[PATCH] Realtime LSM

Jack O'Quin
2004-11-09T22:42:06Z

The realtime-lsm Linux Security Module, written by Torben Hohn and
Jack O'Quin with significant help from Chris Wright and Jody McIntyre,
selectively grants realtime capabilities to specific user groups or
applications. One typical use is low latency audio, and the patch has
been extensively field tested by Linux audio users.

We developed this LSM in response to a serious need. Audio developers
have been struggling for years with having to apply specialized kernel
patches to get reliable realtime operation. Audio is very intolerant
of scheduling glitches. They cause nasty pops in the output. Also,
large audio applications should not run as `root'. The 2.4
"capabilities patch" was never a satisfactory solution for this
problem, but it was the best we could do at the time.

Now, thanks to the good work being done on 2.6, we are now close to
being able to do serious realtime work with standard kernels available
everywhere. The LSM framework is an important element of that
solution with the realtime LSM a small but essential component,
because it makes these features available without excessive
administrative burden. Many musicians have a Mac or Windows
background, systems which grant realtime privileges to all tasks
indiscriminantly. The realtime LSM allows us to grant similar
privileges while maintaining better control over who gets them.

This version was originally applied against linux-2.6.10-rc1-mm2, and
retested with linux-2.6.10-rc1-mm4.

Signed-Off-By: Jack O'Quin <joq@joq.us>

diff -ruN -X /home/joq/bin/kdiff.exclude linux-2.6.10-rc1-mm2/Documentation/realtime-lsm.txt linux-2.6.10-rc1-mm2-rt1/Documentation/realtime-lsm.txt
--- linux-2.6.10-rc1-mm2/Documentation/realtime-lsm.txt Wed Dec 31 18:00:00 1969
+++ linux-2.6.10-rc1-mm2-rt1/Documentation/realtime-lsm.txt Fri Oct 29 21:29:10 2004
@@ -0,0 +1,39 @@
+
+ Realtime Linux Security Module
+
+
+This Linux Security Module (LSM) enables realtime capabilities. It
+was written by Torben Hohn and Jack O'Quin, under the provisions of
+the GPL (see the COPYING file). We make no warranty concerning the
+safety, security or even stability of your system when using it. But,
+we will fix problems if you report them.
+
+Once the LSM has been installed and the kernel for which it was built
+is running, the root user can load it and pass parameters as follows:
+
+ # modprobe realtime any=1
+
+ Any program can request realtime privileges. This allows any local
+ user to crash the system by hogging the CPU in a tight loop or
+ locking down too much memory. But, it is simple to administer. :-)
+
+ # modprobe realtime gid=29
+
+ All users belonging to group 29 and programs that are setgid to that
+ group have realtime privileges. Use any group number you like. A
+ `gid' of -1 disables group access.
+
+ # modprobe realtime mlock=0
+
+ Grants realtime scheduling privileges without the ability to lock
+ memory using mlock() or mlockall() system calls. This option can be
+ used in conjunction with any of the other options.
+
+After the module is loaded, its parameters can be changed dynamically
+via sysfs.
+
+ # echo 1 > /sys/module/realtime/parameters/any
+ # echo 29 > /sys/module/realtime/parameters/gid
+ # echo 1 > /sys/module/realtime/parameters/mlock
+
+Jack O'Quin, joq@joq.us
diff -ruN -X /home/joq/bin/kdiff.exclude linux-2.6.10-rc1-mm2/security/Kconfig linux-2.6.10-rc1-mm2-rt1/security/Kconfig
--- linux-2.6.10-rc1-mm2/security/Kconfig Fri Oct 29 21:12:59 2004
+++ linux-2.6.10-rc1-mm2-rt1/security/Kconfig Fri Oct 29 23:55:03 2004
@@ -84,6 +84,17 @@

If you are unsure how to answer this question, answer N.

+config SECURITY_REALTIME
+ tristate "Realtime Capabilities"
+ depends on SECURITY && SECURITY_CAPABILITIES!=y
+ default n
+ help
+ This module selectively grants realtime privileges
+ controlled by parameters set at load time or via files in
+ /sys/module/realtime/parameters.
+
+ If you are unsure how to answer this question, answer N.
+
source security/selinux/Kconfig

endmenu
diff -ruN -X /home/joq/bin/kdiff.exclude linux-2.6.10-rc1-mm2/security/Makefile linux-2.6.10-rc1-mm2-rt1/security/Makefile
--- linux-2.6.10-rc1-mm2/security/Makefile Fri Oct 29 21:12:59 2004
+++ linux-2.6.10-rc1-mm2-rt1/security/Makefile Fri Oct 29 21:29:10 2004
@@ -17,3 +17,4 @@
obj-$(CONFIG_SECURITY_CAPABILITIES) += commoncap.o capability.o
obj-$(CONFIG_SECURITY_ROOTPLUG) += commoncap.o root_plug.o
obj-$(CONFIG_SECURITY_SECLVL) += seclvl.o
+obj-$(CONFIG_SECURITY_REALTIME) += commoncap.o realtime.o
diff -ruN -X /home/joq/bin/kdiff.exclude linux-2.6.10-rc1-mm2/security/realtime.c linux-2.6.10-rc1-mm2-rt1/security/realtime.c
--- linux-2.6.10-rc1-mm2/security/realtime.c Wed Dec 31 18:00:00 1969
+++ linux-2.6.10-rc1-mm2-rt1/security/realtime.c Fri Oct 29 23:55:55 2004
@@ -0,0 +1,161 @@
+/*
+ * Realtime Capabilities Linux Security Module
+ *
+ * Copyright (C) 2003 Torben Hohn
+ * Copyright (C) 2003, 2004 Jack O'Quin
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ */
+
+#include <linux/config.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/security.h>
+#include <linux/file.h>
+#include <linux/mm.h>
+#include <linux/mman.h>
+#include <linux/pagemap.h>
+#include <linux/swap.h>
+#include <linux/smp_lock.h>
+#include <linux/skbuff.h>
+#include <linux/netlink.h>
+#include <linux/ptrace.h>
+#include <linux/sysctl.h>
+#include <linux/moduleparam.h>
+
+#define RT_LSM "Realtime LSM " /* syslog module name prefix */
+#define RT_ERR "Realtime: " /* syslog error message prefix */
+
+#include <linux/vermagic.h>
+MODULE_INFO(vermagic,VERMAGIC_STRING);
+
+/* module parameters
+ *
+ * These values could change at any time due to some process writing
+ * a new value in /sys/module/realtime/parameters. This is OK,
+ * because each is referenced only once in each function call.
+ * Nothing depends on parameters having the same value every time.
+ */
+
+/* if TRUE, any process is realtime */
+static int rt_any;
+module_param_named(any, rt_any, int, 0644);
+MODULE_PARM_DESC(any, " grant realtime privileges to any process.");
+
+/* realtime group id, or NO_GROUP */
+static int rt_gid = -1;
+module_param_named(gid, rt_gid, int, 0644);
+MODULE_PARM_DESC(gid, " the group ID with access to realtime privileges.");
+
+/* enable mlock() privileges */
+static int rt_mlock = 1;
+module_param_named(mlock, rt_mlock, int, 0644);
+MODULE_PARM_DESC(mlock, " enable memory locking privileges.");
+
+/* helper function for testing group membership */
+static inline int gid_ok(int gid)
+{
+ if (gid == -1)
+ return 0;
+
+ if (gid == current->gid)
+ return 1;
+
+ return in_egroup_p(gid);
+}
+
+static void realtime_bprm_apply_creds(struct linux_binprm *bprm, int unsafe)
+{
+ cap_bprm_apply_creds(bprm, unsafe);
+
+ /* If a non-zero `any' parameter was specified, we grant
+ * realtime privileges to every process. If the `gid'
+ * parameter was specified and it matches the group id of the
+ * executable, of the current process or any supplementary
+ * groups, we grant realtime capabilites.
+ */
+
+ if (rt_any || gid_ok(rt_gid)) {
+ cap_raise(current->cap_effective, CAP_SYS_NICE);
+ if (rt_mlock) {
+ cap_raise(current->cap_effective, CAP_IPC_LOCK);
+ cap_raise(current->cap_effective, CAP_SYS_RESOURCE);
+ }
+ }
+}
+
+static struct security_operations capability_ops = {
+ .ptrace = cap_ptrace,
+ .capget = cap_capget,
+ .capset_check = cap_capset_check,
+ .capset_set = cap_capset_set,
+ .capable = cap_capable,
+ .netlink_send = cap_netlink_send,
+ .netlink_recv = cap_netlink_recv,
+ .bprm_apply_creds = realtime_bprm_apply_creds,
+ .bprm_set_security = cap_bprm_set_security,
+ .bprm_secureexec = cap_bprm_secureexec,
+ .task_post_setuid = cap_task_post_setuid,
+ .task_reparent_to_init = cap_task_reparent_to_init,
+ .syslog = cap_syslog,
+ .vm_enough_memory = cap_vm_enough_memory,
+};
+
+#define MY_NAME __stringify(KBUILD_MODNAME)
+
+static int secondary; /* flag to keep track of how we were registered */
+
+static int __init realtime_init(void)
+{
+ /* register ourselves with the security framework */
+ if (register_security(&capability_ops)) {
+
+ /* try registering with primary module */
+ if (mod_reg_security(MY_NAME, &capability_ops)) {
+ printk(KERN_INFO RT_ERR "Failure registering "
+ "capabilities with primary security module.\n");
+ printk(KERN_INFO RT_ERR "Is kernel configured "
+ "with CONFIG_SECURITY_CAPABILITIES=m?\n");
+ return -EINVAL;
+ }
+ secondary = 1;
+ }
+
+ if (rt_any)
+ printk(KERN_INFO RT_LSM
+ "initialized (all groups, mlock=%d)\n", rt_mlock);
+ else if (rt_gid == -1)
+ printk(KERN_INFO RT_LSM
+ "initialized (no groups, mlock=%d)\n", rt_mlock);
+ else
+ printk(KERN_INFO RT_LSM
+ "initialized (group %d, mlock=%d)\n", rt_gid, rt_mlock);
+
+ return 0;
+}
+
+static void __exit realtime_exit(void)
+{
+ /* remove ourselves from the security framework */
+ if (secondary) {
+ if (mod_unreg_security(MY_NAME, &capability_ops))
+ printk(KERN_INFO RT_ERR "Failure unregistering "
+ "capabilities with primary module.\n");
+
+ } else if (unregister_security(&capability_ops)) {
+ printk(KERN_INFO RT_ERR
+ "Failure unregistering capabilities with the kernel\n");
+ }
+ printk(KERN_INFO "Realtime Capability LSM exiting\n");
+}
+
+late_initcall(realtime_init);
+module_exit(realtime_exit);
+
+MODULE_DESCRIPTION("Realtime Capabilities Security Module");
+MODULE_LICENSE("GPL");

--
joq