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// 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/lsm_hooks.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/sched/task_stack.h>
#include <linux/security.h>
#include "lsm.h"
/* Flag indicating whether initialization completed */
int safesetid_initialized;
struct setuid_ruleset __rcu *safesetid_setuid_rules;
/* Compute a decision for a transition from @src to @dst under @policy. */
enum sid_policy_type _setuid_policy_lookup(struct setuid_ruleset *policy,
kuid_t src, kuid_t dst)
{
struct setuid_rule *rule;
enum sid_policy_type result = SIDPOL_DEFAULT;
hash_for_each_possible(policy->rules, rule, next, __kuid_val(src)) {
if (!uid_eq(rule->src_uid, src))
continue;
if (uid_eq(rule->dst_uid, dst))
return SIDPOL_ALLOWED;
result = SIDPOL_CONSTRAINED;
}
return result;
}
/*
* Compute a decision for a transition from @src to @dst under the active
* policy.
*/
static enum sid_policy_type setuid_policy_lookup(kuid_t src, kuid_t dst)
{
enum sid_policy_type result = SIDPOL_DEFAULT;
struct setuid_ruleset *pol;
rcu_read_lock();
pol = rcu_dereference(safesetid_setuid_rules);
if (pol)
result = _setuid_policy_lookup(pol, src, dst);
rcu_read_unlock();
return result;
}
static int safesetid_security_capable(const struct cred *cred,
struct user_namespace *ns,
int cap,
unsigned int opts)
{
/* We're only interested in CAP_SETUID. */
if (cap != CAP_SETUID)
return 0;
/*
* If CAP_SETUID is currently used for a set*uid() syscall, we want to
* let it go through here; the real security check happens later, in the
* task_fix_setuid hook.
*/
if ((opts & CAP_OPT_INSETID) != 0)
return 0;
/*
* If no policy applies to this task, allow the use of CAP_SETUID for
* other purposes.
*/
if (setuid_policy_lookup(cred->uid, INVALID_UID) == SIDPOL_DEFAULT)
return 0;
/*
* Reject use of CAP_SETUID for functionality other than calling
* set*uid() (e.g. setting 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 -EPERM;
}
/*
* 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 =
setuid_policy_lookup(old->uid, new_uid) != SIDPOL_CONSTRAINED;
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 (setuid_policy_lookup(old->uid, INVALID_UID) == SIDPOL_DEFAULT)
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;
}
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",
};
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