diff options
| -rw-r--r-- | Documentation/admin-guide/LSM/SafeSetID.rst | 107 | ||||
| -rw-r--r-- | Documentation/admin-guide/LSM/index.rst | 1 | ||||
| -rw-r--r-- | security/Kconfig | 3 | ||||
| -rw-r--r-- | security/Makefile | 2 | ||||
| -rw-r--r-- | security/safesetid/Kconfig | 12 | ||||
| -rw-r--r-- | security/safesetid/Makefile | 7 | ||||
| -rw-r--r-- | security/safesetid/lsm.c | 277 | ||||
| -rw-r--r-- | security/safesetid/lsm.h | 33 | ||||
| -rw-r--r-- | security/safesetid/securityfs.c | 193 |
9 files changed, 634 insertions, 1 deletions
diff --git a/Documentation/admin-guide/LSM/SafeSetID.rst b/Documentation/admin-guide/LSM/SafeSetID.rst new file mode 100644 index 000000000000..212434ef65ad --- /dev/null +++ b/Documentation/admin-guide/LSM/SafeSetID.rst @@ -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. diff --git a/Documentation/admin-guide/LSM/index.rst b/Documentation/admin-guide/LSM/index.rst index 9842e21afd4a..a6ba95fbaa9f 100644 --- a/Documentation/admin-guide/LSM/index.rst +++ b/Documentation/admin-guide/LSM/index.rst @@ -46,3 +46,4 @@ subdirectories. Smack tomoyo Yama + SafeSetID 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); |