security/safesetid/lsm.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/hashtable.h>
#include <linux/lsm_hooks.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/sched/task_stack.h>
#include <linux/security.h>

/* Flag indicating whether initialization completed */
int safesetid_initialized;

#define NUM_BITS 8 /* 128 buckets in hash table */

static DEFINE_HASHTABLE(safesetid_whitelist_hashtable, NUM_BITS);

/*
 * Hash table entry to store safesetid policy signifying that 'parent' user
 * can setid to 'child' user.
 */
struct entry {
	struct hlist_node next;
	struct hlist_node dlist; /* for deletion cleanup */
	uint64_t parent_kuid;
	uint64_t child_kuid;
};

static DEFINE_SPINLOCK(safesetid_whitelist_hashtable_spinlock);

static bool check_setuid_policy_hashtable_key(kuid_t parent)
{
	struct entry *entry;

	rcu_read_lock();
	hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
				   entry, next, __kuid_val(parent)) {
		if (entry->parent_kuid == __kuid_val(parent)) {
			rcu_read_unlock();
			return true;
		}
	}
	rcu_read_unlock();

	return false;
}

static bool check_setuid_policy_hashtable_key_value(kuid_t parent,
						    kuid_t child)
{
	struct entry *entry;

	rcu_read_lock();
	hash_for_each_possible_rcu(safesetid_whitelist_hashtable,
				   entry, next, __kuid_val(parent)) {
		if (entry->parent_kuid == __kuid_val(parent) &&
		    entry->child_kuid == __kuid_val(child)) {
			rcu_read_unlock();
			return true;
		}
	}
	rcu_read_unlock();

	return false;
}

static int safesetid_security_capable(const struct cred *cred,
				      struct user_namespace *ns,
				      int cap,
				      unsigned int opts)
{
	if (cap == CAP_SETUID &&
	    check_setuid_policy_hashtable_key(cred->uid)) {
		if (!(opts & CAP_OPT_INSETID)) {
			/*
			 * Deny if we're not in a set*uid() syscall to avoid
			 * giving powers gated by CAP_SETUID that are related
			 * to functionality other than calling set*uid() (e.g.
			 * allowing user to set up userns uid mappings).
			 */
			pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
				__kuid_val(cred->uid));
			return -1;
		}
	}
	return 0;
}

/*
 * Check whether a caller with old credentials @old is allowed to switch to
 * credentials that contain @new_uid.
 */
static bool uid_permitted_for_cred(const struct cred *old, kuid_t new_uid)
{
	bool permitted;

	/* If our old creds already had this UID in it, it's fine. */
	if (uid_eq(new_uid, old->uid) || uid_eq(new_uid, old->euid) ||
	    uid_eq(new_uid, old->suid))
		return true;

	/*
	 * Transitions to new UIDs require a check against the policy of the old
	 * RUID.
	 */
	permitted = check_setuid_policy_hashtable_key_value(old->uid, new_uid);
	if (!permitted) {
		pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
			__kuid_val(old->uid), __kuid_val(old->euid),
			__kuid_val(old->suid), __kuid_val(new_uid));
	}
	return permitted;
}

/*
 * Check whether there is either an exception for user under old cred struct to
 * set*uid to user under new cred struct, or the UID transition is allowed (by
 * Linux set*uid rules) even without CAP_SETUID.
 */
static int safesetid_task_fix_setuid(struct cred *new,
				     const struct cred *old,
				     int flags)
{

	/* Do nothing if there are no setuid restrictions for our old RUID. */
	if (!check_setuid_policy_hashtable_key(old->uid))
		return 0;

	if (uid_permitted_for_cred(old, new->uid) &&
	    uid_permitted_for_cred(old, new->euid) &&
	    uid_permitted_for_cred(old, new->suid) &&
	    uid_permitted_for_cred(old, new->fsuid))
		return 0;

	/*
	 * Kill this process to avoid potential security vulnerabilities
	 * that could arise from a missing whitelist entry preventing a
	 * privileged process from dropping to a lesser-privileged one.
	 */
	force_sig(SIGKILL);
	return -EACCES;
}

int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child)
{
	struct entry *new;

	/* Return if entry already exists */
	if (check_setuid_policy_hashtable_key_value(parent, child))
		return 0;

	new = kzalloc(sizeof(struct entry), GFP_KERNEL);
	if (!new)
		return -ENOMEM;
	new->parent_kuid = __kuid_val(parent);
	new->child_kuid = __kuid_val(child);
	spin_lock(&safesetid_whitelist_hashtable_spinlock);
	hash_add_rcu(safesetid_whitelist_hashtable,
		     &new->next,
		     __kuid_val(parent));
	spin_unlock(&safesetid_whitelist_hashtable_spinlock);
	return 0;
}

void flush_safesetid_whitelist_entries(void)
{
	struct entry *entry;
	struct hlist_node *hlist_node;
	unsigned int bkt_loop_cursor;
	HLIST_HEAD(free_list);

	/*
	 * Could probably use hash_for_each_rcu here instead, but this should
	 * be fine as well.
	 */
	spin_lock(&safesetid_whitelist_hashtable_spinlock);
	hash_for_each_safe(safesetid_whitelist_hashtable, bkt_loop_cursor,
			   hlist_node, entry, next) {
		hash_del_rcu(&entry->next);
		hlist_add_head(&entry->dlist, &free_list);
	}
	spin_unlock(&safesetid_whitelist_hashtable_spinlock);
	synchronize_rcu();
	hlist_for_each_entry_safe(entry, hlist_node, &free_list, dlist) {
		hlist_del(&entry->dlist);
		kfree(entry);
	}
}

static struct security_hook_list safesetid_security_hooks[] = {
	LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
	LSM_HOOK_INIT(capable, safesetid_security_capable)
};

static int __init safesetid_security_init(void)
{
	security_add_hooks(safesetid_security_hooks,
			   ARRAY_SIZE(safesetid_security_hooks), "safesetid");

	/* Report that SafeSetID successfully initialized */
	safesetid_initialized = 1;

	return 0;
}

DEFINE_LSM(safesetid_security_init) = {
	.init = safesetid_security_init,
	.name = "safesetid",
};