logind: validate /run/user/1000 before we set it

Let's be safe than sorry, in particular as logind doesn't set it up
anymore, but user-runtime-dir@.service does, and logind doesn't really
track success of that.

(cherry picked from commit b92171124819305985ed292cc472f6668a027425)

1 files changed, 42 insertions, 8 deletions

diff --git a/src/login/pam_elogind.c b/src/login/pam_elogind.c
index bb6da2aac..c55066e23 100644
--- a/src/login/pam_elogind.c
+++ b/src/login/pam_elogind.c
@@ -274,6 +274,36 @@ static int append_session_cg_weight(pam_handle_t *handle, sd_bus_message *m, con
         return 0;
 }
 
+static bool validate_runtime_directory(pam_handle_t *handle, const char *path, uid_t uid) {
+        struct stat st;
+
+        assert(path);
+
+        /* Just some extra paranoia: let's not set $XDG_RUNTIME_DIR if the directory we'd set it to isn't actually set
+         * up properly for us. */
+
+        if (lstat(path, &st) < 0) {
+                pam_syslog(handle, LOG_ERR, "Failed to stat() runtime directory '%s': %s", path, strerror(errno));
+                goto fail;
+        }
+
+        if (!S_ISDIR(st.st_mode)) {
+                pam_syslog(handle, LOG_ERR, "Runtime directory '%s' is not actually a directory.", path);
+                goto fail;
+        }
+
+        if (st.st_uid != uid) {
+                pam_syslog(handle, LOG_ERR, "Runtime directory '%s' is not owned by UID " UID_FMT ", as it should.", path, uid);
+                goto fail;
+        }
+
+        return true;
+
+fail:
+        pam_syslog(handle, LOG_WARNING, "Not setting $XDG_RUNTIME_DIR, as the directory is not in order.");
+        return false;
+}
+
 _public_ PAM_EXTERN int pam_sm_open_session(
                 pam_handle_t *handle,
                 int flags,
@@ -334,10 +364,12 @@ _public_ PAM_EXTERN int pam_sm_open_session(
                 if (asprintf(&rt, "/run/user/"UID_FMT, pw->pw_uid) < 0)
                         return PAM_BUF_ERR;
 
-                r = pam_misc_setenv(handle, "XDG_RUNTIME_DIR", rt, 0);
-                if (r != PAM_SUCCESS) {
-                        pam_syslog(handle, LOG_ERR, "Failed to set runtime dir.");
-                        return r;
+                if (validate_runtime_directory(handle, rt, pw->pw_uid)) {
+                        r = pam_misc_setenv(handle, "XDG_RUNTIME_DIR", rt, 0);
+                        if (r != PAM_SUCCESS) {
+                                pam_syslog(handle, LOG_ERR, "Failed to set runtime dir.");
+                                return r;
+                        }
                 }
 
                 r = export_legacy_dbus_address(handle, pw->pw_uid, rt);
@@ -565,10 +597,12 @@ _public_ PAM_EXTERN int pam_sm_open_session(
                  * in privileged apps clobbering the runtime directory
                  * unnecessarily. */
 
-                r = pam_misc_setenv(handle, "XDG_RUNTIME_DIR", runtime_path, 0);
-                if (r != PAM_SUCCESS) {
-                        pam_syslog(handle, LOG_ERR, "Failed to set runtime dir.");
-                        return r;
+                if (validate_runtime_directory(handle, runtime_path, pw->pw_uid)) {
+                        r = pam_misc_setenv(handle, "XDG_RUNTIME_DIR", runtime_path, 0);
+                        if (r != PAM_SUCCESS) {
+                                pam_syslog(handle, LOG_ERR, "Failed to set runtime dir.");
+                                return r;
+                        }
                 }
 
                 r = export_legacy_dbus_address(handle, pw->pw_uid, runtime_path);