feature: resolving snap desktop files using app armor as the primary …

…source (#3426)

## What's new?
- AppArmor is now the primary source of snap desktop files in the
`DesktopFileManager`

CMakeLists.txt

@@ -206,6 +206,17 @@ CMAKE_DEPENDENT_OPTION(
   MIR_SIGBUS_HANDLER_ENVIRONMENT_BROKEN "Expect WLCS BadBuffer tests to fail" OFF
   "MIR_RUN_WLCS_TESTS" OFF
 )
+
+pkg_check_modules(APPARMOR IMPORTED_TARGET libapparmor)
+
+CMAKE_DEPENDENT_OPTION(
+  MIR_USE_APPARMOR "Whether or not Mir should build with support for AppArmor" ON
+  "APPARMOR_FOUND" OFF
+)
+if (MIR_USE_APPARMOR)
+  add_compile_definitions(MIR_USE_APPARMOR)
+endif()
+
 SET(MIR_EXCLUDE_TESTS "" CACHE STRING "Semicolon-separated list of tests to exclude (wildcards accepted)")
 
 

debian/control

@@ -55,7 +55,8 @@ Build-Depends: cmake,
                libnvidia-egl-wayland-dev,
                eglexternalplatform-dev,
                systemtap-sdt-dev,
-               wlcs
+               wlcs,
+               libapparmor-dev
 Standards-Version: 4.6.1
 Homepage: https://mir-server.io/
 Vcs-Browser: https://github.com/MirServer/mir/

snap/snapcraft.yaml

@@ -52,6 +52,7 @@ parts:
     - nettle-dev
     - python3-pil
     - systemtap-sdt-dev
+    - libapparmor-dev
     stage-packages:
     - libboost-filesystem1.83.0
     - libboost-iostreams1.83.0
@@ -98,3 +99,4 @@ parts:
     - libxml2
     - libxrender1
     - libyaml-cpp0.8
+    - libapparmor1

spread/build/alpine/task.yaml

@@ -31,7 +31,8 @@ execute: |
         py3-pillow \
         umockdev-dev \
         wayland-dev \
-        yaml-cpp-dev
+        yaml-cpp-dev \
+        libapparmor-dev
 
     BUILD_DIR=$PWD/../build
     cmake \

src/server/CMakeLists.txt

@@ -149,6 +149,10 @@ target_link_libraries(mirserver
     atomic
 )
 
+if (MIR_USE_APPARMOR)
+  target_link_libraries(mirserver PUBLIC PkgConfig::APPARMOR)
+endif()
+
 set(MIRSERVER_INCLUDE_DIRS ${UUID_INCLUDE_DIRS} PARENT_SCOPE)
 
 install(TARGETS mirserver

src/server/frontend_wayland/desktop_file_manager.cpp

@@ -22,6 +22,10 @@
 #include <fstream>
 #include <filesystem>
 
+#ifdef MIR_USE_APPARMOR
+#include <sys/apparmor.h>
+#endif
+
 namespace mf = mir::frontend;
 
 namespace
@@ -89,9 +93,10 @@ std::string mf::DesktopFileManager::resolve_app_id(const scene::Surface* surface
 
     auto session = surface->session().lock();
     auto pid = session->process_id();
+    auto socket_fd = session->socket_fd();
 
     // Fourth, check if the window belongs to snap
-    found = resolve_if_snap(pid);
+    found = resolve_if_snap(pid, socket_fd);
     if (found)
         return found->id;
 
@@ -157,8 +162,31 @@ std::string mf::DesktopFileManager::parse_snap_security_profile_to_desktop_id(st
     return sandboxed_app_id + DESKTOP_FILE_POSTFIX;
 }
 
-std::shared_ptr<mf::DesktopFile> mf::DesktopFileManager::resolve_if_snap(int pid)
+std::shared_ptr<mf::DesktopFile> mf::DesktopFileManager::resolve_if_snap(int pid, mir::Fd const& socket_fd)
 {
+#ifdef MIR_USE_APPARMOR
+    // First, try to resolve the AppArmor profile
+    char* label_cstr;
+    char* mode_cstr;
+
+    if (aa_getpeercon(socket_fd, &label_cstr, &mode_cstr) >= 0)
+    {
+        std::string const label{label_cstr};
+        free(label_cstr);
+        // mode_cstr should NOT be freed, as it's from the same buffer as label_cstr
+
+        auto sandboxed_app_id = parse_snap_security_profile_to_desktop_id(label);
+        if (!sandboxed_app_id.empty())
+        {
+            if (auto file = cache->lookup_by_app_id(sandboxed_app_id))
+                return file;
+        }
+    }
+#else
+    (void)socket_fd;
+#endif
+
+    // If that fails, try to read /proc/<PID>/attr/current
     std::string attr_file = "/proc/" + std::to_string(pid) + "/attr/current";
     if (!std::filesystem::exists(attr_file))
         return nullptr;

src/server/frontend_wayland/desktop_file_manager.h

@@ -64,7 +64,7 @@ class DesktopFileManager
     std::shared_ptr<DesktopFileCache> cache;
     std::shared_ptr<DesktopFile> resolve_from_wayland_app_id(std::string& app_id);
     std::shared_ptr<DesktopFile> lookup_basename(std::string& name);
-    std::shared_ptr<DesktopFile> resolve_if_snap(int pid);
+    std::shared_ptr<DesktopFile> resolve_if_snap(int pid, mir::Fd const& socket_fd);
     std::shared_ptr<DesktopFile> resolve_if_flatpak(int pid);
     std::shared_ptr<DesktopFile> resolve_if_executable_matches(int pid);
 };