don't raise all privs on setuid-root file with fE set (v2)

Distributions face a backward compatibility problem with starting to use
file capabilities.  For instance, removing setuid root from ping and
doing setcap cap_net_raw=pe means that booting with an older kernel
or one compiled without file capabilities means ping won't work for
non-root users.

In order to replace the setuid root bit on a capability-unaware
program, one has to set the effective, or legacy, file capability,
which makes the capability effective immediately.  This patch
uses the legacy bit as a queue to not automatically add full
privilege to a setuid-root program.

So, with this patch, an ordinary setuid-root program will run with
privilege.  But if /bin/ping has both setuid-root and cap_net_raw in
fP and fE, then ping (when run by non-root user) will not run
with only cap_net_raw.

Changelog:
	Apr 2 2009: Print a message once when such a binary is loaded,
		as per James Morris' suggestion.
	Apr 2 2009: Fix the condition to only catch uid!=0 && euid==0.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: James Morris <jmorris@namei.org>

security/commoncap.c

@@ -28,6 +28,28 @@
 #include <linux/prctl.h>
 #include <linux/securebits.h>
 
+/*
+ * If a non-root user executes a setuid-root binary in
+ * !secure(SECURE_NOROOT) mode, then we raise capabilities.
+ * However if fE is also set, then the intent is for only
+ * the file capabilities to be applied, and the setuid-root
+ * bit is left on either to change the uid (plausible) or
+ * to get full privilege on a kernel without file capabilities
+ * support.  So in that case we do not raise capabilities.
+ *
+ * Warn if that happens, once per boot.
+ */
+static void warn_setuid_and_fcaps_mixed(char *fname)
+{
+	static int warned;
+	if (!warned) {
+		printk(KERN_INFO "warning: `%s' has both setuid-root and"
+			" effective capabilities. Therefore not raising all"
+			" capabilities.\n", fname);
+		warned = 1;
+	}
+}
+
 int cap_netlink_send(struct sock *sk, struct sk_buff *skb)
 {
 	NETLINK_CB(skb).eff_cap = current_cap();
@@ -463,6 +485,15 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 		return ret;
 
 	if (!issecure(SECURE_NOROOT)) {
+		/*
+		 * If the legacy file capability is set, then don't set privs
+		 * for a setuid root binary run by a non-root user.  Do set it
+		 * for a root user just to cause least surprise to an admin.
+		 */
+		if (effective && new->uid != 0 && new->euid == 0) {
+			warn_setuid_and_fcaps_mixed(bprm->filename);
+			goto skip;
+		}
 		/*
 		 * To support inheritance of root-permissions and suid-root
 		 * executables under compatibility mode, we override the
@@ -478,6 +509,7 @@ int cap_bprm_set_creds(struct linux_binprm *bprm)
 		if (new->euid == 0)
 			effective = true;
 	}
+skip:
 
 	/* Don't let someone trace a set[ug]id/setpcap binary with the revised
 	 * credentials unless they have the appropriate permit