268 lines
8.0 KiB
C
268 lines
8.0 KiB
C
// SPDX-License-Identifier: GPL-2.0
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/*
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* SafeSetID Linux Security Module
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*
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* Author: Micah Morton <mortonm@chromium.org>
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*
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* Copyright (C) 2018 The Chromium OS Authors.
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2, as
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* published by the Free Software Foundation.
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*
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*/
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#define pr_fmt(fmt) "SafeSetID: " fmt
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#include <linux/lsm_hooks.h>
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#include <linux/module.h>
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#include <linux/ptrace.h>
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#include <linux/sched/task_stack.h>
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#include <linux/security.h>
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#include "lsm.h"
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/* Flag indicating whether initialization completed */
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int safesetid_initialized;
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struct setid_ruleset __rcu *safesetid_setuid_rules;
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struct setid_ruleset __rcu *safesetid_setgid_rules;
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/* Compute a decision for a transition from @src to @dst under @policy. */
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enum sid_policy_type _setid_policy_lookup(struct setid_ruleset *policy,
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kid_t src, kid_t dst)
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{
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struct setid_rule *rule;
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enum sid_policy_type result = SIDPOL_DEFAULT;
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if (policy->type == UID) {
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hash_for_each_possible(policy->rules, rule, next, __kuid_val(src.uid)) {
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if (!uid_eq(rule->src_id.uid, src.uid))
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continue;
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if (uid_eq(rule->dst_id.uid, dst.uid))
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return SIDPOL_ALLOWED;
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result = SIDPOL_CONSTRAINED;
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}
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} else if (policy->type == GID) {
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hash_for_each_possible(policy->rules, rule, next, __kgid_val(src.gid)) {
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if (!gid_eq(rule->src_id.gid, src.gid))
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continue;
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if (gid_eq(rule->dst_id.gid, dst.gid)){
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return SIDPOL_ALLOWED;
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}
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result = SIDPOL_CONSTRAINED;
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}
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} else {
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/* Should not reach here, report the ID as contrainsted */
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result = SIDPOL_CONSTRAINED;
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}
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return result;
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}
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/*
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* Compute a decision for a transition from @src to @dst under the active
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* policy.
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*/
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static enum sid_policy_type setid_policy_lookup(kid_t src, kid_t dst, enum setid_type new_type)
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{
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enum sid_policy_type result = SIDPOL_DEFAULT;
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struct setid_ruleset *pol;
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rcu_read_lock();
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if (new_type == UID)
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pol = rcu_dereference(safesetid_setuid_rules);
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else if (new_type == GID)
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pol = rcu_dereference(safesetid_setgid_rules);
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else { /* Should not reach here */
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result = SIDPOL_CONSTRAINED;
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rcu_read_unlock();
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return result;
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}
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if (pol) {
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pol->type = new_type;
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result = _setid_policy_lookup(pol, src, dst);
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}
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rcu_read_unlock();
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return result;
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}
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static int safesetid_security_capable(const struct cred *cred,
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struct user_namespace *ns,
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int cap,
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unsigned int opts)
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{
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/* We're only interested in CAP_SETUID and CAP_SETGID. */
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if (cap != CAP_SETUID && cap != CAP_SETGID)
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return 0;
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/*
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* If CAP_SET{U/G}ID is currently used for a setid() syscall, we want to
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* let it go through here; the real security check happens later, in the
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* task_fix_set{u/g}id hook.
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*
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* NOTE:
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* Until we add support for restricting setgroups() calls, GID security
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* policies offer no meaningful security since we always return 0 here
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* when called from within the setgroups() syscall and there is no
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* additional hook later on to enforce security policies for setgroups().
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*/
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if ((opts & CAP_OPT_INSETID) != 0)
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return 0;
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switch (cap) {
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case CAP_SETUID:
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/*
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* If no policy applies to this task, allow the use of CAP_SETUID for
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* other purposes.
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*/
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if (setid_policy_lookup((kid_t){.uid = cred->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
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return 0;
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/*
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* Reject use of CAP_SETUID for functionality other than calling
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* set*uid() (e.g. setting up userns uid mappings).
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*/
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pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
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__kuid_val(cred->uid));
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return -EPERM;
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break;
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case CAP_SETGID:
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/*
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* If no policy applies to this task, allow the use of CAP_SETGID for
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* other purposes.
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*/
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if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
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return 0;
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/*
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* Reject use of CAP_SETUID for functionality other than calling
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* set*gid() (e.g. setting up userns gid mappings).
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*/
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pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n",
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__kuid_val(cred->uid));
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return -EPERM;
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break;
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default:
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/* Error, the only capabilities were checking for is CAP_SETUID/GID */
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return 0;
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break;
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}
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return 0;
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}
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/*
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* Check whether a caller with old credentials @old is allowed to switch to
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* credentials that contain @new_id.
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*/
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static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type)
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{
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bool permitted;
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/* If our old creds already had this ID in it, it's fine. */
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if (new_type == UID) {
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if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) ||
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uid_eq(new_id.uid, old->suid))
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return true;
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} else if (new_type == GID){
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if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) ||
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gid_eq(new_id.gid, old->sgid))
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return true;
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} else /* Error, new_type is an invalid type */
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return false;
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/*
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* Transitions to new UIDs require a check against the policy of the old
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* RUID.
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*/
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permitted =
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setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED;
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if (!permitted) {
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if (new_type == UID) {
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pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
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__kuid_val(old->uid), __kuid_val(old->euid),
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__kuid_val(old->suid), __kuid_val(new_id.uid));
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} else if (new_type == GID) {
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pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n",
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__kgid_val(old->gid), __kgid_val(old->egid),
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__kgid_val(old->sgid), __kgid_val(new_id.gid));
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} else /* Error, new_type is an invalid type */
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return false;
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}
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return permitted;
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}
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/*
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* Check whether there is either an exception for user under old cred struct to
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* set*uid to user under new cred struct, or the UID transition is allowed (by
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* Linux set*uid rules) even without CAP_SETUID.
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*/
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static int safesetid_task_fix_setuid(struct cred *new,
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const struct cred *old,
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int flags)
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{
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/* Do nothing if there are no setuid restrictions for our old RUID. */
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if (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
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return 0;
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if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) &&
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id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) &&
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id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) &&
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id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID))
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return 0;
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/*
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* Kill this process to avoid potential security vulnerabilities
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* that could arise from a missing allowlist entry preventing a
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* privileged process from dropping to a lesser-privileged one.
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*/
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force_sig(SIGKILL);
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return -EACCES;
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}
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static int safesetid_task_fix_setgid(struct cred *new,
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const struct cred *old,
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int flags)
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{
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/* Do nothing if there are no setgid restrictions for our old RGID. */
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if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
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return 0;
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if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) &&
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id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) &&
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id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) &&
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id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID))
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return 0;
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/*
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* Kill this process to avoid potential security vulnerabilities
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* that could arise from a missing allowlist entry preventing a
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* privileged process from dropping to a lesser-privileged one.
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*/
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force_sig(SIGKILL);
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return -EACCES;
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}
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static struct security_hook_list safesetid_security_hooks[] = {
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LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
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LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
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LSM_HOOK_INIT(capable, safesetid_security_capable)
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};
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static int __init safesetid_security_init(void)
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{
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security_add_hooks(safesetid_security_hooks,
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ARRAY_SIZE(safesetid_security_hooks), "safesetid");
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/* Report that SafeSetID successfully initialized */
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safesetid_initialized = 1;
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return 0;
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}
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DEFINE_LSM(safesetid_security_init) = {
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.init = safesetid_security_init,
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.name = "safesetid",
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};
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