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; 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. */ 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 */ 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 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() 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 case CAP_SETGID: 129 /* 130 * If no policy applies to this task, allow the use of CAP_SETGID for 131 * other purposes. 132 */ 133 if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT) 134 return 0; 135 /* 136 * Reject use of CAP_SETUID for functionality other than calling 137 * set*gid() (e.g. setting up userns gid mappings). 138 */ 139 pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n", 140 __kuid_val(cred->uid)); 141 return -EPERM; 142 default: 143 /* Error, the only capabilities were checking for is CAP_SETUID/GID */ 144 return 0; 145 } 146 return 0; 147 } 148 149 /* 150 * Check whether a caller with old credentials @old is allowed to switch to 151 * credentials that contain @new_id. 152 */ 153 static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type) 154 { 155 bool permitted; 156 157 /* If our old creds already had this ID in it, it's fine. */ 158 if (new_type == UID) { 159 if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) || 160 uid_eq(new_id.uid, old->suid)) 161 return true; 162 } else if (new_type == GID){ 163 if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) || 164 gid_eq(new_id.gid, old->sgid)) 165 return true; 166 } else /* Error, new_type is an invalid type */ 167 return false; 168 169 /* 170 * Transitions to new UIDs require a check against the policy of the old 171 * RUID. 172 */ 173 permitted = 174 setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED; 175 176 if (!permitted) { 177 if (new_type == UID) { 178 pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n", 179 __kuid_val(old->uid), __kuid_val(old->euid), 180 __kuid_val(old->suid), __kuid_val(new_id.uid)); 181 } else if (new_type == GID) { 182 pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n", 183 __kgid_val(old->gid), __kgid_val(old->egid), 184 __kgid_val(old->sgid), __kgid_val(new_id.gid)); 185 } else /* Error, new_type is an invalid type */ 186 return false; 187 } 188 return permitted; 189 } 190 191 /* 192 * Check whether there is either an exception for user under old cred struct to 193 * set*uid to user under new cred struct, or the UID transition is allowed (by 194 * Linux set*uid rules) even without CAP_SETUID. 195 */ 196 static int safesetid_task_fix_setuid(struct cred *new, 197 const struct cred *old, 198 int flags) 199 { 200 201 /* Do nothing if there are no setuid restrictions for our old RUID. */ 202 if (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT) 203 return 0; 204 205 if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) && 206 id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) && 207 id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) && 208 id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID)) 209 return 0; 210 211 /* 212 * Kill this process to avoid potential security vulnerabilities 213 * that could arise from a missing allowlist entry preventing a 214 * privileged process from dropping to a lesser-privileged one. 215 */ 216 force_sig(SIGKILL); 217 return -EACCES; 218 } 219 220 static int safesetid_task_fix_setgid(struct cred *new, 221 const struct cred *old, 222 int flags) 223 { 224 225 /* Do nothing if there are no setgid restrictions for our old RGID. */ 226 if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT) 227 return 0; 228 229 if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) && 230 id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) && 231 id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) && 232 id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID)) 233 return 0; 234 235 /* 236 * Kill this process to avoid potential security vulnerabilities 237 * that could arise from a missing allowlist entry preventing a 238 * privileged process from dropping to a lesser-privileged one. 239 */ 240 force_sig(SIGKILL); 241 return -EACCES; 242 } 243 244 static struct security_hook_list safesetid_security_hooks[] = { 245 LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid), 246 LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid), 247 LSM_HOOK_INIT(capable, safesetid_security_capable) 248 }; 249 250 static int __init safesetid_security_init(void) 251 { 252 security_add_hooks(safesetid_security_hooks, 253 ARRAY_SIZE(safesetid_security_hooks), "safesetid"); 254 255 /* Report that SafeSetID successfully initialized */ 256 safesetid_initialized = 1; 257 258 return 0; 259 } 260 261 DEFINE_LSM(safesetid_security_init) = { 262 .init = safesetid_security_init, 263 .name = "safesetid", 264 }; 265