Reject non-empty directories for new WSLC session storage

localization/strings/en-US/Resources.resw

@@ -2361,6 +2361,10 @@ For privacy information about this product please visit https://aka.ms/privacy.<
     <value>No WSLC session found in '{}'</value>
     <comment>{FixedPlaceholder="{}"}Command line arguments, file names and string inserts should not be translated</comment>
   </data>
+  <data name="MessageWslcSessionStorageMustBeEmpty" xml:space="preserve">
+    <value>Cannot use '{}' as session storage because the directory is not empty</value>
+    <comment>{FixedPlaceholder="{}"}Command line arguments, file names and string inserts should not be translated</comment>
+  </data>
   <data name="MessageWslcTagImageInvalidFormat" xml:space="preserve">
     <value>Invalid image tag format: '{}'. Expected format is 'name:tag'</value>
     <comment>{FixedPlaceholder="{}"}Command line arguments, file names and string inserts should not be translated</comment>

src/windows/service/exe/WSLCSessionManager.cpp

@@ -273,7 +273,18 @@ void WSLCSessionManagerImpl::CreateSession(
         const auto sessionSettings = CreateSessionSettings(sessionId, creatorPid, Settings, resolvedDisplayName.c_str());
         wil::com_ptr<IWSLCSession> session;
         wil::com_ptr<IWSLCSessionReference> serviceRef;
-        THROW_IF_FAILED(factory->CreateSession(&sessionSettings, vm.Get(), notifier.Get(), WarningCallback, &session, &serviceRef));
+        {
+            const auto factoryHr = factory->CreateSession(&sessionSettings, vm.Get(), notifier.Get(), WarningCallback, &session, &serviceRef);
+            if (FAILED(factoryHr))
+            {
+                if (auto comError = wslutil::GetCOMErrorInfo(); comError && comError->Message)
+                {
+                    THROW_HR_WITH_USER_ERROR(factoryHr, comError->Message.get());
+                }
+
+                THROW_IF_FAILED(factoryHr);
+            }
+        }
 
         // Track the session via its service ref, along with metadata and security info.
         m_sessions.push_back(SessionEntry{
@@ -591,6 +602,11 @@ HRESULT WSLCSessionManager::OpenSessionByName(_In_ LPCWSTR DisplayName, _Out_ IW
     return CallImpl(&WSLCSessionManagerImpl::OpenSessionByName, DisplayName, Session);
 }
 
+HRESULT WSLCSessionManager::InterfaceSupportsErrorInfo(_In_ REFIID riid)
+{
+    return riid == __uuidof(IWSLCSessionManager) ? S_OK : S_FALSE;
+}
+
 namespace wsl::windows::service::wslc {
 
 WSLCSessionManagerImpl* WSLCSessionManagerImpl::Instance() noexcept

src/windows/service/exe/WSLCSessionManager.h

@@ -191,7 +191,7 @@ class WSLCSessionManagerImpl
 } // namespace wsl::windows::service::wslc
 
 class DECLSPEC_UUID("a9b7a1b9-0671-405c-95f1-e0612cb4ce8f") WSLCSessionManager
-    : public Microsoft::WRL::RuntimeClass<Microsoft::WRL::RuntimeClassFlags<Microsoft::WRL::ClassicCom>, IWSLCSessionManager, IFastRundown>,
+    : public Microsoft::WRL::RuntimeClass<Microsoft::WRL::RuntimeClassFlags<Microsoft::WRL::ClassicCom>, IWSLCSessionManager, IFastRundown, ISupportErrorInfo>,
       public wsl::windows::service::wslc::COMImplClass<wsl::windows::service::wslc::WSLCSessionManagerImpl>
 {
 public:
@@ -208,4 +208,7 @@ class DECLSPEC_UUID("a9b7a1b9-0671-405c-95f1-e0612cb4ce8f") WSLCSessionManager
     IFACEMETHOD(ListSessions)(_Out_ WSLCSessionListEntry** Sessions, _Out_ ULONG* SessionsCount) override;
     IFACEMETHOD(OpenSession)(_In_ ULONG Id, _Out_ IWSLCSession** Session) override;
     IFACEMETHOD(OpenSessionByName)(_In_ LPCWSTR DisplayName, _Out_ IWSLCSession** Session) override;
+
+    // ISupportErrorInfo: enables IErrorInfo marshaling across COM boundaries.
+    IFACEMETHOD(InterfaceSupportsErrorInfo)(_In_ REFIID riid) override;
 };

src/windows/wslcsession/WSLCSession.cpp

@@ -34,6 +34,7 @@ using wsl::windows::service::wslc::WSLCVirtualMachine;
 constexpr auto c_containerdStorage = "/var/lib/docker";
 constexpr auto c_containerdSocket = "/run/containerd/containerd.sock";
 constexpr auto c_dockerdReadyLogLine = "API listen on /var/run/docker.sock";
+constexpr auto c_storageVhdFilename = L"storage.vhdx";
 constexpr DWORD c_processTerminateTimeoutMs = 30 * 1000;
 constexpr DWORD c_processKillTimeoutMs = 10 * 1000;
 
@@ -285,6 +286,52 @@ try
         TraceLoggingValue(m_displayName.c_str(), "DisplayName"),
         TraceLoggingValue(Settings->CreatorPid, "CreatorPid"));
 
+    // Validate storage path before creating VM resources (Terminate() would clobber the error).
+    if (Settings->StoragePath != nullptr)
+    {
+        std::filesystem::path storagePath{Settings->StoragePath};
+        THROW_HR_WITH_USER_ERROR_IF(E_INVALIDARG, Localization::MessagePathNotAbsolute(Settings->StoragePath), !storagePath.is_absolute());
+
+        // MSVC's std::filesystem sets ec to ERROR_FILE_NOT_FOUND / ERROR_PATH_NOT_FOUND
+        // for non-existent paths. Filter those out — only throw on real failures.
+        auto throwOnRealFsError = [](const std::error_code& ec, const std::filesystem::path& path, const char* op) {
+            if (ec && ec.value() != ERROR_FILE_NOT_FOUND && ec.value() != ERROR_PATH_NOT_FOUND)
+            {
+                THROW_IF_WIN32_ERROR_MSG(ec.value(), "%hs failed for %ls", op, path.c_str());
+            }
+        };
+
+        std::error_code ec;
+        const auto vhdPath = storagePath / c_storageVhdFilename;
+        const bool vhdExists = std::filesystem::exists(vhdPath, ec);
+        throwOnRealFsError(ec, vhdPath, "exists");
+
+        THROW_HR_WITH_USER_ERROR_IF(
+            HRESULT_FROM_WIN32(ERROR_PATH_NOT_FOUND),
+            Localization::MessageWslcSessionStorageNotFound(Settings->StoragePath),
+            !vhdExists && WI_IsFlagSet(Settings->StorageFlags, WSLCSessionStorageFlagsNoCreate));
+
+        // Only check emptiness for new sessions. Existing sessions are identified by their VHD.
+        if (!vhdExists)
+        {
+            const bool isDir = std::filesystem::is_directory(storagePath, ec);
+            throwOnRealFsError(ec, storagePath, "is_directory");
+
+            if (isDir)
+            {
+                const bool empty = std::filesystem::is_empty(storagePath, ec);
+                THROW_IF_WIN32_ERROR_MSG(ec.value(), "is_empty failed for %ls", storagePath.c_str());
+
+                THROW_HR_WITH_USER_ERROR_IF(E_INVALIDARG, Localization::MessageWslcSessionStorageMustBeEmpty(storagePath.c_str()), !empty);
+            }
+            else
+            {
+                std::filesystem::create_directories(storagePath, ec);
+                THROW_IF_WIN32_ERROR_MSG(ec.value(), "create_directories failed for %ls", storagePath.c_str());
+            }
+        }
+    }
+
     // Create the VM.
     m_virtualMachine.emplace(Vm, Settings, m_sessionTerminatingEvent.get());
 
@@ -356,10 +403,9 @@ void WSLCSession::ConfigureStorage(const WSLCSessionInitSettings& Settings, PSID
         return;
     }
 
+    // Storage path validation is done in Initialize() before any VM resources are created.
     std::filesystem::path storagePath{Settings.StoragePath};
-    THROW_HR_WITH_USER_ERROR_IF(E_INVALIDARG, Localization::MessagePathNotAbsolute(Settings.StoragePath), !storagePath.is_absolute());
-
-    m_storageVhdPath = storagePath / "storage.vhdx";
+    m_storageVhdPath = storagePath / c_storageVhdFilename;
 
     std::string diskDevice;
     std::optional<ULONG> diskLun{};
@@ -388,15 +434,9 @@ void WSLCSession::ConfigureStorage(const WSLCSessionInitSettings& Settings, PSID
             "Failed to attach vhd: %ls",
             m_storageVhdPath.c_str());
 
-        THROW_HR_WITH_USER_ERROR_IF(
-            HRESULT_FROM_WIN32(ERROR_PATH_NOT_FOUND),
-            Localization::MessageWslcSessionStorageNotFound(Settings.StoragePath),
-            WI_IsFlagSet(Settings.StorageFlags, WSLCSessionStorageFlagsNoCreate));
-
         // If the VHD wasn't found, create it.
         WSL_LOG("CreateStorageVhd", TraceLoggingValue(m_storageVhdPath.c_str(), "StorageVhdPath"));
 
-        std::filesystem::create_directories(storagePath);
         wsl::core::filesystem::CreateVhd(m_storageVhdPath.c_str(), Settings.MaximumStorageSizeMb * _1MB, UserSid, false, false);
         vhdCreated = true;
 

src/windows/wslcsession/WSLCSessionFactory.cpp

@@ -19,9 +19,7 @@ Module Name:
 #include "WSLCSessionFactory.h"
 #include "WSLCSession.h"
 #include "WSLCSessionReference.h"
-#include "wslutil.h"
 
-namespace wslutil = wsl::windows::common::wslutil;
 namespace wslc = wsl::windows::service::wslc;
 
 void wslc::WSLCSessionFactory::SetDestructionCallback(std::function<void()>&& callback)
@@ -68,6 +66,11 @@ try
 }
 CATCH_RETURN()
 
+HRESULT wslc::WSLCSessionFactory::InterfaceSupportsErrorInfo(_In_ REFIID riid)
+{
+    return riid == __uuidof(IWSLCSessionFactory) ? S_OK : S_FALSE;
+}
+
 HRESULT wslc::WSLCSessionFactory::GetProcessHandle(_Out_ HANDLE* ProcessHandle)
 try
 {

src/windows/wslcsession/WSLCSessionFactory.h

@@ -30,7 +30,7 @@ Module Name:
 namespace wsl::windows::service::wslc {
 
 class DECLSPEC_UUID("9FCD2067-9FC6-4EFA-9EB0-698169EBF7D3") WSLCSessionFactory
-    : public Microsoft::WRL::RuntimeClass<Microsoft::WRL::RuntimeClassFlags<Microsoft::WRL::ClassicCom>, IWSLCSessionFactory, IFastRundown>
+    : public Microsoft::WRL::RuntimeClass<Microsoft::WRL::RuntimeClassFlags<Microsoft::WRL::ClassicCom>, IWSLCSessionFactory, IFastRundown, ISupportErrorInfo>
 {
 public:
     NON_COPYABLE(WSLCSessionFactory);
@@ -53,6 +53,9 @@ class DECLSPEC_UUID("9FCD2067-9FC6-4EFA-9EB0-698169EBF7D3") WSLCSessionFactory
 
     IFACEMETHOD(GetProcessHandle)(_Out_ HANDLE* ProcessHandle) override;
 
+    // ISupportErrorInfo: enables IErrorInfo marshaling across COM boundaries.
+    IFACEMETHOD(InterfaceSupportsErrorInfo)(_In_ REFIID riid) override;
+
 private:
     std::function<void()> m_destructionCallback;
 };

test/windows/WSLCTests.cpp

@@ -451,6 +451,26 @@ class WSLCTests
             wil::com_ptr<IWSLCSession> session;
             VERIFY_ARE_EQUAL(sessionManager->CreateSession(&settings, WSLCSessionFlagsNone, nullptr, &session), E_INVALIDARG);
         }
+
+        // Reject non-empty storage directory that doesn't contain a session VHD.
+        {
+            const auto storagePath = std::filesystem::temp_directory_path() /
+                                     std::format(L"wslc-test-storage-{}-{}", GetCurrentProcessId(), GetTickCount64());
+            std::filesystem::create_directories(storagePath);
+            auto cleanup = wil::scope_exit([&]() {
+                std::error_code ignored;
+                std::filesystem::remove_all(storagePath, ignored);
+            });
+
+            std::ofstream{storagePath / L"userfile.txt"} << "data";
+
+            auto settings = GetDefaultSessionSettings(L"storage-not-empty");
+            const auto storagePathString = storagePath.wstring();
+            settings.StoragePath = storagePathString.c_str();
+            wil::com_ptr<IWSLCSession> session;
+            VERIFY_ARE_EQUAL(sessionManager->CreateSession(&settings, WSLCSessionFlagsNone, nullptr, &session), E_INVALIDARG);
+            ValidateCOMErrorMessage(std::format(L"Cannot use '{}' as session storage because the directory is not empty", storagePathString));
+        }
     }
 
     struct VmInfo

test/windows/wslc/e2e/WSLCE2ESessionEnterTests.cpp

@@ -109,8 +109,9 @@ class WSLCE2ESessionEnterTests
     WSLC_TEST_METHOD(WSLCE2E_SessionEnter_StoragePathNotFound)
     {
         auto result = RunWslc(L"system session enter does-not-exist");
+        const auto expectedPath = std::filesystem::absolute(L"does-not-exist").wstring();
         result.Verify({
-            .Stderr = L"The system cannot find the path specified. \r\nError code: ERROR_PATH_NOT_FOUND\r\n",
+            .Stderr = std::format(L"No WSLC session found in '{}'\r\nError code: ERROR_PATH_NOT_FOUND\r\n", expectedPath),
             .ExitCode = 1,
         });
     }