gssd: have process_krb5_upcall fork before handling upcall

Most krb5 installations use credcache locations that contain %{uid},
which expands to the real UID of the current process. In order for
GSSAPI to find those properly, we need to be able to switch the real UID
of the process to the designated one. That however, opens the door to
allowing gssd to be killed or reniced during the window where we've
switched credentials.

To combat this, change gssd to fork before trying to handle each upcall.
The child will do the work to establish the context and the parent task
will just wait for it to exit. It's still possible for the child to be
killed or reniced, but that would only affect a single upcall instead of
the entire daemon. Also, If the process is killed prematurely, then log
an error to tip off the admin that there was a problem.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>

1 files changed, 23 insertions, 1 deletions

diff --git a/utils/gssd/gssd_proc.c b/utils/gssd/gssd_proc.c
index e58c341..99537d9 100644
--- a/utils/gssd/gssd_proc.c
+++ b/utils/gssd/gssd_proc.c
@@ -67,6 +67,8 @@
 #include <errno.h>
 #include <gssapi/gssapi.h>
 #include <netdb.h>
+#include <sys/types.h>
+#include <sys/wait.h>
 
 #include "gssd.h"
 #include "err_util.h"
@@ -982,6 +984,26 @@ process_krb5_upcall(struct clnt_info *clp, uid_t uid, int fd, char *tgtname,
 	int			err, downcall_err = -EACCES;
 	gss_cred_id_t		gss_cred;
 	OM_uint32		maj_stat, min_stat, lifetime_rec;
+	pid_t			pid;
+
+	pid = fork();
+	switch(pid) {
+	case 0:
+		/* Child: fall through to rest of function */
+		break;
+	case -1:
+		/* fork() failed! */
+		printerr(0, "WARNING: unable to fork() to handle upcall: %s\n",
+				strerror(errno));
+		return;
+	default:
+		/* Parent: just wait on child to exit and return */
+		wait(&err);
+		if (WIFSIGNALED(err))
+			printerr(0, "WARNING: forked child was killed with signal %d\n",
+					WTERMSIG(err));
+		return;
+	}
 
 	printerr(1, "handling krb5 upcall (%s)\n", clp->dirname);
 
@@ -1121,7 +1143,7 @@ out:
 		AUTH_DESTROY(auth);
 	if (rpc_clnt)
 		clnt_destroy(rpc_clnt);
-	return;
+	exit(0);
 
 out_return_error:
 	do_error_downcall(fd, uid, downcall_err);