Chromoting: Implemented security attention sequence (SAS) emulation on Windows.

remoting/host/sas_sender_win.cc

Index: remoting/host/sas_sender_win.cc
diff --git a/remoting/host/sas_sender_win.cc b/remoting/host/sas_sender_win.cc
new file mode 100644
index 0000000000000000000000000000000000000000..12ffa0b6cbc252503e65f8e3f53a642bdb48444c
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+++ b/remoting/host/sas_sender_win.cc
@@ -0,0 +1,185 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "remoting/host/sas_sender_win.h"
+
+#include <string>
+
+#include "base/logging.h"
+#include "base/file_path.h"
+#include "base/native_library.h"
+#include "base/path_service.h"
+#include "base/utf_string_conversions.h"
+#include "base/win/registry.h"
+#include "base/win/windows_version.h"
+
+namespace remoting {
+
+namespace {
+
+// Names of the API and library implementing software SAS generation.
+const FilePath::CharType kSasDllFileName[] =
+    FILE_PATH_LITERAL("sas.dll");
+const char kSendSasName[] = "SendSAS";
+
+// The prototype of SendSAS().
+typedef VOID (WINAPI *SendSasFunc)(BOOL);
+
+// The registry key and value holding the policy controlling software SAS
+// generation.
+const char kSystemPolicyKeyName[] =
+    "Software\\Microsoft\\Windows\\CurrentVersion\\Policies\\System";
+const char kSoftwareSasValueName[] = "SoftwareSASGeneration";
+
+const DWORD kEnableSoftwareSasByServices = 1;
+
+// Toggles the default software SAS generation policy to enable SAS generation
+// by services. Non-default policy is not channged.
+class ScopedSoftwareSasPolicy {
+ public:
+  ScopedSoftwareSasPolicy();
+  ~ScopedSoftwareSasPolicy();
+
+  bool Apply();
+
+ private:
+  // The handle of the registry key were SoftwareSASGeneration policy is stored.
+  base::win::RegKey system_policy_;
+
+  // Name of the registry value holding the policy.
+  string16 value_name_;
+
+  // True if the policy needs to be restored.
+  bool restore_policy_;
+
+  DISALLOW_COPY_AND_ASSIGN(ScopedSoftwareSasPolicy);
+};
+
+ScopedSoftwareSasPolicy::ScopedSoftwareSasPolicy()
+    : restore_policy_(false) {
+}
+
+ScopedSoftwareSasPolicy::~ScopedSoftwareSasPolicy() {
+  // Restore the default policy by deleting the value that we have set.
+  if (restore_policy_) {
+    LONG result = system_policy_.DeleteValue(value_name_.c_str());
+    if (result != ERROR_SUCCESS) {
+      SetLastError(result);
+      LOG_GETLASTERROR(ERROR)
+          << "Failed to restore the software SAS generation policy";
+    }
+  }
+}
+
+bool ScopedSoftwareSasPolicy::Apply() {
+  // Query the currently set SoftwareSASGeneration policy.
+  LONG result = system_policy_.Open(HKEY_LOCAL_MACHINE,
+                                    ASCIIToUTF16(kSystemPolicyKeyName).c_str(),
+                                    KEY_QUERY_VALUE | KEY_SET_VALUE |
+                                        KEY_WOW64_64KEY);
+  if (result != ERROR_SUCCESS) {
+    SetLastError(result);
+    LOG_GETLASTERROR(ERROR) << "Failed to open 'HKLM\\"
+                            << kSystemPolicyKeyName << "'";
+    return false;
+  }
+
+  value_name_ = ASCIIToUTF16(kSoftwareSasValueName);
+  bool custom_policy = system_policy_.HasValue(value_name_.c_str());
+
+  // Override the default policy (i.e. there is no value in the registry) only.
+  if (!custom_policy) {
+    result = system_policy_.WriteValue(value_name_.c_str(),
+                                       kEnableSoftwareSasByServices);
+    if (result != ERROR_SUCCESS) {
+      SetLastError(result);
+      LOG_GETLASTERROR(ERROR)
+          << "Failed to enable software SAS generation by services";
+      return false;
+    } else {
+      restore_policy_ = true;
+    }
+  }
+
+  return true;
+}
+
+} // namespace
+
+// Sends the security attention sequence using the SendSAS() function from
+// sas.dll. This library is shipped starting from Win7/W2K8 R2 only. However
+// Win7 SDK includes a redistributable verion of the same library that works on
+// Vista/W2K8. We install the latter along with our binaries.
+class SasSenderVista : public SasSender {

[Wez, 2012/03/07 01:56:13]

Why is this SasSanderVista rather than SasSenderWin?

[alexeypa (please no reviews), 2012/03/07 19:59:08]

Because, potentially, we can have a different implementation for Windows XP.

[Wez, 2012/03/08 00:01:33]

That's fine - this version is SasSenderWin, and one we add for older systems can
be SasSenderXP.

[alexeypa (please no reviews), 2012/03/08 01:52:54]

Once we do that the naming will become odd: why is XP not Windows? Or why is
Windows does not include XP?

+ public:
+  SasSenderVista();
+  virtual ~SasSenderVista();
+
+  // SasSender implementation.
+  virtual bool Send() OVERRIDE;
+
+ private:
+  base::NativeLibrary sas_dll_;
+  SendSasFunc send_sas_;
+};
+
+SasSenderVista::SasSenderVista() : sas_dll_(NULL), send_sas_(NULL) {
+}
+
+SasSenderVista::~SasSenderVista() {
+  if (sas_dll_ != NULL) {
+    base::UnloadNativeLibrary(sas_dll_);
+  }
+}
+
+bool SasSenderVista::Send() {
+  // Load sas.dll. The library is expected to be in
+  // the same folder as this binary.

[Wez, 2012/03/07 01:56:13]

nit: Premature line-wrap.

[alexeypa (please no reviews), 2012/03/07 19:59:08]

Done.

+  if (sas_dll_ == NULL) {
+    FilePath exe_path;
+    if (!PathService::Get(base::FILE_EXE, &exe_path)) {
+      LOG(ERROR) << "Failed to get the executable file name.";
+      return false;
+    }
+
+    std::string error;
+    sas_dll_ = base::LoadNativeLibrary(
+                   exe_path.DirName().Append(kSasDllFileName),
+                   &error);
+    if (sas_dll_ == NULL) {
+      LOG(ERROR) << "Failed to load '" << kSasDllFileName << "'";
+      return false;
+    }
+  }
+
+  // Get the pointer to sas!SendSAS().
+  if (send_sas_ == NULL) {
+    send_sas_ = reinterpret_cast<SendSasFunc>(
+      base::GetFunctionPointerFromNativeLibrary(sas_dll_, kSendSasName));
+    if (send_sas_ == NULL) {
+      LOG(ERROR) << "Failed to retrieve the address of '" << kSendSasName
+                 << "()'";
+      return false;
+    }
+  }
+
+  // Enable software SAS generation by services and send SAS. SAS can still fail
+  // if the policy does not applow services to generate software SAS.

[Wez, 2012/03/07 01:56:13]

typo: allow

[alexeypa (please no reviews), 2012/03/07 19:59:08]

Done.

+  ScopedSoftwareSasPolicy enable_sas;
+  if (enable_sas.Apply()) {
+    (*send_sas_)(FALSE);

[Wez, 2012/03/07 01:56:13]

Is it a problem for us to call SendSAS if the policy tweaking didn't work?

[alexeypa (please no reviews), 2012/03/07 19:59:08]

Yes. It does nothing if the policy is not enabled. We have to call SendSAS()
regardless what we think the current state of the policy is. Winlogon is the
only one who can make the final policy decision.

[Wez, 2012/03/08 00:01:33]

So we shouldn't be testing the result of |enable_sas.Apply()| in that case.

[alexeypa (please no reviews), 2012/03/08 01:52:54]

No, we should. False means 'something is badly wrong, let's not touch anything
and return'. True means we didn't see any error, so we can try sending SAS.

Note I said nothing about tweaking the policy by us above. It does not matter,
really. The policy could be in default state and we enabled it. It could be in
non default state (and we didn't touch it), but can still allows services to
generate SAS. IN either case we need to call SendSAS().

+  }
+
+  return true;

[Wez, 2012/03/07 01:56:13]

Do you mean to return true whether or not the policy tweaking worked?

[alexeypa (please no reviews), 2012/03/07 19:59:08]

Yes. False is returned only if any of the operations failed.  A SAS blocked by
the policy is considered a successful operation since nothing is broken.

[Wez, 2012/03/08 00:01:33]

Is it worth returning false, in that case, since there's nothing we can do about
it?

[alexeypa (please no reviews), 2012/03/08 01:52:54]

We should (and do) behave differently. See above.

+}
+
+scoped_ptr<SasSender> SasSender::Create() {
+  if (base::win::OSInfo::GetInstance()->version() >= base::win::VERSION_VISTA) {
+    return scoped_ptr<SasSender>(new SasSenderVista());
+  }
+
+  return scoped_ptr<SasSender>();
+}
+
+} // namespace remoting