journal: call connect() with dropped privileges

When systemd starts a service, it first opened /run/systemd/journal/stdout
socket, and only later switched to the right user.group (if they are
specified). Later on, journald looked at the credentials, and saw
root.root, because credentials are stored at the time the socket is
opened. As a result, all messages passed over _TRANSPORT=stdout were
logged with _UID=0, _GID=0.

Drop real uid and gid temporarily to fix the issue.

1 files changed, 62 insertions, 24 deletions

diff --git a/src/core/execute.c b/src/core/execute.c
index a806d42827..51e1e87236 100644
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -219,12 +219,52 @@ static int open_null_as(int flags, int nfd) {
         return r;
 }
 
-static int connect_logger_as(const ExecContext *context, ExecOutput output, const char *ident, const char *unit_id, int nfd) {
-        int fd, r;
+static int connect_journal_socket(int fd, uid_t uid, gid_t gid) {
         union sockaddr_union sa = {
                 .un.sun_family = AF_UNIX,
                 .un.sun_path = "/run/systemd/journal/stdout",
         };
+        uid_t olduid = UID_INVALID;
+        gid_t oldgid = GID_INVALID;
+        int r;
+
+        if (gid != GID_INVALID) {
+                oldgid = getgid();
+
+                r = setegid(gid);
+                if (r < 0)
+                        return -errno;
+        }
+
+        if (uid != UID_INVALID) {
+                olduid = getuid();
+
+                r = seteuid(uid);
+                if (r < 0) {
+                        r = -errno;
+                        goto restore_gid;
+                }
+        }
+
+        r = connect(fd, &sa.sa, offsetof(struct sockaddr_un, sun_path) + strlen(sa.un.sun_path));
+        if (r < 0)
+                r = -errno;
+
+        /* If we fail to restore the uid or gid, things will likely
+           fail later on. This should only happen if an LSM interferes. */
+
+        if (uid != UID_INVALID)
+                (void) seteuid(olduid);
+
+ restore_gid:
+        if (gid != GID_INVALID)
+                (void) setegid(oldgid);
+
+        return r;
+}
+
+static int connect_logger_as(const ExecContext *context, ExecOutput output, const char *ident, const char *unit_id, int nfd, uid_t uid, gid_t gid) {
+        int fd, r;
 
         assert(context);
         assert(output < _EXEC_OUTPUT_MAX);
@@ -235,11 +275,9 @@ static int connect_logger_as(const ExecContext *context, ExecOutput output, cons
         if (fd < 0)
                 return -errno;
 
-        r = connect(fd, &sa.sa, offsetof(struct sockaddr_un, sun_path) + strlen(sa.un.sun_path));
-        if (r < 0) {
-                safe_close(fd);
-                return -errno;
-        }
+        r = connect_journal_socket(fd, uid, gid);
+        if (r < 0)
+                return r;
 
         if (shutdown(fd, SHUT_RD) < 0) {
                 safe_close(fd);
@@ -358,7 +396,7 @@ static int setup_input(const ExecContext *context, int socket_fd, bool apply_tty
         }
 }
 
-static int setup_output(const ExecContext *context, int fileno, int socket_fd, const char *ident, const char *unit_id, bool apply_tty_stdin) {
+static int setup_output(const ExecContext *context, int fileno, int socket_fd, const char *ident, const char *unit_id, bool apply_tty_stdin, uid_t uid, gid_t gid) {
         ExecOutput o;
         ExecInput i;
         int r;
@@ -425,10 +463,10 @@ static int setup_output(const ExecContext *context, int fileno, int socket_fd, c
         case EXEC_OUTPUT_KMSG_AND_CONSOLE:
         case EXEC_OUTPUT_JOURNAL:
         case EXEC_OUTPUT_JOURNAL_AND_CONSOLE:
-                r = connect_logger_as(context, o, ident, unit_id, fileno);
+                r = connect_logger_as(context, o, ident, unit_id, fileno, uid, gid);
                 if (r < 0) {
                         log_unit_struct(unit_id,
-                                        LOG_CRIT,
+                                        LOG_ERR,
                                         LOG_MESSAGE("Failed to connect %s of %s to the journal socket: %s",
                                                     fileno == STDOUT_FILENO ? "stdout" : "stderr",
                                                     unit_id, strerror(-r)),
@@ -1327,6 +1365,15 @@ static int exec_child(ExecCommand *command,
                 }
         }
 
+        if (context->user) {
+                username = context->user;
+                err = get_user_creds(&username, &uid, &gid, &home, &shell);
+                if (err < 0) {
+                        *error = EXIT_USER;
+                        return err;
+                }
+        }
+
         /* If a socket is connected to STDIN/STDOUT/STDERR, we
          * must sure to drop O_NONBLOCK */
         if (socket_fd >= 0)
@@ -1338,13 +1385,13 @@ static int exec_child(ExecCommand *command,
                 return err;
         }
 
-        err = setup_output(context, STDOUT_FILENO, socket_fd, basename(command->path), params->unit_id, params->apply_tty_stdin);
+        err = setup_output(context, STDOUT_FILENO, socket_fd, basename(command->path), params->unit_id, params->apply_tty_stdin, uid, gid);
         if (err < 0) {
                 *error = EXIT_STDOUT;
                 return err;
         }
 
-        err = setup_output(context, STDERR_FILENO, socket_fd, basename(command->path), params->unit_id, params->apply_tty_stdin);
+        err = setup_output(context, STDERR_FILENO, socket_fd, basename(command->path), params->unit_id, params->apply_tty_stdin, uid, gid);
         if (err < 0) {
                 *error = EXIT_STDERR;
                 return err;
@@ -1419,21 +1466,12 @@ static int exec_child(ExecCommand *command,
         if (context->utmp_id)
                 utmp_put_init_process(context->utmp_id, getpid(), getsid(0), context->tty_path);
 
-        if (context->user) {
-                username = context->user;
-                err = get_user_creds(&username, &uid, &gid, &home, &shell);
+        if (context->user && is_terminal_input(context->std_input)) {
+                err = chown_terminal(STDIN_FILENO, uid);
                 if (err < 0) {
-                        *error = EXIT_USER;
+                        *error = EXIT_STDIN;
                         return err;
                 }
-
-                if (is_terminal_input(context->std_input)) {
-                        err = chown_terminal(STDIN_FILENO, uid);
-                        if (err < 0) {
-                                *error = EXIT_STDIN;
-                                return err;
-                        }
-                }
         }
 
 #ifdef ENABLE_KDBUS