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authorMicah Morton <mortonm@chromium.org>2019-01-16 07:46:06 -0800
committerJames Morris <james.morris@microsoft.com>2019-01-25 11:22:45 -0800
commitaeca4e2ca65c1aeacfbe520684e6421719d99417 (patch)
tree0497b4bdda5dcd4005293603c1543b9b1a8795a6 /security
parent40852275a94afb3e836be9248399e036982d1a79 (diff)
LSM: add SafeSetID module that gates setid calls
SafeSetID 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. For now, only gating the set*uid family of syscalls is supported, with support for set*gid coming in a future patch set. Signed-off-by: Micah Morton <mortonm@chromium.org> Acked-by: Kees Cook <keescook@chromium.org> Signed-off-by: James Morris <james.morris@microsoft.com>
Diffstat (limited to 'security')
-rw-r--r--security/Kconfig3
-rw-r--r--security/Makefile2
-rw-r--r--security/safesetid/Kconfig12
-rw-r--r--security/safesetid/Makefile7
-rw-r--r--security/safesetid/lsm.c277
-rw-r--r--security/safesetid/lsm.h33
-rw-r--r--security/safesetid/securityfs.c193
7 files changed, 526 insertions, 1 deletions
diff --git a/security/Kconfig b/security/Kconfig
index 78dc12b7eeb3..9555f4914492 100644
--- a/security/Kconfig
+++ b/security/Kconfig
@@ -236,12 +236,13 @@ source "security/tomoyo/Kconfig"
source "security/apparmor/Kconfig"
source "security/loadpin/Kconfig"
source "security/yama/Kconfig"
+source "security/safesetid/Kconfig"
source "security/integrity/Kconfig"
config LSM
string "Ordered list of enabled LSMs"
- default "yama,loadpin,integrity,selinux,smack,tomoyo,apparmor"
+ default "yama,loadpin,safesetid,integrity,selinux,smack,tomoyo,apparmor"
help
A comma-separated list of LSMs, in initialization order.
Any LSMs left off this list will be ignored. This can be
diff --git a/security/Makefile b/security/Makefile
index 4d2d3782ddef..c598b904938f 100644
--- a/security/Makefile
+++ b/security/Makefile
@@ -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-$(CONFIG_CGROUP_DEVICE) += device_cgroup.o
# Object integrity file lists
diff --git a/security/safesetid/Kconfig b/security/safesetid/Kconfig
new file mode 100644
index 000000000000..bf89a47ffcc8
--- /dev/null
+++ b/security/safesetid/Kconfig
@@ -0,0 +1,12 @@
+config SECURITY_SAFESETID
+ bool "Gate setid transitions to limit CAP_SET{U/G}ID capabilities"
+ default n
+ help
+ SafeSetID is an LSM module that gates the setid family of syscalls to
+ restrict UID/GID transitions from a given UID/GID to only those
+ approved by a system-wide whitelist. These restrictions also prohibit
+ the given UIDs/GIDs from obtaining auxiliary privileges associated
+ with CAP_SET{U/G}ID, such as allowing a user to set up user namespace
+ UID mappings.
+
+ If you are unsure how to answer this question, answer N.
diff --git a/security/safesetid/Makefile b/security/safesetid/Makefile
new file mode 100644
index 000000000000..6b0660321164
--- /dev/null
+++ b/security/safesetid/Makefile
@@ -0,0 +1,7 @@
+# SPDX-License-Identifier: GPL-2.0
+#
+# Makefile for the safesetid LSM.
+#
+
+obj-$(CONFIG_SECURITY_SAFESETID) := safesetid.o
+safesetid-y := lsm.o securityfs.o
diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
new file mode 100644
index 000000000000..3a2c75ac810c
--- /dev/null
+++ b/security/safesetid/lsm.c
@@ -0,0 +1,277 @@
+// 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>
+
+/* Flag indicating whether initialization completed */
+int safesetid_initialized;
+
+#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:
+ pr_warn("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");
+
+ /* Report that SafeSetID successfully initialized */
+ safesetid_initialized = 1;
+
+ return 0;
+}
+
+DEFINE_LSM(safesetid_security_init) = {
+ .init = safesetid_security_init,
+};
diff --git a/security/safesetid/lsm.h b/security/safesetid/lsm.h
new file mode 100644
index 000000000000..c1ea3c265fcf
--- /dev/null
+++ b/security/safesetid/lsm.h
@@ -0,0 +1,33 @@
+/* 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>
+
+/* Flag indicating whether initialization completed */
+extern int safesetid_initialized;
+
+/* 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 */
diff --git a/security/safesetid/securityfs.c b/security/safesetid/securityfs.c
new file mode 100644
index 000000000000..61be4ee459cc
--- /dev/null
+++ b/security/safesetid/securityfs.c
@@ -0,0 +1,193 @@
+// 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:
+ pr_warn("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;
+
+ if (!safesetid_initialized)
+ return 0;
+
+ 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);