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 <uapi/linux/lsm.h>
  23 #include "lsm.h"
  24
  25 /* Flag indicating whether initialization completed */
  26 int safesetid_initialized __initdata;
  27
  28 struct setid_ruleset __rcu *safesetid_setuid_rules;
  29 struct setid_ruleset __rcu *safesetid_setgid_rules;
  30
  31
  32 /* Compute a decision for a transition from @src to @dst under @policy. */
  33 enum sid_policy_type _setid_policy_lookup(struct setid_ruleset *policy,
  34                 kid_t src, kid_t dst)
  35 {
  36         struct setid_rule *rule;
  37         enum sid_policy_type result = SIDPOL_DEFAULT;
  38
  39         if (policy->type == UID) {
  40                 hash_for_each_possible(policy->rules, rule, next, __kuid_val(src.uid)) {
  41                         if (!uid_eq(rule->src_id.uid, src.uid))
  42                                 continue;
  43                         if (uid_eq(rule->dst_id.uid, dst.uid))
  44                                 return SIDPOL_ALLOWED;
  45                         result = SIDPOL_CONSTRAINED;
  46                 }
  47         } else if (policy->type == GID) {
  48                 hash_for_each_possible(policy->rules, rule, next, __kgid_val(src.gid)) {
  49                         if (!gid_eq(rule->src_id.gid, src.gid))
  50                                 continue;
  51                         if (gid_eq(rule->dst_id.gid, dst.gid)){
  52                                 return SIDPOL_ALLOWED;
  53                         }
  54                         result = SIDPOL_CONSTRAINED;
  55                 }
  56         } else {
  57                 /* Should not reach here, report the ID as contrainsted */
  58                 result = SIDPOL_CONSTRAINED;
  59         }
  60         return result;
  61 }
  62
  63 /*
  64  * Compute a decision for a transition from @src to @dst under the active
  65  * policy.
  66  */
  67 static enum sid_policy_type setid_policy_lookup(kid_t src, kid_t dst, enum setid_type new_type)
  68 {
  69         enum sid_policy_type result = SIDPOL_DEFAULT;
  70         struct setid_ruleset *pol;
  71
  72         rcu_read_lock();
  73         if (new_type == UID)
  74                 pol = rcu_dereference(safesetid_setuid_rules);
  75         else if (new_type == GID)
  76                 pol = rcu_dereference(safesetid_setgid_rules);
  77         else { /* Should not reach here */
  78                 result = SIDPOL_CONSTRAINED;
  79                 rcu_read_unlock();
  80                 return result;
  81         }
  82
  83         if (pol) {
  84                 pol->type = new_type;
  85                 result = _setid_policy_lookup(pol, src, dst);
  86         }
  87         rcu_read_unlock();
  88         return result;
  89 }
  90
  91 static int safesetid_security_capable(const struct cred *cred,
  92                                       struct user_namespace *ns,
  93                                       int cap,
  94                                       unsigned int opts)
  95 {
  96         /* We're only interested in CAP_SETUID and CAP_SETGID. */
  97         if (cap != CAP_SETUID && cap != CAP_SETGID)
  98                 return 0;
  99
 100         /*
 101          * If CAP_SET{U/G}ID is currently used for a setid or setgroups syscall, we
 102          * want to let it go through here; the real security check happens later, in
 103          * the task_fix_set{u/g}id or task_fix_setgroups hooks.
 104          */
 105         if ((opts & CAP_OPT_INSETID) != 0)
 106                 return 0;
 107
 108         switch (cap) {
 109         case CAP_SETUID:
 110                 /*
 111                 * If no policy applies to this task, allow the use of CAP_SETUID for
 112                 * other purposes.
 113                 */
 114                 if (setid_policy_lookup((kid_t){.uid = cred->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
 115                         return 0;
 116                 /*
 117                  * Reject use of CAP_SETUID for functionality other than calling
 118                  * set*uid() (e.g. setting up userns uid mappings).
 119                  */
 120                 pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
 121                         __kuid_val(cred->uid));
 122                 return -EPERM;
 123         case CAP_SETGID:
 124                 /*
 125                 * If no policy applies to this task, allow the use of CAP_SETGID for
 126                 * other purposes.
 127                 */
 128                 if (setid_policy_lookup((kid_t){.gid = cred->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
 129                         return 0;
 130                 /*
 131                  * Reject use of CAP_SETUID for functionality other than calling
 132                  * set*gid() (e.g. setting up userns gid mappings).
 133                  */
 134                 pr_warn("Operation requires CAP_SETGID, which is not available to GID %u for operations besides approved set*gid transitions\n",
 135                         __kgid_val(cred->gid));
 136                 return -EPERM;
 137         default:
 138                 /* Error, the only capabilities were checking for is CAP_SETUID/GID */
 139                 return 0;
 140         }
 141         return 0;
 142 }
 143
 144 /*
 145  * Check whether a caller with old credentials @old is allowed to switch to
 146  * credentials that contain @new_id.
 147  */
 148 static bool id_permitted_for_cred(const struct cred *old, kid_t new_id, enum setid_type new_type)
 149 {
 150         bool permitted;
 151
 152         /* If our old creds already had this ID in it, it's fine. */
 153         if (new_type == UID) {
 154                 if (uid_eq(new_id.uid, old->uid) || uid_eq(new_id.uid, old->euid) ||
 155                         uid_eq(new_id.uid, old->suid))
 156                         return true;
 157         } else if (new_type == GID){
 158                 if (gid_eq(new_id.gid, old->gid) || gid_eq(new_id.gid, old->egid) ||
 159                         gid_eq(new_id.gid, old->sgid))
 160                         return true;
 161         } else /* Error, new_type is an invalid type */
 162                 return false;
 163
 164         /*
 165          * Transitions to new UIDs require a check against the policy of the old
 166          * RUID.
 167          */
 168         permitted =
 169             setid_policy_lookup((kid_t){.uid = old->uid}, new_id, new_type) != SIDPOL_CONSTRAINED;
 170
 171         if (!permitted) {
 172                 if (new_type == UID) {
 173                         pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
 174                                 __kuid_val(old->uid), __kuid_val(old->euid),
 175                                 __kuid_val(old->suid), __kuid_val(new_id.uid));
 176                 } else if (new_type == GID) {
 177                         pr_warn("GID transition ((%d,%d,%d) -> %d) blocked\n",
 178                                 __kgid_val(old->gid), __kgid_val(old->egid),
 179                                 __kgid_val(old->sgid), __kgid_val(new_id.gid));
 180                 } else /* Error, new_type is an invalid type */
 181                         return false;
 182         }
 183         return permitted;
 184 }
 185
 186 /*
 187  * Check whether there is either an exception for user under old cred struct to
 188  * set*uid to user under new cred struct, or the UID transition is allowed (by
 189  * Linux set*uid rules) even without CAP_SETUID.
 190  */
 191 static int safesetid_task_fix_setuid(struct cred *new,
 192                                      const struct cred *old,
 193                                      int flags)
 194 {
 195
 196         /* Do nothing if there are no setuid restrictions for our old RUID. */
 197         if (setid_policy_lookup((kid_t){.uid = old->uid}, INVALID_ID, UID) == SIDPOL_DEFAULT)
 198                 return 0;
 199
 200         if (id_permitted_for_cred(old, (kid_t){.uid = new->uid}, UID) &&
 201             id_permitted_for_cred(old, (kid_t){.uid = new->euid}, UID) &&
 202             id_permitted_for_cred(old, (kid_t){.uid = new->suid}, UID) &&
 203             id_permitted_for_cred(old, (kid_t){.uid = new->fsuid}, UID))
 204                 return 0;
 205
 206         /*
 207          * Kill this process to avoid potential security vulnerabilities
 208          * that could arise from a missing allowlist entry preventing a
 209          * privileged process from dropping to a lesser-privileged one.
 210          */
 211         force_sig(SIGKILL);
 212         return -EACCES;
 213 }
 214
 215 static int safesetid_task_fix_setgid(struct cred *new,
 216                                      const struct cred *old,
 217                                      int flags)
 218 {
 219
 220         /* Do nothing if there are no setgid restrictions for our old RGID. */
 221         if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
 222                 return 0;
 223
 224         if (id_permitted_for_cred(old, (kid_t){.gid = new->gid}, GID) &&
 225             id_permitted_for_cred(old, (kid_t){.gid = new->egid}, GID) &&
 226             id_permitted_for_cred(old, (kid_t){.gid = new->sgid}, GID) &&
 227             id_permitted_for_cred(old, (kid_t){.gid = new->fsgid}, GID))
 228                 return 0;
 229
 230         /*
 231          * Kill this process to avoid potential security vulnerabilities
 232          * that could arise from a missing allowlist entry preventing a
 233          * privileged process from dropping to a lesser-privileged one.
 234          */
 235         force_sig(SIGKILL);
 236         return -EACCES;
 237 }
 238
 239 static int safesetid_task_fix_setgroups(struct cred *new, const struct cred *old)
 240 {
 241         int i;
 242
 243         /* Do nothing if there are no setgid restrictions for our old RGID. */
 244         if (setid_policy_lookup((kid_t){.gid = old->gid}, INVALID_ID, GID) == SIDPOL_DEFAULT)
 245                 return 0;
 246
 247         get_group_info(new->group_info);
 248         for (i = 0; i < new->group_info->ngroups; i++) {
 249                 if (!id_permitted_for_cred(old, (kid_t){.gid = new->group_info->gid[i]}, GID)) {
 250                         put_group_info(new->group_info);
 251                         /*
 252                          * Kill this process to avoid potential security vulnerabilities
 253                          * that could arise from a missing allowlist entry preventing a
 254                          * privileged process from dropping to a lesser-privileged one.
 255                          */
 256                         force_sig(SIGKILL);
 257                         return -EACCES;
 258                 }
 259         }
 260
 261         put_group_info(new->group_info);
 262         return 0;
 263 }
 264
 265 static const struct lsm_id safesetid_lsmid = {
 266         .name = "safesetid",
 267         .id = LSM_ID_SAFESETID,
 268 };
 269
 270 static struct security_hook_list safesetid_security_hooks[] = {
 271         LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
 272         LSM_HOOK_INIT(task_fix_setgid, safesetid_task_fix_setgid),
 273         LSM_HOOK_INIT(task_fix_setgroups, safesetid_task_fix_setgroups),
 274         LSM_HOOK_INIT(capable, safesetid_security_capable)
 275 };
 276
 277 static int __init safesetid_security_init(void)
 278 {
 279         security_add_hooks(safesetid_security_hooks,
 280                            ARRAY_SIZE(safesetid_security_hooks),
 281                            &safesetid_lsmid);
 282
 283         /* Report that SafeSetID successfully initialized */
 284         safesetid_initialized = 1;
 285
 286         return 0;
 287 }
 288
 289 DEFINE_LSM(safesetid_security_init) = {
 290         .init = safesetid_security_init,
 291         .name = "safesetid",
 292 };