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-rw-r--r--security/safesetid/lsm.c285
1 files changed, 285 insertions, 0 deletions
diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
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index 000000000..e806739f7
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+++ b/security/safesetid/lsm.c
@@ -0,0 +1,285 @@
+// 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 __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",
+ __kuid_val(cred->uid));
+ 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 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");
+
+ /* 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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