PGP: Set a default creation SELinux labels on GnuPG directories

This is another way how to fix mismatching SELinux context on
/run/user directories without moving the directories to
/run/gnupg/user.

librepo used to precreate the directory in /run/user to make sure
a GnuPG agent executed by GPGME library places its socket there.

The directories there are normally created and removed by systemd
(logind PAM session). librepo created them for a case when a package
manager is invoked out of systemd session, before the super user logs
in. E.g. by a timer job to cache repository metadata.

A problem was when this out-of-session process was a SELinux-confined
process creating files with its own SELinux label different from a DNF
program. Then the directory was created with a SELinux label different
from the one expected by systemd and when logging out a corresponding
user, the mismatching label clashed with systemd.

This patch fixes the issue by choosing a SELinux label of those
directories to the label defined in a default SELinux file context
database.

This patch adds a new -DENABLE_SELINUX=OFF CMake option to disable the
new dependency on libselinux. A default behavior is to support SELinux
only if GPGME backend is selected with -DUSE_GPGME=ON.

https://issues.redhat.com/browse/RHEL-10720

CMakeLists.txt

@@ -7,6 +7,7 @@ OPTION (WITH_ZCHUNK "Build with zchunk support" ON)
 OPTION (ENABLE_PYTHON "Build Python bindings" ON)
 OPTION (USE_GPGME "Use GpgMe (instead of rpm library) for OpenPGP key support" ON)
 OPTION (USE_RUN_GNUPG_USER_SOCKET "Create a directory for gpg-agent socket in /run/gnugp/user (instead of /run/user)" OFF)
+OPTION (ENABLE_SELINUX "Restore SELinux labels on GnuPG directories" ON)
 
 INCLUDE (${CMAKE_SOURCE_DIR}/VERSION.cmake)
 SET (VERSION "${LIBREPO_MAJOR}.${LIBREPO_MINOR}.${LIBREPO_PATCH}")
@@ -37,6 +38,9 @@ FIND_PACKAGE(CURL 7.52.0 REQUIRED)
 
 IF (USE_GPGME)
     FIND_PACKAGE(Gpgme REQUIRED)
+    IF (ENABLE_SELINUX)
+        PKG_CHECK_MODULES(SELINUX REQUIRED libselinux)
+    ENDIF(ENABLE_SELINUX)
 ELSE (USE_GPGME)
     PKG_CHECK_MODULES(RPM REQUIRED rpm>=4.18.0)
 
@@ -93,6 +97,10 @@ ENDIF (NOT CURL_FOUND)
 INCLUDE_DIRECTORIES(${LIBXML2_INCLUDE_DIRS})
 INCLUDE_DIRECTORIES(${CURL_INCLUDE_DIR})
 #INCLUDE_DIRECTORIES(${CHECK_INCLUDE_DIR})
+IF (USE_GPGME AND ENABLE_SELINUX)
+    INCLUDE_DIRECTORIES(${SELINUX_INCLUDE_DIRS})
+    ADD_DEFINITIONS(-DENABLE_SELINUX=1)
+ENDIF (USE_GPGME AND ENABLE_SELINUX)
 
 include (GNUInstallDirs)
 # Python stuff

librepo.spec

@@ -10,13 +10,21 @@
 
 %if 0%{?fedora} >= 39
 %bcond_with use_gpgme
+%bcond_with use_selinux
 %else
 %bcond_without use_gpgme
+%bcond_without use_selinux
 %endif
 
 # Needs to match how gnupg2 is compiled
 %bcond_with run_gnupg_user_socket
 
+%if %{with use_gpgme} && %{with use_selinux}
+%global need_selinux 1
+%else
+%global need_selinux 0
+%endif
+
 %global dnf_conflict 2.8.8
 
 Name:           librepo
@@ -42,6 +50,9 @@ BuildRequires:  libattr-devel
 BuildRequires:  libcurl-devel >= %{libcurl_version}
 BuildRequires:  pkgconfig(libxml-2.0)
 BuildRequires:  pkgconfig(libcrypto)
+%if %{need_selinux}
+BuildRequires:  pkgconfig(libselinux)
+%endif
 BuildRequires:  pkgconfig(openssl)
 %if %{with zchunk}
 BuildRequires:  pkgconfig(zck) >= 0.9.11
@@ -82,7 +93,8 @@ Python 3 bindings for the librepo library.
 %cmake \
     -DWITH_ZCHUNK=%{?with_zchunk:ON}%{!?with_zchunk:OFF} \
     -DUSE_GPGME=%{?with_use_gpgme:ON}%{!?with_use_gpgme:OFF} \
-    -DUSE_RUN_GNUPG_USER_SOCKET=%{?with_run_gnupg_user_socket:ON}%{!?with_run_gnupg_user_socket:OFF}
+    -DUSE_RUN_GNUPG_USER_SOCKET=%{?with_run_gnupg_user_socket:ON}%{!?with_run_gnupg_user_socket:OFF} \
+    -DENABLE_SELINUX=%{?need_selinux:ON}%{!?need_selinux:OFF}
 %cmake_build
 
 %check

librepo/CMakeLists.txt

@@ -71,6 +71,9 @@ TARGET_LINK_LIBRARIES(librepo
                      )
 IF (USE_GPGME)
     TARGET_LINK_LIBRARIES(librepo ${GPGME_VANILLA_LIBRARIES})
+    IF (ENABLE_SELINUX)
+        TARGET_LINK_LIBRARIES(librepo ${SELINUX_LIBRARIES})
+    ENDIF(ENABLE_SELINUX)
 ELSE(USE_GPGME)
     TARGET_LINK_LIBRARIES(librepo ${RPM_LIBRARIES})
 ENDIF (USE_GPGME)

librepo/gpg_gpgme.c

@@ -28,6 +28,11 @@
 #include <stdint.h>
 #include <unistd.h>
 
+#if ENABLE_SELINUX
+#include <selinux/selinux.h>
+#include <selinux/label.h>
+#endif
+
 #include "gpg.h"
 #include "gpg_internal.h"
 #include "rcodes.h"
@@ -45,10 +50,14 @@
  * Previous solution was to send the agent a "KILLAGENT" message, but that
  * would cause a race condition with calling gpgme_release(), see [2], [3].
  *
- * Another previous solution used /run/user/$UID which showed problematic when
- * this library was used out of an systemd-logind session. Then /run/user/$UID,
- * normally maintained by systemd, was assigned a SELinux label unexpected by
- * systemd causing errors on a user logout [4].
+ * Current solution with precreating /run/user/$UID showed problematic when
+ * this library was used out of an systemd-logind session. Then
+ * /run/user/$UID, normally maintained by systemd, was assigned a SELinux
+ * label unexpected by systemd causing errors on a user logout [4].
+ *
+ * We remedy it by choosing the label according to a default file context
+ * policy (ENABLE_SELINUX macro) or by using a different path supported by
+ * some GnuPG configurations (DUSE_RUN_GNUPG_USER_SOCKET macro).
  *
  * Since the agent doesn't clean up its sockets properly, by creating this
  * directory we make sure they are in a place that is not causing trouble with
@@ -72,19 +81,69 @@ lr_gpg_ensure_socket_dir_exists()
     const uid_t uid = getuid();
     char dirname[32];
     int res;
+#if ENABLE_SELINUX
+    char *old_default_context = NULL;
+    int old_default_context_was_retrieved = 0;
+    struct selabel_handle *labeling_handle = NULL;
+
+    /* A purpose of this piece of code is to deal with applications whose
+     * security policy overrides a file context for temporary files but don't
+     * know that librepo executes GnuPG which expects a default file context. */
+    if (0 == getfscreatecon(&old_default_context)) {
+        old_default_context_was_retrieved = 1;
+    } else {
+        g_debug("Failed to retrieve a default SELinux context");
+    }
+    labeling_handle = selabel_open(SELABEL_CTX_FILE, NULL, 0);
+    if (labeling_handle == NULL) {
+        g_debug("Failed to open a SELinux labeling handle: %s", strerror(errno));
+    }
+#endif
 
     for (int i = 0; templates[i] != NULL; i++) {
         res = snprintf(dirname, sizeof(dirname), templates[i], (uintmax_t)uid);
         if (res >= sizeof(dirname)) {
             g_debug("Failed to format a GnuPG agent socket path because of a small buffer");
-            return;
+            goto exit;
         }
+
+#if ENABLE_SELINUX
+        if (labeling_handle != NULL) {
+            char *new_default_context = NULL;
+            if (selabel_lookup(labeling_handle, &new_default_context, dirname, modes[i])) {
+                /* Here we could hard-code "system_u:object_r:user_tmp_t:s0", but
+                 * that value should be really defined in default file context
+                 * SELinux policy. Only log that the policy is incomplete. */
+                g_debug("Failed to look up a default SELinux label for \"%s\"", dirname);
+            } else {
+                if (setfscreatecon(new_default_context)) {
+                    g_debug("Failed to set default SELinux context to \"%s\"",
+                            new_default_context);
+                }
+                freecon(new_default_context);
+            }
+        }
+#endif
+
         res = mkdir(dirname, modes[i]);
         if (res != 0 && errno != EEXIST) {
             g_debug("Failed to create \"%s\": %d - %s\n", dirname, errno, strerror(errno));
-            return;
+            goto exit;
+        }
+    }
+
+exit:
+#if ENABLE_SELINUX
+    if (labeling_handle != NULL) {
+        selabel_close(labeling_handle);
+    }
+    if (old_default_context_was_retrieved) {
+        if (setfscreatecon(old_default_context)) {
+            g_debug("Failed to restore a default SELinux context");
         }
     }
+    freecon(old_default_context);
+#endif
 }
 
 static gpgme_ctx_t