yama_lsm.c

/*
 * 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/lsm_hooks.h>
#include <linux/sysctl.h>
#include <linux/ptrace.h>
#include <linux/prctl.h>
#include <linux/ratelimit.h>
#include <linux/workqueue.h>
#include <linux/string_helpers.h>
#include <linux/task_work.h>
#include <linux/sched.h>
#include <linux/spinlock.h>

#define YAMA_SCOPE_DISABLED	0
#define YAMA_SCOPE_RELATIONAL	1
#define YAMA_SCOPE_CAPABILITY	2
#define YAMA_SCOPE_NO_ATTACH	3

static int ptrace_scope = YAMA_SCOPE_RELATIONAL;

/* describe a ptrace relationship for potential exception */
struct ptrace_relation {
	struct task_struct *tracer;
	struct task_struct *tracee;
	bool invalid;
	struct list_head node;
	struct rcu_head rcu;
};

static LIST_HEAD(ptracer_relations);
static DEFINE_SPINLOCK(ptracer_relations_lock);

static void yama_relation_cleanup(struct work_struct *work);
static DECLARE_WORK(yama_relation_work, yama_relation_cleanup);

struct access_report_info {
	struct callback_head work;
	const char *access;
	struct task_struct *target;
	struct task_struct *agent;
};

static void __report_access(struct callback_head *work)
{
	struct access_report_info *info =
		container_of(work, struct access_report_info, work);
	char *target_cmd, *agent_cmd;

	target_cmd = kstrdup_quotable_cmdline(info->target, GFP_KERNEL);
	agent_cmd = kstrdup_quotable_cmdline(info->agent, GFP_KERNEL);

	pr_notice_ratelimited(
		"ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
		info->access, target_cmd, info->target->pid, agent_cmd,
		info->agent->pid);

	kfree(agent_cmd);
	kfree(target_cmd);

	put_task_struct(info->agent);
	put_task_struct(info->target);
	kfree(info);
}

/* defers execution because cmdline access can sleep */
static void report_access(const char *access, struct task_struct *target,
				struct task_struct *agent)
{
	struct access_report_info *info;
	char agent_comm[sizeof(agent->comm)];

	assert_spin_locked(&target->alloc_lock); /* for target->comm */

	if (current->flags & PF_KTHREAD) {
		/* I don't think kthreads call task_work_run() before exiting.
		 * Imagine angry ranting about procfs here.
		 */
		pr_notice_ratelimited(
		    "ptrace %s of \"%s\"[%d] was attempted by \"%s\"[%d]\n",
		    access, target->comm, target->pid,
		    get_task_comm(agent_comm, agent), agent->pid);
		return;
	}

	info = kmalloc(sizeof(*info), GFP_ATOMIC);
	if (!info)
		return;
	init_task_work(&info->work, __report_access);
	get_task_struct(target);
	get_task_struct(agent);
	info->access = access;
	info->target = target;
	info->agent = agent;
	if (task_work_add(current, &info->work, true) == 0)
		return; /* success */

	WARN(1, "report_access called from exiting task");
	put_task_struct(target);
	put_task_struct(agent);
	kfree(info);
}

/**
 * yama_relation_cleanup - remove invalid entries from the relation list
 *
 */
static void yama_relation_cleanup(struct work_struct *work)
{
	struct ptrace_relation *relation;

	spin_lock(&ptracer_relations_lock);
	rcu_read_lock();
	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
		if (relation->invalid) {
			list_del_rcu(&relation->node);
			kfree_rcu(relation, rcu);
		}
	}
	rcu_read_unlock();
	spin_unlock(&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)
{
	struct ptrace_relation *relation, *added;

	added = kmalloc(sizeof(*added), GFP_KERNEL);
	if (!added)
		return -ENOMEM;

	added->tracee = tracee;
	added->tracer = tracer;
	added->invalid = false;

	spin_lock(&ptracer_relations_lock);
	rcu_read_lock();
	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
		if (relation->invalid)
			continue;
		if (relation->tracee == tracee) {
			list_replace_rcu(&relation->node, &added->node);
			kfree_rcu(relation, rcu);
			goto out;
		}
	}

	list_add_rcu(&added->node, &ptracer_relations);

out:
	rcu_read_unlock();
	spin_unlock(&ptracer_relations_lock);
	return 0;
}

/**
 * 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;
	bool marked = false;

	rcu_read_lock();
	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
		if (relation->invalid)
			continue;
		if (relation->tracee == tracee ||
		    (tracer && relation->tracer == tracer)) {
			relation->invalid = true;
			marked = true;
		}
	}
	rcu_read_unlock();

	if (marked)
		schedule_work(&yama_relation_work);
}

/**
 * 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 = -ENOSYS;
	struct task_struct *myself = current;

	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;

			tracer = find_get_task_by_vpid(arg2);
			if (!tracer) {
				rc = -EINVAL;
			} else {
				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 a 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;

	rcu_read_lock();

	/*
	 * If there's already an active tracing relationship, then make an
	 * exception for the sake of other accesses, like process_vm_rw().
	 */
	parent = ptrace_parent(tracee);
	if (parent != NULL && same_thread_group(parent, tracer)) {
		rc = 1;
		goto unlock;
	}

	/* Look for a PR_SET_PTRACER relationship. */
	if (!thread_group_leader(tracee))
		tracee = rcu_dereference(tracee->group_leader);
	list_for_each_entry_rcu(relation, &ptracer_relations, node) {
		if (relation->invalid)
			continue;
		if (relation->tracee == tracee) {
			parent = relation->tracer;
			found = true;
			break;
		}
	}

	if (found && (parent == NULL || task_is_descendant(parent, tracer)))
		rc = 1;

unlock:
	rcu_read_unlock();

	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 = 0;

	/* require ptrace target be a child of ptracer on attach */
	if (mode & PTRACE_MODE_ATTACH) {
		switch (ptrace_scope) {
		case YAMA_SCOPE_DISABLED:
			/* No additional restrictions. */
			break;
		case YAMA_SCOPE_RELATIONAL:
			rcu_read_lock();
			if (!pid_alive(child))
				rc = -EPERM;
			if (!rc && !task_is_descendant(current, child) &&
			    !ptracer_exception_found(current, child) &&
			    !ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
				rc = -EPERM;
			rcu_read_unlock();
			break;
		case YAMA_SCOPE_CAPABILITY:
			rcu_read_lock();
			if (!ns_capable(__task_cred(child)->user_ns, CAP_SYS_PTRACE))
				rc = -EPERM;
			rcu_read_unlock();
			break;
		case YAMA_SCOPE_NO_ATTACH:
		default:
			rc = -EPERM;
			break;
		}
	}

	if (rc && (mode & PTRACE_MODE_NOAUDIT) == 0)
		report_access("attach", child, current);

	return rc;
}

/**
 * yama_ptrace_traceme - validate PTRACE_TRACEME calls
 * @parent: task that will become the ptracer of the current task
 *
 * Returns 0 if following the ptrace is allowed, -ve on error.
 */
static int yama_ptrace_traceme(struct task_struct *parent)
{
	int rc = 0;

	/* Only disallow PTRACE_TRACEME on more aggressive settings. */
	switch (ptrace_scope) {
	case YAMA_SCOPE_CAPABILITY:
		if (!has_ns_capability(parent, current_user_ns(), CAP_SYS_PTRACE))
			rc = -EPERM;
		break;
	case YAMA_SCOPE_NO_ATTACH:
		rc = -EPERM;
		break;
	}

	if (rc) {
		task_lock(current);
		report_access("traceme", current, parent);
		task_unlock(current);
	}

	return rc;
}

static struct security_hook_list yama_hooks[] __lsm_ro_after_init = {
	LSM_HOOK_INIT(ptrace_access_check, yama_ptrace_access_check),
	LSM_HOOK_INIT(ptrace_traceme, yama_ptrace_traceme),
	LSM_HOOK_INIT(task_prctl, yama_task_prctl),
	LSM_HOOK_INIT(task_free, yama_task_free),
};

#ifdef CONFIG_SYSCTL
static int yama_dointvec_minmax(struct ctl_table *table, int write,
				void __user *buffer, size_t *lenp, loff_t *ppos)
{
	struct ctl_table table_copy;

	if (write && !capable(CAP_SYS_PTRACE))
		return -EPERM;

	/* Lock the max value if it ever gets set. */
	table_copy = *table;
	if (*(int *)table_copy.data == *(int *)table_copy.extra2)
		table_copy.extra1 = table_copy.extra2;

	return proc_dointvec_minmax(&table_copy, write, buffer, lenp, ppos);
}

static int zero;
static int max_scope = YAMA_SCOPE_NO_ATTACH;

static 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   = yama_dointvec_minmax,
		.extra1         = &zero,
		.extra2         = &max_scope,
	},
	{ }
};
static void __init yama_init_sysctl(void)
{
	if (!register_sysctl_paths(yama_sysctl_path, yama_sysctl_table))
		panic("Yama: sysctl registration failed.\n");
}
#else
static inline void yama_init_sysctl(void) { }
#endif /* CONFIG_SYSCTL */

static int __init yama_init(void)
{
	pr_info("Yama: becoming mindful.\n");
	security_add_hooks(yama_hooks, ARRAY_SIZE(yama_hooks), "yama");
	yama_init_sysctl();
	return 0;
}

DEFINE_LSM(yama) = {
	.name = "yama",
	.init = yama_init,
};