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//== MIGChecker.cpp - MIG calling convention checker ------------*- C++ -*--==//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file defines MIGChecker, a Mach Interface Generator calling convention
// checker. Namely, in MIG callback implementation the following rules apply:
// - When a server routine returns an error code that represents success, it
// must take ownership of resources passed to it (and eventually release
// them).
// - Additionally, when returning success, all out-parameters must be
// initialized.
// - When it returns any other error code, it must not take ownership,
// because the message and its out-of-line parameters will be destroyed
// by the client that called the function.
// For now we only check the last rule, as its violations lead to dangerous
// use-after-free exploits.
#include "clang/Analysis/AnyCall.h"
#include "clang/StaticAnalyzer/Checkers/BuiltinCheckerRegistration.h"
#include "clang/StaticAnalyzer/Core/BugReporter/BugType.h"
#include "clang/StaticAnalyzer/Core/Checker.h"
#include "clang/StaticAnalyzer/Core/CheckerManager.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CallEvent.h"
#include "clang/StaticAnalyzer/Core/PathSensitive/CheckerContext.h"
using namespace clang;
using namespace ento;
namespace {
class MIGChecker : public Checker<check::PostCall, check::PreStmt<ReturnStmt>,
check::EndFunction> {
BugType BT{this, "Use-after-free (MIG calling convention violation)",
// The checker knows that an out-of-line object is deallocated if it is
// passed as an argument to one of these functions. If this object is
// additionally an argument of a MIG routine, the checker keeps track of that
// information and issues a warning when an error is returned from the
// respective routine.
std::vector<std::pair<CallDescription, unsigned>> Deallocators = {
#define CALL(required_args, deallocated_arg, ...) \
{{{__VA_ARGS__}, required_args}, deallocated_arg}
// E.g., if the checker sees a C function 'vm_deallocate' that is
// defined on class 'IOUserClient' that has exactly 3 parameters, it knows
// that argument #1 (starting from 0, i.e. the second argument) is going
// to be consumed in the sense of the MIG consume-on-success convention.
CALL(3, 1, "vm_deallocate"),
CALL(3, 1, "mach_vm_deallocate"),
CALL(2, 0, "mig_deallocate"),
CALL(2, 1, "mach_port_deallocate"),
CALL(1, 0, "device_deallocate"),
CALL(1, 0, "iokit_remove_connect_reference"),
CALL(1, 0, "iokit_remove_reference"),
CALL(1, 0, "iokit_release_port"),
CALL(1, 0, "ipc_port_release"),
CALL(1, 0, "ipc_port_release_sonce"),
CALL(1, 0, "ipc_voucher_attr_control_release"),
CALL(1, 0, "ipc_voucher_release"),
CALL(1, 0, "lock_set_dereference"),
CALL(1, 0, "memory_object_control_deallocate"),
CALL(1, 0, "pset_deallocate"),
CALL(1, 0, "semaphore_dereference"),
CALL(1, 0, "space_deallocate"),
CALL(1, 0, "space_inspect_deallocate"),
CALL(1, 0, "task_deallocate"),
CALL(1, 0, "task_inspect_deallocate"),
CALL(1, 0, "task_name_deallocate"),
CALL(1, 0, "thread_deallocate"),
CALL(1, 0, "thread_inspect_deallocate"),
CALL(1, 0, "upl_deallocate"),
CALL(1, 0, "vm_map_deallocate"),
// E.g., if the checker sees a method 'releaseAsyncReference64()' that is
// defined on class 'IOUserClient' that takes exactly 1 argument, it knows
// that the argument is going to be consumed in the sense of the MIG
// consume-on-success convention.
CALL(1, 0, "IOUserClient", "releaseAsyncReference64"),
CALL(1, 0, "IOUserClient", "releaseNotificationPort"),
#undef CALL
CallDescription OsRefRetain{"os_ref_retain", 1};
void checkReturnAux(const ReturnStmt *RS, CheckerContext &C) const;
void checkPostCall(const CallEvent &Call, CheckerContext &C) const;
// HACK: We're making two attempts to find the bug: checkEndFunction
// should normally be enough but it fails when the return value is a literal
// that never gets put into the Environment and ends of function with multiple
// returns get agglutinated across returns, preventing us from obtaining
// the return value. The problem is similar to
// but now we step into it in the top-level function.
void checkPreStmt(const ReturnStmt *RS, CheckerContext &C) const {
checkReturnAux(RS, C);
void checkEndFunction(const ReturnStmt *RS, CheckerContext &C) const {
checkReturnAux(RS, C);
} // end anonymous namespace
// A flag that says that the programmer has called a MIG destructor
// for at least one parameter.
// A set of parameters for which the check is suppressed because
// reference counting is being performed.
REGISTER_SET_WITH_PROGRAMSTATE(RefCountedParameters, const ParmVarDecl *)
static const ParmVarDecl *getOriginParam(SVal V, CheckerContext &C,
bool IncludeBaseRegions = false) {
// TODO: We should most likely always include base regions here.
SymbolRef Sym = V.getAsSymbol(IncludeBaseRegions);
if (!Sym)
return nullptr;
// If we optimistically assume that the MIG routine never re-uses the storage
// that was passed to it as arguments when it invalidates it (but at most when
// it assigns to parameter variables directly), this procedure correctly
// determines if the value was loaded from the transitive closure of MIG
// routine arguments in the heap.
while (const MemRegion *MR = Sym->getOriginRegion()) {
const auto *VR = dyn_cast<VarRegion>(MR);
if (VR && VR->hasStackParametersStorage() &&
return cast<ParmVarDecl>(VR->getDecl());
const SymbolicRegion *SR = MR->getSymbolicBase();
if (!SR)
return nullptr;
Sym = SR->getSymbol();
return nullptr;
static bool isInMIGCall(CheckerContext &C) {
const LocationContext *LC = C.getLocationContext();
assert(LC && "Unknown location context");
const StackFrameContext *SFC;
// Find the top frame.
while (LC) {
SFC = LC->getStackFrame();
LC = SFC->getParent();
const Decl *D = SFC->getDecl();
if (Optional<AnyCall> AC = AnyCall::forDecl(D)) {
// Even though there's a Sema warning when the return type of an annotated
// function is not a kern_return_t, this warning isn't an error, so we need
// an extra sanity check here.
// FIXME: AnyCall doesn't support blocks yet, so they remain unchecked
// for now.
if (!AC->getReturnType(C.getASTContext())
return false;
if (D->hasAttr<MIGServerRoutineAttr>())
return true;
// See if there's an annotated method in the superclass.
if (const auto *MD = dyn_cast<CXXMethodDecl>(D))
for (const auto *OMD: MD->overridden_methods())
if (OMD->hasAttr<MIGServerRoutineAttr>())
return true;
return false;
void MIGChecker::checkPostCall(const CallEvent &Call, CheckerContext &C) const {
if (Call.isCalled(OsRefRetain)) {
// If the code is doing reference counting over the parameter,
// it opens up an opportunity for safely calling a destructor function.
// TODO: We should still check for over-releases.
if (const ParmVarDecl *PVD =
getOriginParam(Call.getArgSVal(0), C, /*IncludeBaseRegions=*/true)) {
// We never need to clean up the program state because these are
// top-level parameters anyway, so they're always live.
if (!isInMIGCall(C))
auto I = llvm::find_if(Deallocators,
[&](const std::pair<CallDescription, unsigned> &Item) {
return Call.isCalled(Item.first);
if (I == Deallocators.end())
ProgramStateRef State = C.getState();
unsigned ArgIdx = I->second;
SVal Arg = Call.getArgSVal(ArgIdx);
const ParmVarDecl *PVD = getOriginParam(Arg, C);
if (!PVD || State->contains<RefCountedParameters>(PVD))
const NoteTag *T = C.getNoteTag([this, PVD](BugReport &BR) -> std::string {
if (&BR.getBugType() != &BT)
return "";
SmallString<64> Str;
llvm::raw_svector_ostream OS(Str);
OS << "Value passed through parameter '" << PVD->getName()
<< "\' is deallocated";
return OS.str();
C.addTransition(State->set<ReleasedParameter>(true), T);
// Returns true if V can potentially represent a "successful" kern_return_t.
static bool mayBeSuccess(SVal V, CheckerContext &C) {
ProgramStateRef State = C.getState();
// Can V represent KERN_SUCCESS?
if (!State->isNull(V).isConstrainedFalse())
return true;
SValBuilder &SVB = C.getSValBuilder();
ASTContext &ACtx = C.getASTContext();
// Can V represent MIG_NO_REPLY?
static const int MigNoReply = -305;
V = SVB.evalEQ(C.getState(), V, SVB.makeIntVal(MigNoReply, ACtx.IntTy));
if (!State->isNull(V).isConstrainedTrue())
return true;
// If none of the above, it's definitely an error.
return false;
void MIGChecker::checkReturnAux(const ReturnStmt *RS, CheckerContext &C) const {
// It is very unlikely that a MIG callback will be called from anywhere
// within the project under analysis and the caller isn't itself a routine
// that follows the MIG calling convention. Therefore we're safe to believe
// that it's always the top frame that is of interest. There's a slight chance
// that the user would want to enforce the MIG calling convention upon
// a random routine in the middle of nowhere, but given that the convention is
// fairly weird and hard to follow in the first place, there's relatively
// little motivation to spread it this way.
if (!C.inTopFrame())
if (!isInMIGCall(C))
// We know that the function is non-void, but what if the return statement
// is not there in the code? It's not a compile error, we should not crash.
if (!RS)
ProgramStateRef State = C.getState();
if (!State->get<ReleasedParameter>())
SVal V = C.getSVal(RS);
if (mayBeSuccess(V, C))
ExplodedNode *N = C.generateErrorNode();
if (!N)
auto R = std::make_unique<PathSensitiveBugReport>(
"MIG callback fails with error after deallocating argument value. "
"This is a use-after-free vulnerability because the caller will try to "
"deallocate it again",
bugreporter::trackExpressionValue(N, RS->getRetValue(), *R,
bugreporter::TrackingKind::Thorough, false);
void ento::registerMIGChecker(CheckerManager &Mgr) {
bool ento::shouldRegisterMIGChecker(const LangOptions &LO) {
return true;