// SPDX-License-Identifier: GPL-2.0
/*
* SafeSetID Linux Security Module
*
* Author: Micah Morton <[email protected]>
*
* 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 <uapi/linux/lsm.h>
#include "lsm.h"
/* Flag indicating whether initialization completed */
int safesetid_initialized __initdata;
struct setid_ruleset __rcu *safesetid_setuid_rules;
struct setid_ruleset __rcu *safesetid_setgid_rules;
/* Compute a decision for a transition from @src to @dst under @policy. */
enum sid_policy_type _setid_policy_lookup(struct setid_ruleset *policy,
kid_t src, kid_t dst)
{
struct setid_rule *rule;
enum sid_policy_type result = SIDPOL_DEFAULT;
if (policy->type == UID) {
hash_for_each_possible(policy->rules, rule, next, __kuid_val(src.uid)) {
if (!uid_eq(rule->src_id.uid, src.uid))
continue;
if (uid_eq(rule->dst_id.uid, dst.uid))
return SIDPOL_ALLOWED;
result = SIDPOL_CONSTRAINED;
}
} else if (policy->type == GID) {
hash_for_each_possible(policy->rules, rule, next, __kgid_val(src.gid)) {
if (!gid_eq(rule->src_id.gid, src.gid))
continue;
if (gid_eq(rule->dst_id.gid, dst.gid)){
return SIDPOL_ALLOWED;
}
result = SIDPOL_CONSTRAINED;
}
} else {
/* Should not reach here, report the ID as contrainsted */
result = SIDPOL_CONSTRAINED;
}
return result;
}
/*
* Compute a decision for a transition from @src to @dst under the active
* policy.
*/
static enum sid_policy_type setid_policy_lookup(kid_t src, kid_t dst, enum setid_type new_type)
{
enum sid_policy_type result = SIDPOL_DEFAULT;
struct setid_ruleset *pol;
rcu_read_lock();
if (new_type == UID)
pol = rcu_dereference(safesetid_setuid_rules);
else if (new_type == GID)
pol = rcu_dereference(safesetid_setgid_rules);
else { /* Should not reach here */
result = SIDPOL_CONSTRAINED;
rcu_read_unlock();
return result;
}
if (pol) {
pol->type = new_type;
result = _setid_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 and CAP_SETGID. */
if (cap != CAP_SETUID && cap != CAP_SETGID)
return 0;
/*
* If CAP_SET{U/G}ID is currently used for a setid or setgroups syscall, we
* want to let it go through here; the real security check happens later, in
* the task_fix_set{u/g}id or task_fix_setgroups hooks.
*/
if ((opts & CAP_OPT_INSETID) != 0)
return 0;
switch (cap) {
case CAP_SETUID:
/*
* If no policy applies to this task, allow the use of CAP_SETUID for
* other purposes.
*/
if (setid_policy_lookup((kid_t){.uid = cred->uid}, INVALID_ID, 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;
case CAP_SETGID:
/*
* If no policy applies to this task, allow the use of CAP_SETGID for
* other purposes.
*/
if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
return 0;
/*
* Reject use of CAP_SETUID for functionality other than calling
* set*gid() (e.g. setting up userns gid mappings).
*/
pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n",
__kgid_val(cred->gid));
return -EPERM;
default:
/* Error, the only capabilities were checking for is CAP_SETUID/GID */
return 0;
}
return 0;
}
/*
* Check whether a caller with old credentials @old is allowed to switch to
* credentials that contain @new_id.
*/
static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type)
{
bool permitted;
/* If our old creds already had this ID in it, it's fine. */
if (new_type == UID) {
if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) ||
uid_eq(new_id.uid, old->suid))
return true;
} else if (new_type == GID){
if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) ||
gid_eq(new_id.gid, old->sgid))
return true;
} else /* Error, new_type is an invalid type */
return false;
/*
* Transitions to new UIDs require a check against the policy of the old
* RUID.
*/
permitted =
setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED;
if (!permitted) {
if (new_type == UID) {
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_id.uid));
} else if (new_type == GID) {
pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n",
__kgid_val(old->gid), __kgid_val(old->egid),
__kgid_val(old->sgid), __kgid_val(new_id.gid));
} else /* Error, new_type is an invalid type */
return false;
}
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 (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
return 0;
if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) &&
id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) &&
id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) &&
id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID))
return 0;
/*
* Kill this process to avoid potential security vulnerabilities
* that could arise from a missing allowlist entry preventing a
* privileged process from dropping to a lesser-privileged one.
*/
force_sig(SIGKILL);
return -EACCES;
}
static int safesetid_task_fix_setgid(struct cred *new,
const struct cred *old,
int flags)
{
/* Do nothing if there are no setgid restrictions for our old RGID. */
if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
return 0;
if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) &&
id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) &&
id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) &&
id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID))
return 0;
/*
* Kill this process to avoid potential security vulnerabilities
* that could arise from a missing allowlist entry preventing a
* privileged process from dropping to a lesser-privileged one.
*/
force_sig(SIGKILL);
return -EACCES;
}
static int safesetid_task_fix_setgroups(struct cred *new, const struct cred *old)
{
int i;
/* Do nothing if there are no setgid restrictions for our old RGID. */
if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
return 0;
get_group_info(new->group_info);
for (i = 0; i < new->group_info->ngroups; i++) {
if (!id_permitted_for_cred(old, (kid_t){.gid = new->group_info->gid[i]}, GID)) {
put_group_info(new->group_info);
/*
* Kill this process to avoid potential security vulnerabilities
* that could arise from a missing allowlist entry preventing a
* privileged process from dropping to a lesser-privileged one.
*/
force_sig(SIGKILL);
return -EACCES;
}
}
put_group_info(new->group_info);
return 0;
}
static const struct lsm_id safesetid_lsmid = {
.name = "safesetid",
.id = LSM_ID_SAFESETID,
};
static struct security_hook_list safesetid_security_hooks[] = {
LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
LSM_HOOK_INIT(task_fix_setgroups, safesetid_task_fix_setgroups),
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_lsmid);
/* 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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