# HG changeset patch
# User Bob Owen <bobowencode@gmail.com>
# Date 1611849321 0
#      Thu Jan 28 15:55:21 2021 +0000
# Node ID c9195d88e6c67ef2c23c12e307bc16b94d696f50
# Parent  37557864a6845bb8068904e44e8a7dd16746d211
Bug 1716024 p1: Add MITIGATION_CET_COMPAT_MODE to chromium sandbox code. r=handyman!

diff --git a/security/sandbox/chromium/sandbox/win/src/process_mitigations.cc b/security/sandbox/chromium/sandbox/win/src/process_mitigations.cc
--- a/security/sandbox/chromium/sandbox/win/src/process_mitigations.cc
+++ b/security/sandbox/chromium/sandbox/win/src/process_mitigations.cc
@@ -80,16 +80,37 @@ bool IsRunning32bitEmulatedOnArm64() {
   if (!retval)
     return false;
   if (native_machine == IMAGE_FILE_MACHINE_ARM64)
     return true;
 #endif  // defined(ARCH_CPU_X86)
   return false;
 }
 
+// Returns true if user-mode Hardware-enforced Stack Protection is available for
+// the Win32 environment.
+bool IsUserCetWin32Available() {
+  static bool cetAvailable = []() -> bool {
+    using IsUserCetAvailableInEnvironmentFunction =
+      decltype(&IsUserCetAvailableInEnvironment);
+
+    IsUserCetAvailableInEnvironmentFunction is_user_cet_available =
+        reinterpret_cast<IsUserCetAvailableInEnvironmentFunction>(
+            ::GetProcAddress(::GetModuleHandleW(L"kernel32.dll"),
+                             "IsUserCetAvailableInEnvironment"));
+    if (!is_user_cet_available) {
+      return false;
+    }
+
+    return is_user_cet_available(USER_CET_ENVIRONMENT_WIN32_PROCESS);
+  }();
+
+  return cetAvailable;
+}
+
 }  // namespace
 
 namespace sandbox {
 
 bool ApplyProcessMitigationsToCurrentProcess(MitigationFlags flags) {
   if (!CanSetProcessMitigationsPostStartup(flags))
     return false;
 
@@ -487,16 +508,25 @@ void ConvertProcessMitigationsToPolicy(M
     //       the underlying hardware does not support the implementation.
     //       Windows just does its best under the hood for the given hardware.
     if (flags & MITIGATION_RESTRICT_INDIRECT_BRANCH_PREDICTION) {
       *policy_value_2 |=
           PROCESS_CREATION_MITIGATION_POLICY2_RESTRICT_INDIRECT_BRANCH_PREDICTION_ALWAYS_ON;
     }
   }
 
+  // Mitigations >= Win10 20H1
+  //----------------------------------------------------------------------------
+  if (version >= base::win::Version::WIN10_20H1) {
+    if (flags & MITIGATION_CET_COMPAT_MODE && IsUserCetWin32Available()) {
+      *policy_value_2 |=
+          PROCESS_CREATION_MITIGATION_POLICY2_CET_USER_SHADOW_STACKS_ALWAYS_ON;
+    }
+  }
+
   // When done setting policy flags, sanity check supported policies on this
   // machine, and then update |size|.
 
   const ULONG64* supported = GetSupportedMitigations();
 
   *policy_value_1 = *policy_value_1 & supported[0];
   *policy_value_2 = *policy_value_2 & supported[1];
 
diff --git a/security/sandbox/chromium/sandbox/win/src/security_level.h b/security/sandbox/chromium/sandbox/win/src/security_level.h
--- a/security/sandbox/chromium/sandbox/win/src/security_level.h
+++ b/security/sandbox/chromium/sandbox/win/src/security_level.h
@@ -286,11 +286,15 @@ const MitigationFlags MITIGATION_RESTRIC
 // Working down from the high bit to avoid conflict with new upstream flags.
 
 // Disable Control Flow Guard. This may seem more like an anti-mitigation, but
 // this flag allows code to make targeted changes to CFG to avoid bugs, while
 // leaving it enabled in the common case. Corresponds to
 // PROCESS_CREATION_MITIGATION_POLICY_CONTROL_FLOW_GUARD_ALWAYS_ON.
 const MitigationFlags MITIGATION_CONTROL_FLOW_GUARD_DISABLE = 0x80000000;
 
+// This enables CET User Shadow Stack for compatible modules and corresponds to
+// PROCESS_CREATION_MITIGATION_POLICY2_CET_USER_SHADOW_STACKS_ALWAYS_ON.
+const MitigationFlags MITIGATION_CET_COMPAT_MODE = 0x40000000;
+
 }  // namespace sandbox
 
 #endif  // SANDBOX_SRC_SECURITY_LEVEL_H_