[mips] Spectre variant two mitigation for MIPSR2

This patch provides mitigation for CVE-2017-5715, Spectre variant two,
which affects the P5600 and P6600. It provides the option
-mindirect-jump=hazard, which instructs the LLVM backend to replace
indirect branches with their hazard barrier variants.

This option is accepted when targeting MIPS revision two or later.

The migitation strategy suggested by MIPS for these processors is to
use two hazard barrier instructions. 'jalr.hb' and 'jr.hb' are hazard
barrier variants of the 'jalr' and 'jr' instructions respectively.

These instructions impede the execution of instruction stream until
architecturally defined hazards (changes to the instruction stream,
privileged registers which may affect execution) are cleared. These
instructions in MIPS' designs are not speculated past.

These instructions are used with the option -mindirect-jump=hazard
when branching indirectly and for indirect function calls.

These instructions are defined by the MIPS32R2 ISA, so this mitigation
method is not compatible with processors which implement an earlier
revision of the MIPS ISA.

Implementation note: I've opted to provide this as an
-mindirect-jump={hazard,...} style option in case alternative
mitigation methods are required for other implementations of the MIPS
ISA in future, e.g. retpoline style solutions.

Reviewers: atanasyan

Differential Revision: https://reviews.llvm.org/D43487

llvm-svn: 325651

clang/include/clang/Basic/DiagnosticDriverKinds.td

@@ -333,6 +333,10 @@ def warn_drv_unsupported_abicalls : Warning<
   "ignoring '-mabicalls' option as it cannot be used with "
   "non position-independent code and the N64 ABI">,
   InGroup<OptionIgnored>;
+def err_drv_unsupported_indirect_jump_opt : Error<
+  "'-mindirect-jump=%0' is unsupported with the '%1' architecture">;
+def err_drv_unknown_indirect_jump_opt : Error<
+  "unknown '-mindirect-jump=' option '%0'">;
 
 def warn_drv_unable_to_find_directory_expected : Warning<
   "unable to find %0 directory, expected to be in '%1'">,

clang/include/clang/Driver/Options.td

@@ -2022,6 +2022,9 @@ def mbranch_likely : Flag<["-"], "mbranch-likely">, Group<m_Group>,
   IgnoredGCCCompat;
 def mno_branch_likely : Flag<["-"], "mno-branch-likely">, Group<m_Group>,
   IgnoredGCCCompat;
+def mindirect_jump_EQ : Joined<["-"], "mindirect-jump=">,
+  Group<m_Group>,
+  HelpText<"Change indirect jump instructions to inhibit speculation">;
 def mdsp : Flag<["-"], "mdsp">, Group<m_Group>;
 def mno_dsp : Flag<["-"], "mno-dsp">, Group<m_Group>;
 def mdspr2 : Flag<["-"], "mdspr2">, Group<m_Group>;

clang/lib/Basic/Targets/Mips.h

@@ -54,6 +54,7 @@ class LLVM_LIBRARY_VISIBILITY MipsTargetInfo : public TargetInfo {
   enum DspRevEnum { NoDSP, DSP1, DSP2 } DspRev;
   bool HasMSA;
   bool DisableMadd4;
+  bool UseIndirectJumpHazard;
 
 protected:
   bool HasFP64;
@@ -64,7 +65,8 @@ public:
       : TargetInfo(Triple), IsMips16(false), IsMicromips(false),
         IsNan2008(false), IsAbs2008(false), IsSingleFloat(false),
         IsNoABICalls(false), CanUseBSDABICalls(false), FloatABI(HardFloat),
-        DspRev(NoDSP), HasMSA(false), DisableMadd4(false), HasFP64(false) {
+        DspRev(NoDSP), HasMSA(false), DisableMadd4(false),
+        UseIndirectJumpHazard(false), HasFP64(false) {
     TheCXXABI.set(TargetCXXABI::GenericMIPS);
 
     setABI((getTriple().getArch() == llvm::Triple::mips ||
@@ -339,6 +341,8 @@ public:
         IsAbs2008 = false;
       else if (Feature == "+noabicalls")
         IsNoABICalls = true;
+      else if (Feature == "+use-indirect-jump-hazard")
+        UseIndirectJumpHazard = true;
     }
 
     setDataLayout();

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

@@ -343,6 +343,28 @@ void mips::getMIPSTargetFeatures(const Driver &D, const llvm::Triple &Triple,
   AddTargetFeature(Args, Features, options::OPT_mno_madd4, options::OPT_mmadd4,
                    "nomadd4");
   AddTargetFeature(Args, Features, options::OPT_mmt, options::OPT_mno_mt, "mt");
+
+  if (Arg *A = Args.getLastArg(options::OPT_mindirect_jump_EQ)) {
+    StringRef Val = StringRef(A->getValue());
+    if (Val == "hazard") {
+      Arg *B =
+          Args.getLastArg(options::OPT_mmicromips, options::OPT_mno_micromips);
+      Arg *C = Args.getLastArg(options::OPT_mips16, options::OPT_mno_mips16);
+
+      if (B && B->getOption().matches(options::OPT_mmicromips))
+        D.Diag(diag::err_drv_unsupported_indirect_jump_opt)
+            << "hazard" << "micromips";
+      else if (C && C->getOption().matches(options::OPT_mips16))
+        D.Diag(diag::err_drv_unsupported_indirect_jump_opt)
+            << "hazard" << "mips16";
+      else if (mips::supportsIndirectJumpHazardBarrier(CPUName))
+        Features.push_back("+use-indirect-jump-hazard");
+      else
+        D.Diag(diag::err_drv_unsupported_indirect_jump_opt)
+            << "hazard" << CPUName;
+    } else
+      D.Diag(diag::err_drv_unknown_indirect_jump_opt) << Val;
+  }
 }
 
 mips::IEEE754Standard mips::getIEEE754Standard(StringRef &CPU) {
@@ -447,3 +469,20 @@ bool mips::shouldUseFPXX(const ArgList &Args, const llvm::Triple &Triple,
 
   return UseFPXX;
 }
+
+bool mips::supportsIndirectJumpHazardBarrier(StringRef &CPU) {
+  // Supporting the hazard barrier method of dealing with indirect
+  // jumps requires MIPSR2 support.
+  return llvm::StringSwitch<bool>(CPU)
+      .Case("mips32r2", true)
+      .Case("mips32r3", true)
+      .Case("mips32r5", true)
+      .Case("mips32r6", true)
+      .Case("mips64r2", true)
+      .Case("mips64r3", true)
+      .Case("mips64r5", true)
+      .Case("mips64r6", true)
+      .Case("octeon", true)
+      .Case("p5600", true)
+      .Default(false);
+}

clang/lib/Driver/ToolChains/Arch/Mips.h

@@ -53,6 +53,7 @@ bool isFPXXDefault(const llvm::Triple &Triple, StringRef CPUName,
 bool shouldUseFPXX(const llvm::opt::ArgList &Args, const llvm::Triple &Triple,
                    StringRef CPUName, StringRef ABIName,
                    mips::FloatABI FloatABI);
+bool supportsIndirectJumpHazardBarrier(StringRef &CPU);
 
 } // end namespace mips
 } // end namespace target

clang/test/Driver/mips-features.c

@@ -402,3 +402,9 @@
 // RUN: %clang -target -mips-mti-linux-gnu -### -c %s -mno-branch-likely 2>&1 \
 // RUN:   | FileCheck --check-prefix=NO-BRANCH-LIKELY %s
 // NO-BRANCH-LIKELY: argument unused during compilation: '-mno-branch-likely'
+
+// -mindirect-jump=hazard
+// RUN: %clang -target mips-unknown-linux-gnu -### -c %s \
+// RUN:        -mindirect-jump=hazard 2>&1 \
+// RUN:   | FileCheck --check-prefix=INDIRECT-BH %s
+// INDIRECT-BH: "-target-feature" "+use-indirect-jump-hazard"

clang/test/Driver/mips-indirect-branch.c

@@ -0,0 +1,23 @@
+// REQUIRES: mips-registered-target
+// -mindirect-jump=hazard -mips32
+// RUN: %clang -target mips-unknown-linux-gnu -mips32 -### -c %s \
+// RUN:        -mindirect-jump=hazard 2>&1 | FileCheck %s --check-prefix=MIPS32
+// MIPS32: error: '-mindirect-jump=hazard' is unsupported with the 'mips32' architecture
+
+// -mindirect-jump=hazard -mmicromips
+// RUN: %clang -target mips-unknown-linux-gnu -mmicromips -### -c %s \
+// RUN:        -mindirect-jump=hazard 2>&1 | FileCheck %s --check-prefix=MICROMIPS
+// MICROMIPS: error: '-mindirect-jump=hazard' is unsupported with the 'micromips' architecture
+
+// -mindirect-jump=hazard -mips16
+// RUN: %clang -target mips-unknown-linux-gnu -mips16 -### -c %s \
+// RUN:        -mindirect-jump=hazard 2>&1 | FileCheck %s --check-prefix=MIPS16
+// MIPS16: error: '-mindirect-jump=hazard' is unsupported with the 'mips16' architecture
+
+// RUN: %clang -target mips-unknown-linux-gnu  -### -c %s \
+// RUN:        -mindirect-jump=retopline 2>&1 | FileCheck %s --check-prefix=RETOPLINE
+// RETOPLINE: error: unknown '-mindirect-jump=' option 'retopline'
+
+// RUN: %clang -target mips-unknown-linux-gnu  -### -mips32 -c %s \
+// RUN:        -mindirect-jump=retopline 2>&1 | FileCheck %s --check-prefix=MIXED
+// MIXED: error: unknown '-mindirect-jump=' option 'retopline'