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Summary: First, we need to explain the core of the vulnerability. Note that this is a very incomplete description, please see the Project Zero blog post for details: https://googleprojectzero.blogspot.com/2018/01/reading-privileged-memory-with-side.html The basis for branch target injection is to direct speculative execution of the processor to some "gadget" of executable code by poisoning the prediction of indirect branches with the address of that gadget. The gadget in turn contains an operation that provides a side channel for reading data. Most commonly, this will look like a load of secret data followed by a branch on the loaded value and then a load of some predictable cache line. The attacker then uses timing of the processors cache to determine which direction the branch took *in the speculative execution*, and in turn what one bit of the loaded value was. Due to the nature of these timing side channels and the branch predictor on Intel processors, this allows an attacker to leak data only accessible to a privileged domain (like the kernel) back into an unprivileged domain. The goal is simple: avoid generating code which contains an indirect branch that could have its prediction poisoned by an attacker. In many cases, the compiler can simply use directed conditional branches and a small search tree. LLVM already has support for lowering switches in this way and the first step of this patch is to disable jump-table lowering of switches and introduce a pass to rewrite explicit indirectbr sequences into a switch over integers. However, there is no fully general alternative to indirect calls. We introduce a new construct we call a "retpoline" to implement indirect calls in a non-speculatable way. It can be thought of loosely as a trampoline for indirect calls which uses the RET instruction on x86. Further, we arrange for a specific call->ret sequence which ensures the processor predicts the return to go to a controlled, known location. The retpoline then "smashes" the return address pushed onto the stack by the call with the desired target of the original indirect call. The result is a predicted return to the next instruction after a call (which can be used to trap speculative execution within an infinite loop) and an actual indirect branch to an arbitrary address. On 64-bit x86 ABIs, this is especially easily done in the compiler by using a guaranteed scratch register to pass the target into this device. For 32-bit ABIs there isn't a guaranteed scratch register and so several different retpoline variants are introduced to use a scratch register if one is available in the calling convention and to otherwise use direct stack push/pop sequences to pass the target address. This "retpoline" mitigation is fully described in the following blog post: https://support.google.com/faqs/answer/7625886 We also support a target feature that disables emission of the retpoline thunk by the compiler to allow for custom thunks if users want them. These are particularly useful in environments like kernels that routinely do hot-patching on boot and want to hot-patch their thunk to different code sequences. They can write this custom thunk and use `-mretpoline-external-thunk` *in addition* to `-mretpoline`. In this case, on x86-64 thu thunk names must be: ``` __llvm_external_retpoline_r11 ``` or on 32-bit: ``` __llvm_external_retpoline_eax __llvm_external_retpoline_ecx __llvm_external_retpoline_edx __llvm_external_retpoline_push ``` And the target of the retpoline is passed in the named register, or in the case of the `push` suffix on the top of the stack via a `pushl` instruction. There is one other important source of indirect branches in x86 ELF binaries: the PLT. These patches also include support for LLD to generate PLT entries that perform a retpoline-style indirection. The only other indirect branches remaining that we are aware of are from precompiled runtimes (such as crt0.o and similar). The ones we have found are not really attackable, and so we have not focused on them here, but eventually these runtimes should also be replicated for retpoline-ed configurations for completeness. For kernels or other freestanding or fully static executables, the compiler switch `-mretpoline` is sufficient to fully mitigate this particular attack. For dynamic executables, you must compile *all* libraries with `-mretpoline` and additionally link the dynamic executable and all shared libraries with LLD and pass `-z retpolineplt` (or use similar functionality from some other linker). We strongly recommend also using `-z now` as non-lazy binding allows the retpoline-mitigated PLT to be substantially smaller. When manually apply similar transformations to `-mretpoline` to the Linux kernel we observed very small performance hits to applications running typical workloads, and relatively minor hits (approximately 2%) even for extremely syscall-heavy applications. This is largely due to the small number of indirect branches that occur in performance sensitive paths of the kernel. When using these patches on statically linked applications, especially C++ applications, you should expect to see a much more dramatic performance hit. For microbenchmarks that are switch, indirect-, or virtual-call heavy we have seen overheads ranging from 10% to 50%. However, real-world workloads exhibit substantially lower performance impact. Notably, techniques such as PGO and ThinLTO dramatically reduce the impact of hot indirect calls (by speculatively promoting them to direct calls) and allow optimized search trees to be used to lower switches. If you need to deploy these techniques in C++ applications, we *strongly* recommend that you ensure all hot call targets are statically linked (avoiding PLT indirection) and use both PGO and ThinLTO. Well tuned servers using all of these techniques saw 5% - 10% overhead from the use of retpoline. We will add detailed documentation covering these components in subsequent patches, but wanted to make the core functionality available as soon as possible. Happy for more code review, but we'd really like to get these patches landed and backported ASAP for obvious reasons. We're planning to backport this to both 6.0 and 5.0 release streams and get a 5.0 release with just this cherry picked ASAP for distros and vendors. This patch is the work of a number of people over the past month: Eric, Reid, Rui, and myself. I'm mailing it out as a single commit due to the time sensitive nature of landing this and the need to backport it. Huge thanks to everyone who helped out here, and everyone at Intel who helped out in discussions about how to craft this. Also, credit goes to Paul Turner (at Google, but not an LLVM contributor) for much of the underlying retpoline design. Reviewers: echristo, rnk, ruiu, craig.topper, DavidKreitzer Subscribers: sanjoy, emaste, mcrosier, mgorny, mehdi_amini, hiraditya, llvm-commits Differential Revision: https://reviews.llvm.org/D41723 llvm-svn: 323155
277 lines
9.8 KiB
C++
277 lines
9.8 KiB
C++
//======- X86RetpolineThunks.cpp - Construct retpoline thunks for x86 --=====//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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/// \file
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///
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/// Pass that injects an MI thunk implementing a "retpoline". This is
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/// a RET-implemented trampoline that is used to lower indirect calls in a way
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/// that prevents speculation on some x86 processors and can be used to mitigate
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/// security vulnerabilities due to targeted speculative execution and side
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/// channels such as CVE-2017-5715.
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///
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/// TODO(chandlerc): All of this code could use better comments and
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/// documentation.
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///
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//===----------------------------------------------------------------------===//
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#include "X86.h"
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#include "X86InstrBuilder.h"
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#include "X86Subtarget.h"
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#include "llvm/CodeGen/MachineFunction.h"
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#include "llvm/CodeGen/MachineInstrBuilder.h"
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#include "llvm/CodeGen/MachineModuleInfo.h"
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#include "llvm/CodeGen/Passes.h"
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#include "llvm/CodeGen/TargetPassConfig.h"
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#include "llvm/IR/IRBuilder.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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using namespace llvm;
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#define DEBUG_TYPE "x86-retpoline-thunks"
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namespace {
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class X86RetpolineThunks : public ModulePass {
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public:
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static char ID;
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X86RetpolineThunks() : ModulePass(ID) {}
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StringRef getPassName() const override { return "X86 Retpoline Thunks"; }
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bool runOnModule(Module &M) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<MachineModuleInfo>();
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AU.addPreserved<MachineModuleInfo>();
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}
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private:
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MachineModuleInfo *MMI;
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const TargetMachine *TM;
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bool Is64Bit;
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const X86Subtarget *STI;
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const X86InstrInfo *TII;
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Function *createThunkFunction(Module &M, StringRef Name);
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void insertRegReturnAddrClobber(MachineBasicBlock &MBB, unsigned Reg);
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void insert32BitPushReturnAddrClobber(MachineBasicBlock &MBB);
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void createThunk(Module &M, StringRef NameSuffix,
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Optional<unsigned> Reg = None);
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};
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} // end anonymous namespace
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ModulePass *llvm::createX86RetpolineThunksPass() {
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return new X86RetpolineThunks();
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}
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char X86RetpolineThunks::ID = 0;
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bool X86RetpolineThunks::runOnModule(Module &M) {
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DEBUG(dbgs() << getPassName() << '\n');
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auto *TPC = getAnalysisIfAvailable<TargetPassConfig>();
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assert(TPC && "X86-specific target pass should not be run without a target "
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"pass config!");
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MMI = &getAnalysis<MachineModuleInfo>();
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TM = &TPC->getTM<TargetMachine>();
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Is64Bit = TM->getTargetTriple().getArch() == Triple::x86_64;
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// Only add a thunk if we have at least one function that has the retpoline
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// feature enabled in its subtarget.
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// FIXME: Conditionalize on indirect calls so we don't emit a thunk when
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// nothing will end up calling it.
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// FIXME: It's a little silly to look at every function just to enumerate
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// the subtargets, but eventually we'll want to look at them for indirect
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// calls, so maybe this is OK.
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if (!llvm::any_of(M, [&](const Function &F) {
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// Save the subtarget we find for use in emitting the subsequent
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// thunk.
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STI = &TM->getSubtarget<X86Subtarget>(F);
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return STI->useRetpoline() && !STI->useRetpolineExternalThunk();
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}))
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return false;
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// If we have a relevant subtarget, get the instr info as well.
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TII = STI->getInstrInfo();
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if (Is64Bit) {
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// __llvm_retpoline_r11:
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// callq .Lr11_call_target
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// .Lr11_capture_spec:
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// pause
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// lfence
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// jmp .Lr11_capture_spec
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// .align 16
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// .Lr11_call_target:
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// movq %r11, (%rsp)
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// retq
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createThunk(M, "r11", X86::R11);
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} else {
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// For 32-bit targets we need to emit a collection of thunks for various
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// possible scratch registers as well as a fallback that is used when
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// there are no scratch registers and assumes the retpoline target has
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// been pushed.
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// __llvm_retpoline_eax:
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// calll .Leax_call_target
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// .Leax_capture_spec:
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// pause
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// jmp .Leax_capture_spec
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// .align 16
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// .Leax_call_target:
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// movl %eax, (%esp) # Clobber return addr
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// retl
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//
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// __llvm_retpoline_ecx:
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// ... # Same setup
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// movl %ecx, (%esp)
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// retl
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//
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// __llvm_retpoline_edx:
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// ... # Same setup
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// movl %edx, (%esp)
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// retl
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//
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// This last one is a bit more special and so needs a little extra
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// handling.
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// __llvm_retpoline_push:
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// calll .Lpush_call_target
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// .Lpush_capture_spec:
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// pause
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// lfence
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// jmp .Lpush_capture_spec
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// .align 16
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// .Lpush_call_target:
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// # Clear pause_loop return address.
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// addl $4, %esp
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// # Top of stack words are: Callee, RA. Exchange Callee and RA.
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// pushl 4(%esp) # Push callee
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// pushl 4(%esp) # Push RA
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// popl 8(%esp) # Pop RA to final RA
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// popl (%esp) # Pop callee to next top of stack
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// retl # Ret to callee
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createThunk(M, "eax", X86::EAX);
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createThunk(M, "ecx", X86::ECX);
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createThunk(M, "edx", X86::EDX);
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createThunk(M, "push");
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}
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return true;
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}
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Function *X86RetpolineThunks::createThunkFunction(Module &M, StringRef Name) {
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LLVMContext &Ctx = M.getContext();
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auto Type = FunctionType::get(Type::getVoidTy(Ctx), false);
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Function *F =
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Function::Create(Type, GlobalValue::LinkOnceODRLinkage, Name, &M);
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F->setVisibility(GlobalValue::HiddenVisibility);
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F->setComdat(M.getOrInsertComdat(Name));
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// Add Attributes so that we don't create a frame, unwind information, or
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// inline.
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AttrBuilder B;
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B.addAttribute(llvm::Attribute::NoUnwind);
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B.addAttribute(llvm::Attribute::Naked);
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F->addAttributes(llvm::AttributeList::FunctionIndex, B);
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// Populate our function a bit so that we can verify.
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BasicBlock *Entry = BasicBlock::Create(Ctx, "entry", F);
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IRBuilder<> Builder(Entry);
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Builder.CreateRetVoid();
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return F;
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}
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void X86RetpolineThunks::insertRegReturnAddrClobber(MachineBasicBlock &MBB,
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unsigned Reg) {
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const unsigned MovOpc = Is64Bit ? X86::MOV64mr : X86::MOV32mr;
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const unsigned SPReg = Is64Bit ? X86::RSP : X86::ESP;
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addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(MovOpc)), SPReg, false, 0)
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.addReg(Reg);
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}
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void X86RetpolineThunks::insert32BitPushReturnAddrClobber(
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MachineBasicBlock &MBB) {
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// The instruction sequence we use to replace the return address without
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// a scratch register is somewhat complicated:
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// # Clear capture_spec from return address.
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// addl $4, %esp
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// # Top of stack words are: Callee, RA. Exchange Callee and RA.
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// pushl 4(%esp) # Push callee
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// pushl 4(%esp) # Push RA
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// popl 8(%esp) # Pop RA to final RA
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// popl (%esp) # Pop callee to next top of stack
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// retl # Ret to callee
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BuildMI(&MBB, DebugLoc(), TII->get(X86::ADD32ri), X86::ESP)
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.addReg(X86::ESP)
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.addImm(4);
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addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(X86::PUSH32rmm)), X86::ESP,
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false, 4);
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addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(X86::PUSH32rmm)), X86::ESP,
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false, 4);
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addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(X86::POP32rmm)), X86::ESP,
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false, 8);
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addRegOffset(BuildMI(&MBB, DebugLoc(), TII->get(X86::POP32rmm)), X86::ESP,
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false, 0);
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}
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void X86RetpolineThunks::createThunk(Module &M, StringRef NameSuffix,
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Optional<unsigned> Reg) {
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Function &F =
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*createThunkFunction(M, (Twine("__llvm_retpoline_") + NameSuffix).str());
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MachineFunction &MF = MMI->getOrCreateMachineFunction(F);
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// Set MF properties. We never use vregs...
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MF.getProperties().set(MachineFunctionProperties::Property::NoVRegs);
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BasicBlock &OrigEntryBB = F.getEntryBlock();
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MachineBasicBlock *Entry = MF.CreateMachineBasicBlock(&OrigEntryBB);
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MachineBasicBlock *CaptureSpec = MF.CreateMachineBasicBlock(&OrigEntryBB);
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MachineBasicBlock *CallTarget = MF.CreateMachineBasicBlock(&OrigEntryBB);
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MF.push_back(Entry);
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MF.push_back(CaptureSpec);
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MF.push_back(CallTarget);
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const unsigned CallOpc = Is64Bit ? X86::CALL64pcrel32 : X86::CALLpcrel32;
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const unsigned RetOpc = Is64Bit ? X86::RETQ : X86::RETL;
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BuildMI(Entry, DebugLoc(), TII->get(CallOpc)).addMBB(CallTarget);
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Entry->addSuccessor(CallTarget);
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Entry->addSuccessor(CaptureSpec);
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CallTarget->setHasAddressTaken();
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// In the capture loop for speculation, we want to stop the processor from
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// speculating as fast as possible. On Intel processors, the PAUSE instruction
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// will block speculation without consuming any execution resources. On AMD
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// processors, the PAUSE instruction is (essentially) a nop, so we also use an
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// LFENCE instruction which they have advised will stop speculation as well
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// with minimal resource utilization. We still end the capture with a jump to
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// form an infinite loop to fully guarantee that no matter what implementation
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// of the x86 ISA, speculating this code path never escapes.
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BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::PAUSE));
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BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::LFENCE));
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BuildMI(CaptureSpec, DebugLoc(), TII->get(X86::JMP_1)).addMBB(CaptureSpec);
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CaptureSpec->setHasAddressTaken();
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CaptureSpec->addSuccessor(CaptureSpec);
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CallTarget->setAlignment(4);
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if (Reg) {
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insertRegReturnAddrClobber(*CallTarget, *Reg);
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} else {
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assert(!Is64Bit && "We only support non-reg thunks on 32-bit x86!");
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insert32BitPushReturnAddrClobber(*CallTarget);
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}
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BuildMI(CallTarget, DebugLoc(), TII->get(RetOpc));
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}
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