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 */
25 int safesetid_initialized __initdata;
26
27 struct setid_ruleset __rcu *safesetid_setuid_rules;
28 struct setid_ruleset __rcu *safesetid_setgid_rules;
29
30
31 /* Compute a decision for a transition from @src to @dst under @policy. */
_setid_policy_lookup(struct setid_ruleset * policy,kid_t src,kid_t dst)32 enum 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 */
setid_policy_lookup(kid_t src,kid_t dst,enum setid_type new_type)66 static 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
safesetid_security_capable(const struct cred * cred,struct user_namespace * ns,int cap,unsigned int opts)90 static 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 or setgroups syscall, we
101 * want to let it go through here; the real security check happens later, in
102 * the task_fix_set{u/g}id or task_fix_setgroups hooks.
103 */
104 if ((opts & CAP_OPT_INSETID) != 0)
105 return 0;
106
107 switch (cap) {
108 case CAP_SETUID:
109 /*
110 * If no policy applies to this task, allow the use of CAP_SETUID for
111 * other purposes.
112 */
113 if (setid_policy_lookup((kid_t){.uid = cred->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
114 return 0;
115 /*
116 * Reject use of CAP_SETUID for functionality other than calling
117 * set*uid() (e.g. setting up userns uid mappings).
118 */
119 pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
120 __kuid_val(cred->uid));
121 return -EPERM;
122 case CAP_SETGID:
123 /*
124 * If no policy applies to this task, allow the use of CAP_SETGID for
125 * other purposes.
126 */
127 if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
128 return 0;
129 /*
130 * Reject use of CAP_SETUID for functionality other than calling
131 * set*gid() (e.g. setting up userns gid mappings).
132 */
133 pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n",
134 __kuid_val(cred->uid));
135 return -EPERM;
136 default:
137 /* Error, the only capabilities were checking for is CAP_SETUID/GID */
138 return 0;
139 }
140 return 0;
141 }
142
143 /*
144 * Check whether a caller with old credentials @old is allowed to switch to
145 * credentials that contain @new_id.
146 */
id_permitted_for_cred(const struct cred * old,kid_t new_id,enum setid_type new_type)147 static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type)
148 {
149 bool permitted;
150
151 /* If our old creds already had this ID in it, it's fine. */
152 if (new_type == UID) {
153 if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) ||
154 uid_eq(new_id.uid, old->suid))
155 return true;
156 } else if (new_type == GID){
157 if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) ||
158 gid_eq(new_id.gid, old->sgid))
159 return true;
160 } else /* Error, new_type is an invalid type */
161 return false;
162
163 /*
164 * Transitions to new UIDs require a check against the policy of the old
165 * RUID.
166 */
167 permitted =
168 setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED;
169
170 if (!permitted) {
171 if (new_type == UID) {
172 pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
173 __kuid_val(old->uid), __kuid_val(old->euid),
174 __kuid_val(old->suid), __kuid_val(new_id.uid));
175 } else if (new_type == GID) {
176 pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n",
177 __kgid_val(old->gid), __kgid_val(old->egid),
178 __kgid_val(old->sgid), __kgid_val(new_id.gid));
179 } else /* Error, new_type is an invalid type */
180 return false;
181 }
182 return permitted;
183 }
184
185 /*
186 * Check whether there is either an exception for user under old cred struct to
187 * set*uid to user under new cred struct, or the UID transition is allowed (by
188 * Linux set*uid rules) even without CAP_SETUID.
189 */
safesetid_task_fix_setuid(struct cred * new,const struct cred * old,int flags)190 static int safesetid_task_fix_setuid(struct cred *new,
191 const struct cred *old,
192 int flags)
193 {
194
195 /* Do nothing if there are no setuid restrictions for our old RUID. */
196 if (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
197 return 0;
198
199 if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) &&
200 id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) &&
201 id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) &&
202 id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID))
203 return 0;
204
205 /*
206 * Kill this process to avoid potential security vulnerabilities
207 * that could arise from a missing allowlist entry preventing a
208 * privileged process from dropping to a lesser-privileged one.
209 */
210 force_sig(SIGKILL);
211 return -EACCES;
212 }
213
safesetid_task_fix_setgid(struct cred * new,const struct cred * old,int flags)214 static int safesetid_task_fix_setgid(struct cred *new,
215 const struct cred *old,
216 int flags)
217 {
218
219 /* Do nothing if there are no setgid restrictions for our old RGID. */
220 if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
221 return 0;
222
223 if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) &&
224 id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) &&
225 id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) &&
226 id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID))
227 return 0;
228
229 /*
230 * Kill this process to avoid potential security vulnerabilities
231 * that could arise from a missing allowlist entry preventing a
232 * privileged process from dropping to a lesser-privileged one.
233 */
234 force_sig(SIGKILL);
235 return -EACCES;
236 }
237
safesetid_task_fix_setgroups(struct cred * new,const struct cred * old)238 static int safesetid_task_fix_setgroups(struct cred *new, const struct cred *old)
239 {
240 int i;
241
242 /* Do nothing if there are no setgid restrictions for our old RGID. */
243 if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
244 return 0;
245
246 get_group_info(new->group_info);
247 for (i = 0; i < new->group_info->ngroups; i++) {
248 if (!id_permitted_for_cred(old, (kid_t){.gid = new->group_info->gid[i]}, GID)) {
249 put_group_info(new->group_info);
250 /*
251 * Kill this process to avoid potential security vulnerabilities
252 * that could arise from a missing allowlist entry preventing a
253 * privileged process from dropping to a lesser-privileged one.
254 */
255 force_sig(SIGKILL);
256 return -EACCES;
257 }
258 }
259
260 put_group_info(new->group_info);
261 return 0;
262 }
263
264 static struct security_hook_list safesetid_security_hooks[] = {
265 LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
266 LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
267 LSM_HOOK_INIT(task_fix_setgroups, safesetid_task_fix_setgroups),
268 LSM_HOOK_INIT(capable, safesetid_security_capable)
269 };
270
safesetid_security_init(void)271 static int __init safesetid_security_init(void)
272 {
273 security_add_hooks(safesetid_security_hooks,
274 ARRAY_SIZE(safesetid_security_hooks), "safesetid");
275
276 /* Report that SafeSetID successfully initialized */
277 safesetid_initialized = 1;
278
279 return 0;
280 }
281
282 DEFINE_LSM(safesetid_security_init) = {
283 .init = safesetid_security_init,
284 .name = "safesetid",
285 };
286