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* 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 ||
(tracer && 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 if (arg2 == PR_SET_PTRACER_ANY || (int)arg2 == -1) {
rc = yama_ptracer_add(NULL, myself);
} 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;
bool found = false;
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;
found = true;
break;
}
if (found && (parent == NULL || 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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