[PATCH v4] statd: drop all capabilities from the bounding set as well

[Jeff Layton <jlayton@redhat.com>]

statd drops all capabilities except for CAP_NET_BIND when it starts. It's
possible though that if it ever had a compromise that an attacker would be
able to invoke a setuid process (or something with file capabilities) in
order to reinstate some caps.

This could happen as a result of the daemon becoming compromised, or
possibly as a result of the ha-callout program becoming compromised.

In order to prevent that, have statd also prune the capability bounding
set to nothing prior to dropping capabilities. That ensures that the
process won't be able to reacquire capabilities via any means --
including exec'ing a setuid program.

We do however need to be cognizant of the fact that PR_CAPBSET_DROP was
only added in 2.6.25, so check to make sure that #define exists via
autoconf before we rely on it. In order to do that, we must add
ax_check_define.m4 from the GNU autoconf macro archive.

Furthermore, do a runtime check to see if /proc/sys/kernel/cap-bound
exists before attempting to clear the bounding set. If it does, then
don't bother trying since it won't work. In that event though, do
throw a warning however since the presence of that file indicates that
there is a disconnect between the build and runtime environments.

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
---
 support/nsm/file.c |   57 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 55 insertions(+), 2 deletions(-)

diff --git a/support/nsm/file.c b/support/nsm/file.c
index 5dd52c1..5476446 100644
--- a/support/nsm/file.c
+++ b/support/nsm/file.c
@@ -338,10 +338,10 @@ nsm_is_default_parentdir(void)
  *
  * Returns true if successful, or false if some error occurred.
  */
+#ifdef HAVE_SYS_CAPABILITY_H
 static _Bool
 nsm_clear_capabilities(void)
 {
-#ifdef HAVE_SYS_CAPABILITY_H
 	cap_t caps;
 
 	caps = cap_from_text("cap_net_bind_service=ep");
@@ -357,10 +357,60 @@ nsm_clear_capabilities(void)
 	}
 
 	(void)cap_free(caps);
-#endif
 	return true;
 }
 
+#define CAP_BOUND_PROCFILE "/proc/sys/kernel/cap-bound"
+static _Bool
+prune_bounding_set(void)
+{
+#ifdef PR_CAPBSET_DROP
+	int ret;
+	unsigned long i;
+	struct stat st;
+
+	/*
+	 * Prior to kernel 2.6.25, the capabilities bounding set was a global
+	 * value. Check to see if /proc/sys/kernel/cap-bound exists and don't
+	 * bother to clear the bounding set if it does.
+	 */
+	ret = stat(CAP_BOUND_PROCFILE, &st);
+	if (!ret) {
+		xlog(L_WARNING, "%s exists. Not attempting to clear "
+				"capabilities bounding set.",
+				CAP_BOUND_PROCFILE);
+		return true;
+	} else if (errno != ENOENT) {
+		/* Warn, but attempt to clear the bounding set anyway. */
+		xlog(L_WARNING, "Unable to stat %s: %m", CAP_BOUND_PROCFILE);
+	}
+
+	/* prune the bounding set to nothing */
+	for (i = 0; i <= CAP_LAST_CAP; ++i) {
+		ret = prctl(PR_CAPBSET_DROP, i, 0, 0, 0);
+		if (ret) {
+			xlog(L_ERROR, "Unable to prune capability %lu from "
+				      "bounding set: %m", i);
+			return false;
+		}
+	}
+#endif /* PR_CAPBSET_DROP */
+	return true;
+}
+#else /* !HAVE_SYS_CAPABILITY_H */
+static _Bool
+nsm_clear_capabilities(void)
+{
+	return true;
+}
+
+static _Bool
+prune_bounding_set(void)
+{
+	return true;
+}
+#endif /* HAVE_SYS_CAPABILITY_H */
+
 /**
  * nsm_drop_privileges - drop root privileges
  * @pidfd: file descriptor of a pid file
@@ -393,6 +443,9 @@ nsm_drop_privileges(const int pidfd)
 		return false;
 	}
 
+	if (!prune_bounding_set())
+		return false;
+
 	if (st.st_uid == 0) {
 		xlog_warn("Running as root.  "
 			"chown %s to choose different user", nsm_base_dirname);
-- 
1.7.7.6