security/safesetid/lsm.c

   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. */
  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 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                         __kgid_val(cred->gid));
 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  */
 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  */
 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
 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
 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
 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 };