securityfs.c

// SPDX-License-Identifier: GPL-2.0
/*
 * SafeSetID Linux Security Module
 *
 * Author: Micah Morton <mortonm@chromium.org>
 *
 * Copyright (C) 2018 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.
 *
 */

#define pr_fmt(fmt) "SafeSetID: " fmt

#include <linux/security.h>
#include <linux/cred.h>

#include "lsm.h"

static DEFINE_MUTEX(uid_policy_update_lock);
static DEFINE_MUTEX(gid_policy_update_lock);

/*
 * In the case the input buffer contains one or more invalid IDs, the kid_t
 * variables pointed to by @parent and @child will get updated but this
 * function will return an error.
 * Contents of @buf may be modified.
 */
static int parse_policy_line(struct file *file, char *buf,
	struct setid_rule *rule)
{
	char *child_str;
	int ret;
	u32 parsed_parent, parsed_child;

	/* Format of |buf| string should be <UID>:<UID> or <GID>:<GID> */
	child_str = strchr(buf, ':');
	if (child_str == NULL)
		return -EINVAL;
	*child_str = '\0';
	child_str++;

	ret = kstrtou32(buf, 0, &parsed_parent);
	if (ret)
		return ret;

	ret = kstrtou32(child_str, 0, &parsed_child);
	if (ret)
		return ret;

	if (rule->type == UID){
		rule->src_id.uid = make_kuid(file->f_cred->user_ns, parsed_parent);
		rule->dst_id.uid = make_kuid(file->f_cred->user_ns, parsed_child);
		if (!uid_valid(rule->src_id.uid) || !uid_valid(rule->dst_id.uid))
			return -EINVAL;
	} else if (rule->type == GID){
		rule->src_id.gid = make_kgid(file->f_cred->user_ns, parsed_parent);
		rule->dst_id.gid = make_kgid(file->f_cred->user_ns, parsed_child);
		if (!gid_valid(rule->src_id.gid) || !gid_valid(rule->dst_id.gid))
			return -EINVAL;
	} else {
		/* Error, rule->type is an invalid type */
		return -EINVAL;
	}
	return 0;
}

static void __release_ruleset(struct rcu_head *rcu)
{
	struct setid_ruleset *pol =
		container_of(rcu, struct setid_ruleset, rcu);
	int bucket;
	struct setid_rule *rule;
	struct hlist_node *tmp;

	hash_for_each_safe(pol->rules, bucket, tmp, rule, next)
		kfree(rule);
	kfree(pol->policy_str);
	kfree(pol);
}

static void release_ruleset(struct setid_ruleset *pol){
	call_rcu(&pol->rcu, __release_ruleset);
}

static void insert_rule(struct setid_ruleset *pol, struct setid_rule *rule)
{
	if (pol->type == UID)
		hash_add(pol->rules, &rule->next, __kuid_val(rule->src_id.uid));
	else if (pol->type == GID)
		hash_add(pol->rules, &rule->next, __kgid_val(rule->src_id.gid));
	else /* Error, pol->type is neither UID or GID */
		return;
}

static int verify_ruleset(struct setid_ruleset *pol)
{
	int bucket;
	struct setid_rule *rule, *nrule;
	int res = 0;

	hash_for_each(pol->rules, bucket, rule, next) {
		if (_setid_policy_lookup(pol, rule->dst_id, INVALID_ID) == SIDPOL_DEFAULT) {
			if (pol->type == UID) {
				pr_warn("insecure policy detected: uid %d is constrained but transitively unconstrained through uid %d\n",
					__kuid_val(rule->src_id.uid),
					__kuid_val(rule->dst_id.uid));
			} else if (pol->type == GID) {
				pr_warn("insecure policy detected: gid %d is constrained but transitively unconstrained through gid %d\n",
					__kgid_val(rule->src_id.gid),
					__kgid_val(rule->dst_id.gid));
			} else { /* pol->type is an invalid type */
				res = -EINVAL;
				return res;
			}
			res = -EINVAL;

			/* fix it up */
			nrule = kmalloc(sizeof(struct setid_rule), GFP_KERNEL);
			if (!nrule)
				return -ENOMEM;
			if (pol->type == UID){
				nrule->src_id.uid = rule->dst_id.uid;
				nrule->dst_id.uid = rule->dst_id.uid;
				nrule->type = UID;
			} else { /* pol->type must be GID if we've made it to here */
				nrule->src_id.gid = rule->dst_id.gid;
				nrule->dst_id.gid = rule->dst_id.gid;
				nrule->type = GID;
			}
			insert_rule(pol, nrule);
		}
	}
	return res;
}

static ssize_t handle_policy_update(struct file *file,
				    const char __user *ubuf, size_t len, enum setid_type policy_type)
{
	struct setid_ruleset *pol;
	char *buf, *p, *end;
	int err;

	if (len >= KMALLOC_MAX_SIZE)
		return -EINVAL;

	pol = kmalloc(sizeof(struct setid_ruleset), GFP_KERNEL);
	if (!pol)
		return -ENOMEM;
	pol->policy_str = NULL;
	pol->type = policy_type;
	hash_init(pol->rules);

	p = buf = memdup_user_nul(ubuf, len);
	if (IS_ERR(buf)) {
		err = PTR_ERR(buf);
		goto out_free_pol;
	}
	pol->policy_str = kstrdup(buf, GFP_KERNEL);
	if (pol->policy_str == NULL) {
		err = -ENOMEM;
		goto out_free_buf;
	}

	/* policy lines, including the last one, end with \n */
	while (*p != '\0') {
		struct setid_rule *rule;

		end = strchr(p, '\n');
		if (end == NULL) {
			err = -EINVAL;
			goto out_free_buf;
		}
		*end = '\0';

		rule = kmalloc(sizeof(struct setid_rule), GFP_KERNEL);
		if (!rule) {
			err = -ENOMEM;
			goto out_free_buf;
		}

		rule->type = policy_type;
		err = parse_policy_line(file, p, rule);
		if (err)
			goto out_free_rule;

		if (_setid_policy_lookup(pol, rule->src_id, rule->dst_id) == SIDPOL_ALLOWED) {
			pr_warn("bad policy: duplicate entry\n");
			err = -EEXIST;
			goto out_free_rule;
		}

		insert_rule(pol, rule);
		p = end + 1;
		continue;

out_free_rule:
		kfree(rule);
		goto out_free_buf;
	}

	err = verify_ruleset(pol);
	/* bogus policy falls through after fixing it up */
	if (err && err != -EINVAL)
		goto out_free_buf;

	/*
	 * Everything looks good, apply the policy and release the old one.
	 * What we really want here is an xchg() wrapper for RCU, but since that
	 * doesn't currently exist, just use a spinlock for now.
	 */
	if (policy_type == UID) {
		mutex_lock(&uid_policy_update_lock);
		pol = rcu_replace_pointer(safesetid_setuid_rules, pol,
					  lockdep_is_held(&uid_policy_update_lock));
		mutex_unlock(&uid_policy_update_lock);
	} else if (policy_type == GID) {
		mutex_lock(&gid_policy_update_lock);
		pol = rcu_replace_pointer(safesetid_setgid_rules, pol,
					  lockdep_is_held(&gid_policy_update_lock));
		mutex_unlock(&gid_policy_update_lock);
	} else {
		/* Error, policy type is neither UID or GID */
		pr_warn("error: bad policy type");
	}
	err = len;

out_free_buf:
	kfree(buf);
out_free_pol:
	if (pol)
		release_ruleset(pol);
	return err;
}

static ssize_t safesetid_uid_file_write(struct file *file,
				    const char __user *buf,
				    size_t len,
				    loff_t *ppos)
{
	if (!file_ns_capable(file, &init_user_ns, CAP_MAC_ADMIN))
		return -EPERM;

	if (*ppos != 0)
		return -EINVAL;

	return handle_policy_update(file, buf, len, UID);
}

static ssize_t safesetid_gid_file_write(struct file *file,
				    const char __user *buf,
				    size_t len,
				    loff_t *ppos)
{
	if (!file_ns_capable(file, &init_user_ns, CAP_MAC_ADMIN))
		return -EPERM;

	if (*ppos != 0)
		return -EINVAL;

	return handle_policy_update(file, buf, len, GID);
}

static ssize_t safesetid_file_read(struct file *file, char __user *buf,
				   size_t len, loff_t *ppos, struct mutex *policy_update_lock, struct __rcu setid_ruleset* ruleset)
{
	ssize_t res = 0;
	struct setid_ruleset *pol;
	const char *kbuf;

	mutex_lock(policy_update_lock);
	pol = rcu_dereference_protected(ruleset, lockdep_is_held(policy_update_lock));
	if (pol) {
		kbuf = pol->policy_str;
		res = simple_read_from_buffer(buf, len, ppos,
					      kbuf, strlen(kbuf));
	}
	mutex_unlock(policy_update_lock);

	return res;
}

static ssize_t safesetid_uid_file_read(struct file *file, char __user *buf,
				   size_t len, loff_t *ppos)
{
	return safesetid_file_read(file, buf, len, ppos,
				   &uid_policy_update_lock, safesetid_setuid_rules);
}

static ssize_t safesetid_gid_file_read(struct file *file, char __user *buf,
				   size_t len, loff_t *ppos)
{
	return safesetid_file_read(file, buf, len, ppos,
				   &gid_policy_update_lock, safesetid_setgid_rules);
}



static const struct file_operations safesetid_uid_file_fops = {
	.read = safesetid_uid_file_read,
	.write = safesetid_uid_file_write,
};

static const struct file_operations safesetid_gid_file_fops = {
	.read = safesetid_gid_file_read,
	.write = safesetid_gid_file_write,
};

static int __init safesetid_init_securityfs(void)
{
	int ret;
	struct dentry *policy_dir;
	struct dentry *uid_policy_file;
	struct dentry *gid_policy_file;

	if (!safesetid_initialized)
		return 0;

	policy_dir = securityfs_create_dir("safesetid", NULL);
	if (IS_ERR(policy_dir)) {
		ret = PTR_ERR(policy_dir);
		goto error;
	}

	uid_policy_file = securityfs_create_file("uid_allowlist_policy", 0600,
			policy_dir, NULL, &safesetid_uid_file_fops);
	if (IS_ERR(uid_policy_file)) {
		ret = PTR_ERR(uid_policy_file);
		goto error;
	}

	gid_policy_file = securityfs_create_file("gid_allowlist_policy", 0600,
			policy_dir, NULL, &safesetid_gid_file_fops);
	if (IS_ERR(gid_policy_file)) {
		ret = PTR_ERR(gid_policy_file);
		goto error;
	}


	return 0;

error:
	securityfs_remove(policy_dir);
	return ret;
}
fs_initcall(safesetid_init_securityfs);