be using a 32-bit thunk!");
       return "__llvm_retpoline_edi";
     case X86::R11:
       assert(Subtarget.is64Bit() && "Should not be using a 64-bit thunk!");
       return "__llvm_retpoline_r11";
     }
     llvm_unreachable("unexpected reg for retpoline");
   }
+
+  if (Subtarget.useLVIControlFlowIntegrity()) {
+    assert(Subtarget.is64Bit() && "Should not be using a 64-bit thunk!");
+    return "__llvm_lvi_thunk_r11";
+  }
   llvm_unreachable("getIndirectThunkSymbol() invoked without thunk feature");
 }
 
 MachineBasicBlock *
 X86TargetLowering::EmitLoweredIndirectThunk(MachineInstr &MI,
                                             MachineBasicBlock *BB) const {
   // Copy the virtual register into the R11 physical register and
   // call the retpoline thunk.

llvm/lib/Target/X86/X86IndirectThunks.cpp

 //==- X86IndirectThunks.cpp - Construct indirect call/jump thunks for x86  --=//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 /// \file
 ///
 /// Pass that injects an MI thunk that is used to lower indirect calls in a way
 /// that prevents speculation on some x86 processors and can be used to mitigate
 /// security vulnerabilities due to targeted speculative execution and side
 /// channels such as CVE-2017-5715.
 ///
 /// Currently supported thunks include:
 /// - Retpoline -- A RET-implemented trampoline that lowers indirect calls
+/// - LVI Thunk -- A CALL/JMP-implemented thunk that forces load serialization
+///   before making an indirect call/jump
 ///
 /// Note that the reason that this is implemented as a MachineFunctionPass and
 /// not a ModulePass is that ModulePasses at this point in the LLVM X86 pipeline
 /// serialize all transformations, which can consume lots of memory.
 ///
 /// TODO(chandlerc): All of this code could use better comments and
 /// documentation.
 ///

 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
 #define DEBUG_TYPE "x86-retpoline-thunks"
 
 static const char RetpolineNamePrefix[] = "__llvm_retpoline_";
-static const char R11RetpolineName[]    = "__llvm_retpoline_r11";
-static const char EAXRetpolineName[]    = "__llvm_retpoline_eax";
-static const char ECXRetpolineName[]    = "__llvm_retpoline_ecx";
-static const char EDXRetpolineName[]    = "__llvm_retpoline_edx";
-static const char EDIRetpolineName[]    = "__llvm_retpoline_edi";
+static const char R11RetpolineName[] = "__llvm_retpoline_r11";
+static const char EAXRetpolineName[] = "__llvm_retpoline_eax";
+static const char ECXRetpolineName[] = "__llvm_retpoline_ecx";
+static const char EDXRetpolineName[] = "__llvm_retpoline_edx";
+static const char EDIRetpolineName[] = "__llvm_retpoline_edi";
 
+static const char LVIThunkNamePrefix[] = "__llvm_lvi_thunk_";
+static const char R11LVIThunkName[] = "__llvm_lvi_thunk_r11";
+
 namespace {
 template <typename Derived> class ThunkInserter {
   Derived &getDerived() { return *static_cast<Derived *>(this); }
 
 protected:
   bool InsertedThunks;
   void doInitialization(Module &M) {}
   void createThunkFunction(MachineModuleInfo &MMI, StringRef Name);
bool mayUseThunk(const MachineFunction &MF) {
     return (STI.useRetpolineIndirectCalls() ||
             STI.useRetpolineIndirectBranches()) &&
            !STI.useRetpolineExternalThunk();
   }
   void insertThunks(MachineModuleInfo &MMI);
   void populateThunk(MachineFunction &MF);
 };
 
+struct LVIThunkInserter : ThunkInserter<LVIThunkInserter> {
+  const char *getThunkPrefix() { return LVIThunkNamePrefix; }
+  bool mayUseThunk(const MachineFunction &MF) {
+    return MF.getSubtarget<X86Subtarget>().useLVIControlFlowIntegrity();
+  }
+  void insertThunks(MachineModuleInfo &MMI) {
+    createThunkFunction(MMI, R11LVIThunkName);
+  }
+  void populateThunk(MachineFunction &MF) {
+    // Grab the entry MBB and erase any other blocks. O0 codegen appears to
+    // generate two bbs for the entry block.
+    MachineBasicBlock *Entry = &MF.front();
+    Entry->clear();
+    while (MF.size() > 1)
+      MF.erase(std::next(MF.begin()));
+
+    // This code mitigates LVI by replacing each indirect call/jump with a
+    // direct call/jump to a thunk that looks like:
+    // ```
+    // lfence
+    // jmpq *%r11
+    // ```
+    // This ensures that if the value in register %r11 was loaded from memory,
+    // then the value in %r11 is (architecturally) correct prior to the jump.
+    const TargetInstrInfo *TII = MF.getSubtarget<X86Subtarget>().getInstrInfo();
+    BuildMI(&MF.front(), DebugLoc(), TII->get(X86::LFENCE));
+    BuildMI(&MF.front(), DebugLoc(), TII->get(X86::JMP64r)).addReg(X86::R11);
+    MF.front().addLiveIn(X86::R11);
+    return;
+  }
+};
+
 class X86IndirectThunks : public MachineFunctionPass {
 public:
   static char ID;
 
   X86IndirectThunks() : MachineFunctionPass(ID) {}
 
   StringRef getPassName() const override { return "X86 Indirect Thunks"; }
 
   bool doInitialization(Module &M) override;
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   void getAnalysisUsage(AnalysisUsage &AU) const override {
     MachineFunctionPass::getAnalysisUsage(AU);
     AU.addRequired<MachineModuleInfoWrapperPass>();
     AU.addPreserved<MachineModuleInfoWrapperPass>();
   }
 
 private:
-  std::tuple<RetpolineThunkInserter> TIs;
+  std::tuple<RetpolineThunkInserter, LVIThunkInserter> TIs;
 
   // FIXME: When LLVM moves to C++17, these can become folds
   template <typename... ThunkInserterT>
   static void initTIs(Module &M,
                       std::tuple<ThunkInserterT...> &ThunkInserters) {
     (void)std::initializer_list<int>{
         (std::get<ThunkInserterT>(ThunkInserters).init(M), 0)...};
   }

llvm/lib/Target/X86/X86Subtarget.h

protected:
   /// Deprecated flag, query `UseRetpolineIndirectCalls` and
   /// `UseRetpolineIndirectBranches` instead.
   bool DeprecatedUseRetpoline = false;
 
   /// When using a retpoline thunk, call an externally provided thunk rather
   /// than emitting one inside the compiler.
   bool UseRetpolineExternalThunk = false;
 
+  /// Prevent generation of indirect call/branch instructions from memory,
+  /// and force all indirect call/branch instructions from a register to be
+  /// preceded by an LFENCE. Also decompose RET instructions into a
+  /// POP+LFENCE+JMP sequence.
+  bool UseLVIControlFlowIntegrity = false;
+
   /// Use software floating point for code generation.
   bool UseSoftFloat = false;
 
   /// Use alias analysis during code generation.
   bool UseAA = false;
 
   /// The minimum alignment known to hold of the stack frame on
   /// entry to the function and which must be maintained by every function.
public:
   bool useRetpolineIndirectBranches() const {
     return UseRetpolineIndirectBranches;
   }
   bool useRetpolineExternalThunk() const { return UseRetpolineExternalThunk; }
 
   // These are generic getters that OR together all of the thunk types
   // supported by the subtarget. Therefore useIndirectThunk*() will return true
   // if any respective thunk feature is enabled.
-  bool useIndirectThunkCalls() const { return useRetpolineIndirectCalls(); }
+  bool useIndirectThunkCalls() const {
+    return useRetpolineIndirectCalls() || useLVIControlFlowIntegrity();
+  }
   bool useIndirectThunkBranches() const {
-    return useRetpolineIndirectBranches();
+    return useRetpolineIndirectBranches() || useLVIControlFlowIntegrity();
   }
 
   bool preferMaskRegisters() const { return PreferMaskRegisters; }
   bool useGLMDivSqrtCosts() const { return UseGLMDivSqrtCosts; }
+  bool useLVIControlFlowIntegrity() const { return UseLVIControlFlowIntegrity; }
 
   unsigned getPreferVectorWidth() const { return PreferVectorWidth; }
   unsigned getRequiredVectorWidth() const { return RequiredVectorWidth; }
 
   // Helper functions to determine when we should allow widening to 512-bit
   // during codegen.
   // TODO: Currently we're always allowing widening on CPUs without VLX,
   // because for many cases we don't have a better option.

llvm/test/CodeGen/X86/lvi-hardening-indirectbr.ll

+; RUN: llc -verify-machineinstrs -mtriple=x86_64-unknown -mattr=+lvi-cfi < %s | FileCheck %s --check-prefix=X64
+; RUN: llc -verify-machineinstrs -mtriple=x86_64-unknown -mattr=+lvi-cfi -O0 < %s | FileCheck %s --check-prefix=X64FAST
+;
+; Note that a lot of this code was lifted from retpoline.ll.
+
+declare void @bar(i32)
+
+; Test a simple indirect call and tail call.
+define void @icall_reg(void (i32)* %fp, i32 %x) {
+entry:
+  tail call void @bar(i32 %x)
+  tail call void %fp(i32 %x)
+  tail call void @bar(i32 %x)
+  tail call void %fp(i32 %x)
+  ret void
+}
+
+; X64-LABEL: icall_reg:
+; X64-DAG:   movq %rdi, %[[fp:[^ ]*]]
+; X64-DAG:   movl %esi, %[[x:[^ ]*]]
+; X64:       movl %esi, %edi
+; X64:       callq bar
+; X64-DAG:   movl %[[x]], %edi
+; X64-DAG:   movq %[[fp]], %r11
+; X64:       callq __llvm_lvi_thunk_r11
+; X64:       movl %[[x]], %edi
+; X64:       callq bar
+; X64-DAG:   movl %[[x]], %edi
+; X64-DAG:   movq %[[fp]], %r11
+; X64:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+; X64FAST-LABEL: icall_reg:
+; X64FAST:       callq bar
+; X64FAST:       callq __llvm_lvi_thunk_r11
+; X64FAST:       callq bar
+; X64FAST:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+
+@global_fp = external global void (i32)*
+
+; Test an indirect call through a global variable.
+define void @icall_global_fp(i32 %x, void (i32)** %fpp) #0 {
+  %fp1 = load void (i32)*, void (i32)** @global_fp
+  call void %fp1(i32 %x)
+  %fp2 = load void (i32)*, void (i32)** @global_fp
+  tail call void %fp2(i32 %x)
+  ret void
+}
+
+; X64-LABEL: icall_global_fp:
+; X64-DAG:   movl %edi, %[[x:[^ ]*]]
+; X64-DAG:   movq global_fp(%rip), %r11
+; X64:       callq __llvm_lvi_thunk_r11
+; X64-DAG:   movl %[[x]], %edi
+; X64-DAG:   movq global_fp(%rip), %r11
+; X64:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+; X64FAST-LABEL: icall_global_fp:
+; X64FAST:       movq global_fp(%rip), %r11
+; X64FAST:       callq __llvm_lvi_thunk_r11
+; X64FAST:       movq global_fp(%rip), %r11
+; X64FAST:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+
+%struct.Foo = type { void (%struct.Foo*)** }
+
+; Test an indirect call through a vtable.
+define void @vcall(%struct.Foo* %obj) #0 {
+  %vptr_field = getelementptr %struct.Foo, %struct.Foo* %obj, i32 0, i32 0
+  %vptr = load void (%struct.Foo*)**, void (%struct.Foo*)*** %vptr_field
+  %vslot = getelementptr void(%struct.Foo*)*, void(%struct.Foo*)** %vptr, i32 1
+  %fp = load void(%struct.Foo*)*, void(%struct.Foo*)** %vslot
+  tail call void %fp(%struct.Foo* %obj)
+  tail call void %fp(%struct.Foo* %obj)
+  ret void
+}
+
+; X64-LABEL: vcall:
+; X64:       movq %rdi, %[[obj:[^ ]*]]
+; X64:       movq (%rdi), %[[vptr:[^ ]*]]
+; X64:       movq 8(%[[vptr]]), %[[fp:[^ ]*]]
+; X64:       movq %[[fp]], %r11
+; X64:       callq __llvm_lvi_thunk_r11
+; X64-DAG:   movq %[[obj]], %rdi
+; X64-DAG:   movq %[[fp]], %r11
+; X64:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+; X64FAST-LABEL: vcall:
+; X64FAST:       callq __llvm_lvi_thunk_r11
+; X64FAST:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+
+declare void @direct_callee()
+
+define void @direct_tail() #0 {
+  tail call void @direct_callee()
+  ret void
+}
+
+; X64-LABEL: direct_tail:
+; X64:       jmp direct_callee # TAILCALL
+; X64FAST-LABEL: direct_tail:
+; X64FAST:   jmp direct_callee # TAILCALL
+
+
+declare void @nonlazybind_callee() #1
+
+define void @nonlazybind_caller() #0 {
+  call void @nonlazybind_callee()
+  tail call void @nonlazybind_callee()
+  ret void
+}
+
+; X64-LABEL: nonlazybind_caller:
+; X64:       movq nonlazybind_callee@GOTPCREL(%rip), %[[REG:.*]]
+; X64:       movq %[[REG]], %r11
+; X64:       callq __llvm_lvi_thunk_r11
+; X64:       movq %[[REG]], %r11
+; X64:       jmp __llvm_lvi_thunk_r11 # TAILCALL
+; X64FAST-LABEL: nonlazybind_caller:
+; X64FAST:   movq nonlazybind_callee@GOTPCREL(%rip), %r11
+; X64FAST:   callq __llvm_lvi_thunk_r11
+; X64FAST:   movq nonlazybind_callee@GOTPCREL(%rip), %r11
+; X64FAST:   jmp __llvm_lvi_thunk_r11 # TAILCALL
+
+
+; Check that a switch gets lowered using a jump table
+define void @switch_jumptable(i32* %ptr, i64* %sink) #0 {
+; X64-LABEL: switch_jumptable:
+; X64_NOT:      jmpq *
+entry:
+  br label %header
+
+header:
+  %i = load volatile i32, i32* %ptr
+  switch i32 %i, label %bb0 [
+    i32 1, label %bb1
+    i32 2, label %bb2
+    i32 3, label %bb3
+    i32 4, label %bb4
+    i32 5, label %bb5
+    i32 6, label %bb6
+    i32 7, label %bb7
+    i32 8, label %bb8
+    i32 9, label %bb9
+  ]
+
+bb0:
+  store volatile i64 0, i64* %sink
+  br label %header
+
+bb1:
+  store volatile i64 1, i64* %sink
+  br label %header
+
+bb2:
+  store volatile i64 2, i64* %sink
+  br label %header
+
+bb3:
+  store volatile i64 3, i64* %sink
+  br label %header
+
+bb4:
+  store volatile i64 4, i64* %sink
+  br label %header
+
+bb5:
+  store volatile i64 5, i64* %sink
+  br label %header
+
+bb6:
+  store volatile i64 6, i64* %sink
+  br label %header
+
+bb7:
+  store volatile i64 7, i64* %sink
+  br label %header
+
+bb8:
+  store volatile i64 8, i64* %sink
+  br label %header
+
+bb9:
+  store volatile i64 9, i64* %sink
+  br label %header
+}
+
+
+@indirectbr_rewrite.targets = constant [10 x i8*] [i8* blockaddress(@indirectbr_rewrite, %bb0),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb1),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb2),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb3),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb4),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb5),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb6),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb7),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb8),
+                                                   i8* blockaddress(@indirectbr_rewrite, %bb9)]
+
+; Check that when thunks are enabled the indirectbr instruction gets
+; rewritten to use switch, and that in turn doesn't get lowered as a jump
+; table.
+define void @indirectbr_rewrite(i64* readonly %p, i64* %sink) #0 {
+; X64-LABEL: indirectbr_rewrite:
+; X64-NOT:     jmpq *
+entry:
+  %i0 = load i64, i64* %p
+  %target.i0 = getelementptr [10 x i8*], [10 x i8*]* @indirectbr_rewrite.targets, i64 0, i64 %i0
+  %target0 = load i8*, i8** %target.i0
+  indirectbr i8* %target0, [label %bb1, label %bb3]
+
+bb0:
+  store volatile i64 0, i64* %sink
+  br label %latch
+
+bb1:
+  store volatile i64 1, i64* %sink
+  br label %latch
+
+bb2:
+  store volatile i64 2, i64* %sink
+  br label %latch
+
+bb3:
+  store volatile i64 3, i64* %sink
+  br label %latch
+
+bb4:
+  store volatile i64 4, i64* %sink
+  br label %latch
+
+bb5:
+  store volatile i64 5, i64* %sink
+  br label %latch
+
+bb6:
+  store volatile i64 6, i64* %sink
+  br label %latch
+
+bb7:
+  store volatile i64 7, i64* %sink
+  br label %latch
+
+bb8:
+  store volatile i64 8, i64* %sink
+  br label %latch
+
+bb9:
+  store volatile i64 9, i64* %sink
+  br label %latch
+
+latch:
+  %i.next = load i64, i64* %p
+  %target.i.next = getelementptr [10 x i8*], [10 x i8*]* @indirectbr_rewrite.targets, i64 0, i64 %i.next
+  %target.next = load i8*, i8** %target.i.next
+  ; Potentially hit a full 10 successors here so that even if we rewrite as
+  ; a switch it will try to be lowered with a jump table.
+  indirectbr i8* %target.next, [label %bb0,
+                                label %bb1,
+                                label %bb2,
+                                label %bb3,
+                                label %bb4,
+                                label %bb5,
+                                label %bb6,
+                                label %bb7,
+                                label %bb8,
+                                label %bb9]
+}
+
+; Lastly check that the necessary thunks were emitted.
+;
+; X64-LABEL:         .section        .text.__llvm_lvi_thunk_r11,{{.*}},__llvm_lvi_thunk_r11,comdat
+; X64-NEXT:          .hidden __llvm_lvi_thunk_r11
+; X64-NEXT:          .weak   __llvm_lvi_thunk_r11
+; X64:       __llvm_lvi_thunk_r11:
+; X64-NEXT:  # {{.*}}                                # %entry
+; X64-NEXT:          lfence
+; X64-NEXT:          jmpq     *%r11
+
+attributes #1 = { nonlazybind }