new file mode 100644
@@ -0,0 +1,107 @@
+=========
+SafeSetID
+=========
+SafeSetID is an LSM module that gates the setid family of syscalls to restrict
+UID/GID transitions from a given UID/GID to only those approved by a
+system-wide whitelist. These restrictions also prohibit the given UIDs/GIDs
+from obtaining auxiliary privileges associated with CAP_SET{U/G}ID, such as
+allowing a user to set up user namespace UID mappings.
+
+
+Background
+==========
+In absence of file capabilities, processes spawned on a Linux system that need
+to switch to a different user must be spawned with CAP_SETUID privileges.
+CAP_SETUID is granted to programs running as root or those running as a non-root
+user that have been explicitly given the CAP_SETUID runtime capability. It is
+often preferable to use Linux runtime capabilities rather than file
+capabilities, since using file capabilities to run a program with elevated
+privileges opens up possible security holes since any user with access to the
+file can exec() that program to gain the elevated privileges.
+
+While it is possible to implement a tree of processes by giving full
+CAP_SET{U/G}ID capabilities, this is often at odds with the goals of running a
+tree of processes under non-root user(s) in the first place. Specifically,
+since CAP_SETUID allows changing to any user on the system, including the root
+user, it is an overpowered capability for what is needed in this scenario,
+especially since programs often only call setuid() to drop privileges to a
+lesser-privileged user -- not elevate privileges. Unfortunately, there is no
+generally feasible way in Linux to restrict the potential UIDs that a user can
+switch to through setuid() beyond allowing a switch to any user on the system.
+This SafeSetID LSM seeks to provide a solution for restricting setid
+capabilities in such a way.
+
+The main use case for this LSM is to allow a non-root program to transition to
+other untrusted uids without full blown CAP_SETUID capabilities. The non-root
+program would still need CAP_SETUID to do any kind of transition, but the
+additional restrictions imposed by this LSM would mean it is a "safer" version
+of CAP_SETUID since the non-root program cannot take advantage of CAP_SETUID to
+do any unapproved actions (e.g. setuid to uid 0 or create/enter new user
+namespace). The higher level goal is to allow for uid-based sandboxing of system
+services without having to give out CAP_SETUID all over the place just so that
+non-root programs can drop to even-lesser-privileged uids. This is especially
+relevant when one non-root daemon on the system should be allowed to spawn other
+processes as different uids, but its undesirable to give the daemon a
+basically-root-equivalent CAP_SETUID.
+
+
+Other Approaches Considered
+===========================
+
+Solve this problem in userspace
+-------------------------------
+For candidate applications that would like to have restricted setid capabilities
+as implemented in this LSM, an alternative option would be to simply take away
+setid capabilities from the application completely and refactor the process
+spawning semantics in the application (e.g. by using a privileged helper program
+to do process spawning and UID/GID transitions). Unfortunately, there are a
+number of semantics around process spawning that would be affected by this, such
+as fork() calls where the program doesn’t immediately call exec() after the
+fork(), parent processes specifying custom environment variables or command line
+args for spawned child processes, or inheritance of file handles across a
+fork()/exec(). Because of this, as solution that uses a privileged helper in
+userspace would likely be less appealing to incorporate into existing projects
+that rely on certain process-spawning semantics in Linux.
+
+Use user namespaces
+-------------------
+Another possible approach would be to run a given process tree in its own user
+namespace and give programs in the tree setid capabilities. In this way,
+programs in the tree could change to any desired UID/GID in the context of their
+own user namespace, and only approved UIDs/GIDs could be mapped back to the
+initial system user namespace, affectively preventing privilege escalation.
+Unfortunately, it is not generally feasible to use user namespaces in isolation,
+without pairing them with other namespace types, which is not always an option.
+Linux checks for capabilities based off of the user namespace that “owns” some
+entity. For example, Linux has the notion that network namespaces are owned by
+the user namespace in which they were created. A consequence of this is that
+capability checks for access to a given network namespace are done by checking
+whether a task has the given capability in the context of the user namespace
+that owns the network namespace -- not necessarily the user namespace under
+which the given task runs. Therefore spawning a process in a new user namespace
+effectively prevents it from accessing the network namespace owned by the
+initial namespace. This is a deal-breaker for any application that expects to
+retain the CAP_NET_ADMIN capability for the purpose of adjusting network
+configurations. Using user namespaces in isolation causes problems regarding
+other system interactions, including use of pid namespaces and device creation.
+
+Use an existing LSM
+-------------------
+None of the other in-tree LSMs have the capability to gate setid transitions, or
+even employ the security_task_fix_setuid hook at all. SELinux says of that hook:
+"Since setuid only affects the current process, and since the SELinux controls
+are not based on the Linux identity attributes, SELinux does not need to control
+this operation."
+
+
+Directions for use
+==================
+This LSM hooks the setid syscalls to make sure transitions are allowed if an
+applicable restriction policy is in place. Policies are configured through
+securityfs by writing to the safesetid/add_whitelist_policy and
+safesetid/flush_whitelist_policies files at the location where securityfs is
+mounted. The format for adding a policy is '<UID>:<UID>', using literal
+numbers, such as '123:456'. To flush the policies, any write to the file is
+sufficient. Again, configuring a policy for a UID will prevent that UID from
+obtaining auxiliary setid privileges, such as allowing a user to set up user
+namespace UID mappings.
@@ -46,3 +46,4 @@ subdirectories.
Smack
tomoyo
Yama
+ SafeSetID
@@ -236,6 +236,7 @@ source "security/tomoyo/Kconfig"
source "security/apparmor/Kconfig"
source "security/loadpin/Kconfig"
source "security/yama/Kconfig"
+source "security/safesetid/Kconfig"
source "security/integrity/Kconfig"
@@ -10,6 +10,7 @@ subdir-$(CONFIG_SECURITY_TOMOYO) += tomoyo
subdir-$(CONFIG_SECURITY_APPARMOR) += apparmor
subdir-$(CONFIG_SECURITY_YAMA) += yama
subdir-$(CONFIG_SECURITY_LOADPIN) += loadpin
+subdir-$(CONFIG_SECURITY_SAFESETID) += safesetid
# always enable default capabilities
obj-y += commoncap.o
@@ -25,6 +26,7 @@ obj-$(CONFIG_SECURITY_TOMOYO) += tomoyo/
obj-$(CONFIG_SECURITY_APPARMOR) += apparmor/
obj-$(CONFIG_SECURITY_YAMA) += yama/
obj-$(CONFIG_SECURITY_LOADPIN) += loadpin/
+obj-$(CONFIG_SECURITY_SAFESETID) += safesetid/
obj-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
# Object integrity file lists
new file mode 100644
@@ -0,0 +1,12 @@
+config SECURITY_SAFESETID
+ bool "Gate setid transitions to limit CAP_SET{U/G}ID capabilities"
+ default n
+ help
+ SafeSetID is an LSM module that gates the setid family of syscalls to
+ restrict UID/GID transitions from a given UID/GID to only those
+ approved by a system-wide whitelist. These restrictions also prohibit
+ the given UIDs/GIDs from obtaining auxiliary privileges associated
+ with CAP_SET{U/G}ID, such as allowing a user to set up user namespace
+ UID mappings.
+
+ If you are unsure how to answer this question, answer N.
new file mode 100644
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for the safesetid LSM.
+#
+
+obj-$(CONFIG_SECURITY_SAFESETID) := safesetid.o
+safesetid-y := lsm.o securityfs.o
new file mode 100644
@@ -0,0 +1,271 @@
+// 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 <asm/syscall.h>
+#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>
+
+#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",
+ __kuid_val(cred->uid));
+ return -1;
+ }
+ }
+ return 0;
+}
+
+static int check_uid_transition(kuid_t parent, kuid_t child)
+{
+ if (check_setuid_policy_hashtable_key_value(parent, child))
+ return 0;
+ pr_warn("UID transition (%d -> %d) blocked",
+ __kuid_val(parent),
+ __kuid_val(child));
+ /*
+ * 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, current);
+ return -EACCES;
+}
+
+/*
+ * 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 this UID. */
+ if (!check_setuid_policy_hashtable_key(old->uid))
+ return 0;
+
+ switch (flags) {
+ case LSM_SETID_RE:
+ /*
+ * Users for which setuid restrictions exist can only set the
+ * real UID to the real UID or the effective UID, unless an
+ * explicit whitelist policy allows the transition.
+ */
+ if (!uid_eq(old->uid, new->uid) &&
+ !uid_eq(old->euid, new->uid)) {
+ return check_uid_transition(old->uid, new->uid);
+ }
+ /*
+ * Users for which setuid restrictions exist can only set the
+ * effective UID to the real UID, the effective UID, or the
+ * saved set-UID, unless an explicit whitelist policy allows
+ * the transition.
+ */
+ if (!uid_eq(old->uid, new->euid) &&
+ !uid_eq(old->euid, new->euid) &&
+ !uid_eq(old->suid, new->euid)) {
+ return check_uid_transition(old->euid, new->euid);
+ }
+ break;
+ case LSM_SETID_ID:
+ /*
+ * Users for which setuid restrictions exist cannot change the
+ * real UID or saved set-UID unless an explicit whitelist
+ * policy allows the transition.
+ */
+ if (!uid_eq(old->uid, new->uid))
+ return check_uid_transition(old->uid, new->uid);
+ if (!uid_eq(old->suid, new->suid))
+ return check_uid_transition(old->suid, new->suid);
+ break;
+ case LSM_SETID_RES:
+ /*
+ * Users for which setuid restrictions exist cannot change the
+ * real UID, effective UID, or saved set-UID to anything but
+ * one of: the current real UID, the current effective UID or
+ * the current saved set-user-ID unless an explicit whitelist
+ * policy allows the transition.
+ */
+ if (!uid_eq(new->uid, old->uid) &&
+ !uid_eq(new->uid, old->euid) &&
+ !uid_eq(new->uid, old->suid)) {
+ return check_uid_transition(old->uid, new->uid);
+ }
+ if (!uid_eq(new->euid, old->uid) &&
+ !uid_eq(new->euid, old->euid) &&
+ !uid_eq(new->euid, old->suid)) {
+ return check_uid_transition(old->euid, new->euid);
+ }
+ if (!uid_eq(new->suid, old->uid) &&
+ !uid_eq(new->suid, old->euid) &&
+ !uid_eq(new->suid, old->suid)) {
+ return check_uid_transition(old->suid, new->suid);
+ }
+ break;
+ case LSM_SETID_FS:
+ /*
+ * Users for which setuid restrictions exist cannot change the
+ * filesystem UID to anything but one of: the current real UID,
+ * the current effective UID or the current saved set-UID
+ * unless an explicit whitelist policy allows the transition.
+ */
+ if (!uid_eq(new->fsuid, old->uid) &&
+ !uid_eq(new->fsuid, old->euid) &&
+ !uid_eq(new->fsuid, old->suid) &&
+ !uid_eq(new->fsuid, old->fsuid)) {
+ return check_uid_transition(old->fsuid, new->fsuid);
+ }
+ break;
+ default:
+ WARN_ON_ONCE("Unknown setid state %d\n", flags);
+ force_sig(SIGKILL, current);
+ return -EINVAL;
+ }
+ return 0;
+}
+
+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");
+
+ return 0;
+}
+
+DEFINE_LSM(safesetid_security_init) = {
+ .init = safesetid_security_init,
+};
new file mode 100644
@@ -0,0 +1,30 @@
+/* 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.
+ *
+ */
+#ifndef _SAFESETID_H
+#define _SAFESETID_H
+
+#include <linux/types.h>
+
+/* Function type. */
+enum safesetid_whitelist_file_write_type {
+ SAFESETID_WHITELIST_ADD, /* Add whitelist policy. */
+ SAFESETID_WHITELIST_FLUSH, /* Flush whitelist policies. */
+};
+
+/* Add entry to safesetid whitelist to allow 'parent' to setid to 'child'. */
+int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child);
+
+void flush_safesetid_whitelist_entries(void);
+
+#endif /* _SAFESETID_H */
new file mode 100644
@@ -0,0 +1,190 @@
+// 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.
+ *
+ */
+#include <linux/security.h>
+#include <linux/cred.h>
+
+#include "lsm.h"
+
+static struct dentry *safesetid_policy_dir;
+
+struct safesetid_file_entry {
+ const char *name;
+ enum safesetid_whitelist_file_write_type type;
+ struct dentry *dentry;
+};
+
+static struct safesetid_file_entry safesetid_files[] = {
+ {.name = "add_whitelist_policy",
+ .type = SAFESETID_WHITELIST_ADD},
+ {.name = "flush_whitelist_policies",
+ .type = SAFESETID_WHITELIST_FLUSH},
+};
+
+/*
+ * In the case the input buffer contains one or more invalid UIDs, the kuid_t
+ * variables pointed to by 'parent' and 'child' will get updated but this
+ * function will return an error.
+ */
+static int parse_safesetid_whitelist_policy(const char __user *buf,
+ size_t len,
+ kuid_t *parent,
+ kuid_t *child)
+{
+ char *kern_buf;
+ char *parent_buf;
+ char *child_buf;
+ const char separator[] = ":";
+ int ret;
+ size_t first_substring_length;
+ long parsed_parent;
+ long parsed_child;
+
+ /* Duplicate string from user memory and NULL-terminate */
+ kern_buf = memdup_user_nul(buf, len);
+ if (IS_ERR(kern_buf))
+ return PTR_ERR(kern_buf);
+
+ /*
+ * Format of |buf| string should be <UID>:<UID>.
+ * Find location of ":" in kern_buf (copied from |buf|).
+ */
+ first_substring_length = strcspn(kern_buf, separator);
+ if (first_substring_length == 0 || first_substring_length == len) {
+ ret = -EINVAL;
+ goto free_kern;
+ }
+
+ parent_buf = kmemdup_nul(kern_buf, first_substring_length, GFP_KERNEL);
+ if (!parent_buf) {
+ ret = -ENOMEM;
+ goto free_kern;
+ }
+
+ ret = kstrtol(parent_buf, 0, &parsed_parent);
+ if (ret)
+ goto free_both;
+
+ child_buf = kern_buf + first_substring_length + 1;
+ ret = kstrtol(child_buf, 0, &parsed_child);
+ if (ret)
+ goto free_both;
+
+ *parent = make_kuid(current_user_ns(), parsed_parent);
+ if (!uid_valid(*parent)) {
+ ret = -EINVAL;
+ goto free_both;
+ }
+
+ *child = make_kuid(current_user_ns(), parsed_child);
+ if (!uid_valid(*child)) {
+ ret = -EINVAL;
+ goto free_both;
+ }
+
+free_both:
+ kfree(parent_buf);
+free_kern:
+ kfree(kern_buf);
+ return ret;
+}
+
+static ssize_t safesetid_file_write(struct file *file,
+ const char __user *buf,
+ size_t len,
+ loff_t *ppos)
+{
+ struct safesetid_file_entry *file_entry =
+ file->f_inode->i_private;
+ kuid_t parent;
+ kuid_t child;
+ int ret;
+
+ if (!ns_capable(current_user_ns(), CAP_MAC_ADMIN))
+ return -EPERM;
+
+ if (*ppos != 0)
+ return -EINVAL;
+
+ switch (file_entry->type) {
+ case SAFESETID_WHITELIST_FLUSH:
+ flush_safesetid_whitelist_entries();
+ break;
+ case SAFESETID_WHITELIST_ADD:
+ ret = parse_safesetid_whitelist_policy(buf, len, &parent,
+ &child);
+ if (ret)
+ return ret;
+
+ ret = add_safesetid_whitelist_entry(parent, child);
+ if (ret)
+ return ret;
+ break;
+ default:
+ WARN_ON_ONCE("Unknown securityfs file %d\n", file_entry->type);
+ break;
+ }
+
+ /* Return len on success so caller won't keep trying to write */
+ return len;
+}
+
+static const struct file_operations safesetid_file_fops = {
+ .write = safesetid_file_write,
+};
+
+static void safesetid_shutdown_securityfs(void)
+{
+ int i;
+
+ for (i = 0; i < ARRAY_SIZE(safesetid_files); ++i) {
+ struct safesetid_file_entry *entry =
+ &safesetid_files[i];
+ securityfs_remove(entry->dentry);
+ entry->dentry = NULL;
+ }
+
+ securityfs_remove(safesetid_policy_dir);
+ safesetid_policy_dir = NULL;
+}
+
+static int __init safesetid_init_securityfs(void)
+{
+ int i;
+ int ret;
+
+ safesetid_policy_dir = securityfs_create_dir("safesetid", NULL);
+ if (!safesetid_policy_dir) {
+ ret = PTR_ERR(safesetid_policy_dir);
+ goto error;
+ }
+
+ for (i = 0; i < ARRAY_SIZE(safesetid_files); ++i) {
+ struct safesetid_file_entry *entry =
+ &safesetid_files[i];
+ entry->dentry = securityfs_create_file(
+ entry->name, 0200, safesetid_policy_dir,
+ entry, &safesetid_file_fops);
+ if (IS_ERR(entry->dentry)) {
+ ret = PTR_ERR(entry->dentry);
+ goto error;
+ }
+ }
+
+ return 0;
+
+error:
+ safesetid_shutdown_securityfs();
+ return ret;
+}
+fs_initcall(safesetid_init_securityfs);