1// SPDX-License-Identifier: GPL-2.0
2/*
3 * SafeSetID Linux Security Module
4 *
5 * Author: Micah Morton <mortonm@chromium.org>
6 *
7 * Copyright (C) 2018 The Chromium OS Authors.
8 *
9 * This program is free software; you can redistribute it and/or modify
10 * it under the terms of the GNU General Public License version 2, as
11 * published by the Free Software Foundation.
12 *
13 */
14
15#define pr_fmt(fmt) "SafeSetID: " fmt
16
17#include <linux/lsm_hooks.h>
18#include <linux/module.h>
19#include <linux/ptrace.h>
20#include <linux/sched/task_stack.h>
21#include <linux/security.h>
22#include "lsm.h"
23
24/* Flag indicating whether initialization completed */
25int safesetid_initialized;
26
27struct setid_ruleset __rcu *safesetid_setuid_rules;
28struct setid_ruleset __rcu *safesetid_setgid_rules;
29
30
31/* Compute a decision for a transition from @src to @dst under @policy. */
32enum sid_policy_type _setid_policy_lookup(struct setid_ruleset *policy,
33		kid_t src, kid_t dst)
34{
35	struct setid_rule *rule;
36	enum sid_policy_type result = SIDPOL_DEFAULT;
37
38	if (policy->type == UID) {
39		hash_for_each_possible(policy->rules, rule, next, __kuid_val(src.uid)) {
40			if (!uid_eq(rule->src_id.uid, src.uid))
41				continue;
42			if (uid_eq(rule->dst_id.uid, dst.uid))
43				return SIDPOL_ALLOWED;
44			result = SIDPOL_CONSTRAINED;
45		}
46	} else if (policy->type == GID) {
47		hash_for_each_possible(policy->rules, rule, next, __kgid_val(src.gid)) {
48			if (!gid_eq(rule->src_id.gid, src.gid))
49				continue;
50			if (gid_eq(rule->dst_id.gid, dst.gid)){
51				return SIDPOL_ALLOWED;
52			}
53			result = SIDPOL_CONSTRAINED;
54		}
55	} else {
56		/* Should not reach here, report the ID as contrainsted */
57		result = SIDPOL_CONSTRAINED;
58	}
59	return result;
60}
61
62/*
63 * Compute a decision for a transition from @src to @dst under the active
64 * policy.
65 */
66static enum sid_policy_type setid_policy_lookup(kid_t src, kid_t dst, enum setid_type new_type)
67{
68	enum sid_policy_type result = SIDPOL_DEFAULT;
69	struct setid_ruleset *pol;
70
71	rcu_read_lock();
72	if (new_type == UID)
73		pol = rcu_dereference(safesetid_setuid_rules);
74	else if (new_type == GID)
75		pol = rcu_dereference(safesetid_setgid_rules);
76	else { /* Should not reach here */
77		result = SIDPOL_CONSTRAINED;
78		rcu_read_unlock();
79		return result;
80	}
81
82	if (pol) {
83		pol->type = new_type;
84		result = _setid_policy_lookup(pol, src, dst);
85	}
86	rcu_read_unlock();
87	return result;
88}
89
90static int safesetid_security_capable(const struct cred *cred,
91				      struct user_namespace *ns,
92				      int cap,
93				      unsigned int opts)
94{
95	/* We're only interested in CAP_SETUID and CAP_SETGID. */
96	if (cap != CAP_SETUID && cap != CAP_SETGID)
97		return 0;
98
99	/*
100	 * If CAP_SET{U/G}ID is currently used for a setid() syscall, we want to
101	 * let it go through here; the real security check happens later, in the
102	 * task_fix_set{u/g}id hook.
103         *
104         * NOTE:
105         * Until we add support for restricting setgroups() calls, GID security
106         * policies offer no meaningful security since we always return 0 here
107         * when called from within the setgroups() syscall and there is no
108         * additional hook later on to enforce security policies for setgroups().
109	 */
110	if ((opts & CAP_OPT_INSETID) != 0)
111		return 0;
112
113	switch (cap) {
114	case CAP_SETUID:
115		/*
116		* If no policy applies to this task, allow the use of CAP_SETUID for
117		* other purposes.
118		*/
119		if (setid_policy_lookup((kid_t){.uid = cred->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
120			return 0;
121		/*
122		 * Reject use of CAP_SETUID for functionality other than calling
123		 * set*uid() (e.g. setting up userns uid mappings).
124		 */
125		pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
126			__kuid_val(cred->uid));
127		return -EPERM;
128		break;
129	case CAP_SETGID:
130		/*
131		* If no policy applies to this task, allow the use of CAP_SETGID for
132		* other purposes.
133		*/
134		if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
135			return 0;
136		/*
137		 * Reject use of CAP_SETUID for functionality other than calling
138		 * set*gid() (e.g. setting up userns gid mappings).
139		 */
140		pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n",
141			__kuid_val(cred->uid));
142		return -EPERM;
143		break;
144	default:
145		/* Error, the only capabilities were checking for is CAP_SETUID/GID */
146		return 0;
147		break;
148	}
149	return 0;
150}
151
152/*
153 * Check whether a caller with old credentials @old is allowed to switch to
154 * credentials that contain @new_id.
155 */
156static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type)
157{
158	bool permitted;
159
160	/* If our old creds already had this ID in it, it's fine. */
161	if (new_type == UID) {
162		if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) ||
163			uid_eq(new_id.uid, old->suid))
164			return true;
165	} else if (new_type == GID){
166		if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) ||
167			gid_eq(new_id.gid, old->sgid))
168			return true;
169	} else /* Error, new_type is an invalid type */
170		return false;
171
172	/*
173	 * Transitions to new UIDs require a check against the policy of the old
174	 * RUID.
175	 */
176	permitted =
177	    setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED;
178
179	if (!permitted) {
180		if (new_type == UID) {
181			pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
182				__kuid_val(old->uid), __kuid_val(old->euid),
183				__kuid_val(old->suid), __kuid_val(new_id.uid));
184		} else if (new_type == GID) {
185			pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n",
186				__kgid_val(old->gid), __kgid_val(old->egid),
187				__kgid_val(old->sgid), __kgid_val(new_id.gid));
188		} else /* Error, new_type is an invalid type */
189			return false;
190	}
191	return permitted;
192}
193
194/*
195 * Check whether there is either an exception for user under old cred struct to
196 * set*uid to user under new cred struct, or the UID transition is allowed (by
197 * Linux set*uid rules) even without CAP_SETUID.
198 */
199static int safesetid_task_fix_setuid(struct cred *new,
200				     const struct cred *old,
201				     int flags)
202{
203
204	/* Do nothing if there are no setuid restrictions for our old RUID. */
205	if (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
206		return 0;
207
208	if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) &&
209	    id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) &&
210	    id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) &&
211	    id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID))
212		return 0;
213
214	/*
215	 * Kill this process to avoid potential security vulnerabilities
216	 * that could arise from a missing allowlist entry preventing a
217	 * privileged process from dropping to a lesser-privileged one.
218	 */
219	force_sig(SIGKILL);
220	return -EACCES;
221}
222
223static int safesetid_task_fix_setgid(struct cred *new,
224				     const struct cred *old,
225				     int flags)
226{
227
228	/* Do nothing if there are no setgid restrictions for our old RGID. */
229	if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
230		return 0;
231
232	if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) &&
233	    id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) &&
234	    id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) &&
235	    id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID))
236		return 0;
237
238	/*
239	 * Kill this process to avoid potential security vulnerabilities
240	 * that could arise from a missing allowlist entry preventing a
241	 * privileged process from dropping to a lesser-privileged one.
242	 */
243	force_sig(SIGKILL);
244	return -EACCES;
245}
246
247static struct security_hook_list safesetid_security_hooks[] = {
248	LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
249	LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
250	LSM_HOOK_INIT(capable, safesetid_security_capable)
251};
252
253static int __init safesetid_security_init(void)
254{
255	security_add_hooks(safesetid_security_hooks,
256			   ARRAY_SIZE(safesetid_security_hooks), "safesetid");
257
258	/* Report that SafeSetID successfully initialized */
259	safesetid_initialized = 1;
260
261	return 0;
262}
263
264DEFINE_LSM(safesetid_security_init) = {
265	.init = safesetid_security_init,
266	.name = "safesetid",
267};
268