Commit 2d514487 authored by Kees Cook's avatar Kees Cook Committed by James Morris

security: Yama LSM

This adds the Yama Linux Security Module to collect DAC security
improvements (specifically just ptrace restrictions for now) that have
existed in various forms over the years and have been carried outside the
mainline kernel by other Linux distributions like Openwall and grsecurity.
Signed-off-by: default avatarKees Cook <keescook@chromium.org>
Acked-by: default avatarJohn Johansen <john.johansen@canonical.com>
Signed-off-by: default avatarJames Morris <jmorris@namei.org>
parent 1a2a4d06
......@@ -6,6 +6,8 @@ SELinux.txt
- how to get started with the SELinux security enhancement.
Smack.txt
- documentation on the Smack Linux Security Module.
Yama.txt
- documentation on the Yama Linux Security Module.
apparmor.txt
- documentation on the AppArmor security extension.
credentials.txt
......
Yama is a Linux Security Module that collects a number of system-wide DAC
security protections that are not handled by the core kernel itself. To
select it at boot time, specify "security=yama" (though this will disable
any other LSM).
Yama is controlled through sysctl in /proc/sys/kernel/yama:
- ptrace_scope
==============================================================
ptrace_scope:
As Linux grows in popularity, it will become a larger target for
malware. One particularly troubling weakness of the Linux process
interfaces is that a single user is able to examine the memory and
running state of any of their processes. For example, if one application
(e.g. Pidgin) was compromised, it would be possible for an attacker to
attach to other running processes (e.g. Firefox, SSH sessions, GPG agent,
etc) to extract additional credentials and continue to expand the scope
of their attack without resorting to user-assisted phishing.
This is not a theoretical problem. SSH session hijacking
(http://www.storm.net.nz/projects/7) and arbitrary code injection
(http://c-skills.blogspot.com/2007/05/injectso.html) attacks already
exist and remain possible if ptrace is allowed to operate as before.
Since ptrace is not commonly used by non-developers and non-admins, system
builders should be allowed the option to disable this debugging system.
For a solution, some applications use prctl(PR_SET_DUMPABLE, ...) to
specifically disallow such ptrace attachment (e.g. ssh-agent), but many
do not. A more general solution is to only allow ptrace directly from a
parent to a child process (i.e. direct "gdb EXE" and "strace EXE" still
work), or with CAP_SYS_PTRACE (i.e. "gdb --pid=PID", and "strace -p PID"
still work as root).
For software that has defined application-specific relationships
between a debugging process and its inferior (crash handlers, etc),
prctl(PR_SET_PTRACER, pid, ...) can be used. An inferior can declare which
other process (and its descendents) are allowed to call PTRACE_ATTACH
against it. Only one such declared debugging process can exists for
each inferior at a time. For example, this is used by KDE, Chromium, and
Firefox's crash handlers, and by Wine for allowing only Wine processes
to ptrace each other.
0 - classic ptrace permissions: a process can PTRACE_ATTACH to any other
process running under the same uid, as long as it is dumpable (i.e.
did not transition uids, start privileged, or have called
prctl(PR_SET_DUMPABLE...) already).
1 - restricted ptrace: a process must have a predefined relationship
with the inferior it wants to call PTRACE_ATTACH on. By default,
this relationship is that of only its descendants when the above
classic criteria is also met. To change the relationship, an
inferior can call prctl(PR_SET_PTRACER, debugger, ...) to declare
an allowed debugger PID to call PTRACE_ATTACH on the inferior.
The original children-only logic was based on the restrictions in grsecurity.
==============================================================
......@@ -114,4 +114,10 @@
# define PR_SET_MM_START_BRK 6
# define PR_SET_MM_BRK 7
/*
* Set specific pid that is allowed to ptrace the current task.
* A value of 0 mean "no process".
*/
#define PR_SET_PTRACER 0x59616d61
#endif /* _LINUX_PRCTL_H */
......@@ -187,6 +187,7 @@ source security/selinux/Kconfig
source security/smack/Kconfig
source security/tomoyo/Kconfig
source security/apparmor/Kconfig
source security/yama/Kconfig
source security/integrity/Kconfig
......@@ -196,6 +197,7 @@ choice
default DEFAULT_SECURITY_SMACK if SECURITY_SMACK
default DEFAULT_SECURITY_TOMOYO if SECURITY_TOMOYO
default DEFAULT_SECURITY_APPARMOR if SECURITY_APPARMOR
default DEFAULT_SECURITY_YAMA if SECURITY_YAMA
default DEFAULT_SECURITY_DAC
help
......@@ -214,6 +216,9 @@ choice
config DEFAULT_SECURITY_APPARMOR
bool "AppArmor" if SECURITY_APPARMOR=y
config DEFAULT_SECURITY_YAMA
bool "Yama" if SECURITY_YAMA=y
config DEFAULT_SECURITY_DAC
bool "Unix Discretionary Access Controls"
......@@ -225,6 +230,7 @@ config DEFAULT_SECURITY
default "smack" if DEFAULT_SECURITY_SMACK
default "tomoyo" if DEFAULT_SECURITY_TOMOYO
default "apparmor" if DEFAULT_SECURITY_APPARMOR
default "yama" if DEFAULT_SECURITY_YAMA
default "" if DEFAULT_SECURITY_DAC
endmenu
......
......@@ -7,6 +7,7 @@ subdir-$(CONFIG_SECURITY_SELINUX) += selinux
subdir-$(CONFIG_SECURITY_SMACK) += smack
subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
subdir-$(CONFIG_SECURITY_YAMA) += yama
# always enable default capabilities
obj-y += commoncap.o
......@@ -21,6 +22,7 @@ obj-$(CONFIG_SECURITY_SMACK) += smack/built-in.o
obj-$(CONFIG_AUDIT) += lsm_audit.o
obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/built-in.o
obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/built-in.o
obj-$(CONFIG_SECURITY_YAMA) += yama/built-in.o
obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
# Object integrity file lists
......
config SECURITY_YAMA
bool "Yama support"
depends on SECURITY
select SECURITYFS
select SECURITY_PATH
default n
help
This selects Yama, which extends DAC support with additional
system-wide security settings beyond regular Linux discretionary
access controls. Currently available is ptrace scope restriction.
Further information can be found in Documentation/security/Yama.txt.
If you are unsure how to answer this question, answer N.
obj-$(CONFIG_SECURITY_YAMA) := yama.o
yama-y := yama_lsm.o
/*
* Yama Linux Security Module
*
* Author: Kees Cook <keescook@chromium.org>
*
* Copyright (C) 2010 Canonical, Ltd.
* Copyright (C) 2011 The Chromium OS Authors.
*
* This program is free software; you can redistribute it and/or modify
* it under the terms of the GNU General Public License version 2, as
* published by the Free Software Foundation.
*
*/
#include <linux/security.h>
#include <linux/sysctl.h>
#include <linux/ptrace.h>
#include <linux/prctl.h>
#include <linux/ratelimit.h>
static int ptrace_scope = 1;
/* describe a ptrace relationship for potential exception */
struct ptrace_relation {
struct task_struct *tracer;
struct task_struct *tracee;
struct list_head node;
};
static LIST_HEAD(ptracer_relations);
static DEFINE_SPINLOCK(ptracer_relations_lock);
/**
* yama_ptracer_add - add/replace an exception for this tracer/tracee pair
* @tracer: the task_struct of the process doing the ptrace
* @tracee: the task_struct of the process to be ptraced
*
* Each tracee can have, at most, one tracer registered. Each time this
* is called, the prior registered tracer will be replaced for the tracee.
*
* Returns 0 if relationship was added, -ve on error.
*/
static int yama_ptracer_add(struct task_struct *tracer,
struct task_struct *tracee)
{
int rc = 0;
struct ptrace_relation *added;
struct ptrace_relation *entry, *relation = NULL;
added = kmalloc(sizeof(*added), GFP_KERNEL);
if (!added)
return -ENOMEM;
spin_lock_bh(&ptracer_relations_lock);
list_for_each_entry(entry, &ptracer_relations, node)
if (entry->tracee == tracee) {
relation = entry;
break;
}
if (!relation) {
relation = added;
relation->tracee = tracee;
list_add(&relation->node, &ptracer_relations);
}
relation->tracer = tracer;
spin_unlock_bh(&ptracer_relations_lock);
if (added != relation)
kfree(added);
return rc;
}
/**
* yama_ptracer_del - remove exceptions related to the given tasks
* @tracer: remove any relation where tracer task matches
* @tracee: remove any relation where tracee task matches
*/
static void yama_ptracer_del(struct task_struct *tracer,
struct task_struct *tracee)
{
struct ptrace_relation *relation, *safe;
spin_lock_bh(&ptracer_relations_lock);
list_for_each_entry_safe(relation, safe, &ptracer_relations, node)
if (relation->tracee == tracee ||
relation->tracer == tracer) {
list_del(&relation->node);
kfree(relation);
}
spin_unlock_bh(&ptracer_relations_lock);
}
/**
* yama_task_free - check for task_pid to remove from exception list
* @task: task being removed
*/
static void yama_task_free(struct task_struct *task)
{
yama_ptracer_del(task, task);
}
/**
* yama_task_prctl - check for Yama-specific prctl operations
* @option: operation
* @arg2: argument
* @arg3: argument
* @arg4: argument
* @arg5: argument
*
* Return 0 on success, -ve on error. -ENOSYS is returned when Yama
* does not handle the given option.
*/
static int yama_task_prctl(int option, unsigned long arg2, unsigned long arg3,
unsigned long arg4, unsigned long arg5)
{
int rc;
struct task_struct *myself = current;
rc = cap_task_prctl(option, arg2, arg3, arg4, arg5);
if (rc != -ENOSYS)
return rc;
switch (option) {
case PR_SET_PTRACER:
/* Since a thread can call prctl(), find the group leader
* before calling _add() or _del() on it, since we want
* process-level granularity of control. The tracer group
* leader checking is handled later when walking the ancestry
* at the time of PTRACE_ATTACH check.
*/
rcu_read_lock();
if (!thread_group_leader(myself))
myself = rcu_dereference(myself->group_leader);
get_task_struct(myself);
rcu_read_unlock();
if (arg2 == 0) {
yama_ptracer_del(NULL, myself);
rc = 0;
} else {
struct task_struct *tracer;
rcu_read_lock();
tracer = find_task_by_vpid(arg2);
if (tracer)
get_task_struct(tracer);
else
rc = -EINVAL;
rcu_read_unlock();
if (tracer) {
rc = yama_ptracer_add(tracer, myself);
put_task_struct(tracer);
}
}
put_task_struct(myself);
break;
}
return rc;
}
/**
* task_is_descendant - walk up a process family tree looking for a match
* @parent: the process to compare against while walking up from child
* @child: the process to start from while looking upwards for parent
*
* Returns 1 if child is a descendant of parent, 0 if not.
*/
static int task_is_descendant(struct task_struct *parent,
struct task_struct *child)
{
int rc = 0;
struct task_struct *walker = child;
if (!parent || !child)
return 0;
rcu_read_lock();
if (!thread_group_leader(parent))
parent = rcu_dereference(parent->group_leader);
while (walker->pid > 0) {
if (!thread_group_leader(walker))
walker = rcu_dereference(walker->group_leader);
if (walker == parent) {
rc = 1;
break;
}
walker = rcu_dereference(walker->real_parent);
}
rcu_read_unlock();
return rc;
}
/**
* ptracer_exception_found - tracer registered as exception for this tracee
* @tracer: the task_struct of the process attempting ptrace
* @tracee: the task_struct of the process to be ptraced
*
* Returns 1 if tracer has is ptracer exception ancestor for tracee.
*/
static int ptracer_exception_found(struct task_struct *tracer,
struct task_struct *tracee)
{
int rc = 0;
struct ptrace_relation *relation;
struct task_struct *parent = NULL;
spin_lock_bh(&ptracer_relations_lock);
rcu_read_lock();
if (!thread_group_leader(tracee))
tracee = rcu_dereference(tracee->group_leader);
list_for_each_entry(relation, &ptracer_relations, node)
if (relation->tracee == tracee) {
parent = relation->tracer;
break;
}
if (task_is_descendant(parent, tracer))
rc = 1;
rcu_read_unlock();
spin_unlock_bh(&ptracer_relations_lock);
return rc;
}
/**
* yama_ptrace_access_check - validate PTRACE_ATTACH calls
* @child: task that current task is attempting to ptrace
* @mode: ptrace attach mode
*
* Returns 0 if following the ptrace is allowed, -ve on error.
*/
static int yama_ptrace_access_check(struct task_struct *child,
unsigned int mode)
{
int rc;
/* If standard caps disallows it, so does Yama. We should
* only tighten restrictions further.
*/
rc = cap_ptrace_access_check(child, mode);
if (rc)
return rc;
/* require ptrace target be a child of ptracer on attach */
if (mode == PTRACE_MODE_ATTACH &&
ptrace_scope &&
!task_is_descendant(current, child) &&
!ptracer_exception_found(current, child) &&
!capable(CAP_SYS_PTRACE))
rc = -EPERM;
if (rc) {
char name[sizeof(current->comm)];
printk_ratelimited(KERN_NOTICE "ptrace of non-child"
" pid %d was attempted by: %s (pid %d)\n",
child->pid,
get_task_comm(name, current),
current->pid);
}
return rc;
}
static struct security_operations yama_ops = {
.name = "yama",
.ptrace_access_check = yama_ptrace_access_check,
.task_prctl = yama_task_prctl,
.task_free = yama_task_free,
};
#ifdef CONFIG_SYSCTL
static int zero;
static int one = 1;
struct ctl_path yama_sysctl_path[] = {
{ .procname = "kernel", },
{ .procname = "yama", },
{ }
};
static struct ctl_table yama_sysctl_table[] = {
{
.procname = "ptrace_scope",
.data = &ptrace_scope,
.maxlen = sizeof(int),
.mode = 0644,
.proc_handler = proc_dointvec_minmax,
.extra1 = &zero,
.extra2 = &one,
},
{ }
};
#endif /* CONFIG_SYSCTL */
static __init int yama_init(void)
{
if (!security_module_enable(&yama_ops))
return 0;
printk(KERN_INFO "Yama: becoming mindful.\n");
if (register_security(&yama_ops))
panic("Yama: kernel registration failed.\n");
#ifdef CONFIG_SYSCTL
if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
panic("Yama: sysctl registration failed.\n");
#endif
return 0;
}
security_initcall(yama_init);
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