selinux: only invoke capabilities and selinux for CAP_MAC_ADMIN checks

SELinux uses CAP_MAC_ADMIN to control the ability to get or set a raw,
uninterpreted security context unknown to the currently loaded security
policy. When performing these checks, we only want to perform a base
capabilities check and a SELinux permission check.  If any other
modules that implement a capable hook are stacked with SELinux, we do
not want to require them to also have to authorize CAP_MAC_ADMIN,
since it may have different implications for their security model.
Rework the CAP_MAC_ADMIN checks within SELinux to only invoke the
capabilities module and the SELinux permission checking.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -3107,6 +3107,18 @@ static int selinux_inode_setotherxattr(struct dentry *dentry, const char *name)
 	return dentry_has_perm(cred, dentry, FILE__SETATTR);
 }
 
+static bool has_cap_mac_admin(bool audit)
+{
+	const struct cred *cred = current_cred();
+	int cap_audit = audit ? SECURITY_CAP_AUDIT : SECURITY_CAP_NOAUDIT;
+
+	if (cap_capable(cred, &init_user_ns, CAP_MAC_ADMIN, cap_audit))
+		return false;
+	if (cred_has_capability(cred, CAP_MAC_ADMIN, cap_audit, true))
+		return false;
+	return true;
+}
+
 static int selinux_inode_setxattr(struct dentry *dentry, const char *name,
 				  const void *value, size_t size, int flags)
 {
@@ -3138,7 +3150,7 @@ static int selinux_inode_setxattr(struct dentry *dentry, const char *name,
 
 	rc = security_context_to_sid(value, size, &newsid, GFP_KERNEL);
 	if (rc == -EINVAL) {
-		if (!capable(CAP_MAC_ADMIN)) {
+		if (!has_cap_mac_admin(true)) {
 			struct audit_buffer *ab;
 			size_t audit_size;
 			const char *str;
@@ -3264,13 +3276,8 @@ static int selinux_inode_getsecurity(struct inode *inode, const char *name, void
 	 * and lack of permission just means that we fall back to the
 	 * in-core context value, not a denial.
 	 */
-	error = cap_capable(current_cred(), &init_user_ns, CAP_MAC_ADMIN,
-			    SECURITY_CAP_NOAUDIT);
-	if (!error)
-		error = cred_has_capability(current_cred(), CAP_MAC_ADMIN,
-					    SECURITY_CAP_NOAUDIT, true);
 	isec = inode_security(inode);
-	if (!error)
+	if (has_cap_mac_admin(false))
 		error = security_sid_to_context_force(isec->sid, &context,
 						      &size);
 	else
@@ -5919,7 +5926,7 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
 		}
 		error = security_context_to_sid(value, size, &sid, GFP_KERNEL);
 		if (error == -EINVAL && !strcmp(name, "fscreate")) {
-			if (!capable(CAP_MAC_ADMIN)) {
+			if (!has_cap_mac_admin(true)) {
 				struct audit_buffer *ab;
 				size_t audit_size;