core: leave PAM stub process around with GIDs updated

In the process execution code of PID 1, before
096424d1230e0a0339735c51b43949809e972430 the GID settings where changed before
invoking PAM, and the UID settings after. After the change both changes are
made after the PAM session hooks are run. When invoking PAM we fork once, and
leave a stub process around which will invoke the PAM session end hooks when
the session goes away. This code previously was dropping the remaining privs
(which were precisely the UID). Fix this code to do this correctly again, by
really dropping them else (i.e. the GID as well).

While we are at it, also fix error logging of this code.

Fixes: #4238

diff --git a/src/core/execute.c b/src/core/execute.c
index 019ff8490bb12fbde0526732648e60c3aac5bab2..e4a23ac16955e1a1caef78aa7777dbde1de8ae47 100644 (file)
--- a/src/core/execute.c
+++ b/src/core/execute.c
@@ -843,6 +843,7 @@ static int setup_pam(
                 const char *name,
                 const char *user,
                 uid_t uid,
+                gid_t gid,
                 const char *tty,
                 char ***env,
                 int fds[], unsigned n_fds) {
@@ -948,8 +949,13 @@ static int setup_pam(
                  * and this will make PR_SET_PDEATHSIG work in most cases.
                  * If this fails, ignore the error - but expect sd-pam threads
                  * to fail to exit normally */
+
+                if (maybe_setgroups(0, NULL) < 0)
+                        log_warning_errno(errno, "Failed to setgroups() in sd-pam: %m");
+                if (setresgid(gid, gid, gid) < 0)
+                        log_warning_errno(errno, "Failed to setresgid() in sd-pam: %m");
                 if (setresuid(uid, uid, uid) < 0)
-                        log_error_errno(r, "Error: Failed to setresuid() in sd-pam: %m");
+                        log_warning_errno(errno, "Failed to setresuid() in sd-pam: %m");
 
                 (void) ignore_signals(SIGPIPE, -1);
 
@@ -2413,7 +2419,7 @@ static int exec_child(
                 }
 
                 if (context->pam_name && username) {
-                        r = setup_pam(context->pam_name, username, uid, context->tty_path, &accum_env, fds, n_fds);
+                        r = setup_pam(context->pam_name, username, uid, gid, context->tty_path, &accum_env, fds, n_fds);
                         if (r < 0) {
                                 *exit_status = EXIT_PAM;
                                 return r;