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;
  26
  27 struct setuid_ruleset __rcu *safesetid_setuid_rules;
  28
  29 /* Compute a decision for a transition from @src to @dst under @policy. */
  30 enum sid_policy_type _setuid_policy_lookup(struct setuid_ruleset *policy,
  31                 kuid_t src, kuid_t dst)
  32 {
  33         struct setuid_rule *rule;
  34         enum sid_policy_type result = SIDPOL_DEFAULT;
  35
  36         hash_for_each_possible(policy->rules, rule, next, __kuid_val(src)) {
  37                 if (!uid_eq(rule->src_uid, src))
  38                         continue;
  39                 if (uid_eq(rule->dst_uid, dst))
  40                         return SIDPOL_ALLOWED;
  41                 result = SIDPOL_CONSTRAINED;
  42         }
  43         return result;
  44 }
  45
  46 /*
  47  * Compute a decision for a transition from @src to @dst under the active
  48  * policy.
  49  */
  50 static enum sid_policy_type setuid_policy_lookup(kuid_t src, kuid_t dst)
  51 {
  52         enum sid_policy_type result = SIDPOL_DEFAULT;
  53         struct setuid_ruleset *pol;
  54
  55         rcu_read_lock();
  56         pol = rcu_dereference(safesetid_setuid_rules);
  57         if (pol)
  58                 result = _setuid_policy_lookup(pol, src, dst);
  59         rcu_read_unlock();
  60         return result;
  61 }
  62
  63 static int safesetid_security_capable(const struct cred *cred,
  64                                       struct user_namespace *ns,
  65                                       int cap,
  66                                       unsigned int opts)
  67 {
  68         /* We're only interested in CAP_SETUID. */
  69         if (cap != CAP_SETUID)
  70                 return 0;
  71
  72         /*
  73          * If CAP_SETUID is currently used for a set*uid() syscall, we want to
  74          * let it go through here; the real security check happens later, in the
  75          * task_fix_setuid hook.
  76          */
  77         if ((opts & CAP_OPT_INSETID) != 0)
  78                 return 0;
  79
  80         /*
  81          * If no policy applies to this task, allow the use of CAP_SETUID for
  82          * other purposes.
  83          */
  84         if (setuid_policy_lookup(cred->uid, INVALID_UID) == SIDPOL_DEFAULT)
  85                 return 0;
  86
  87         /*
  88          * Reject use of CAP_SETUID for functionality other than calling
  89          * set*uid() (e.g. setting up userns uid mappings).
  90          */
  91         pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
  92                 __kuid_val(cred->uid));
  93         return -EPERM;
  94 }
  95
  96 /*
  97  * Check whether a caller with old credentials @old is allowed to switch to
  98  * credentials that contain @new_uid.
  99  */
 100 static bool uid_permitted_for_cred(const struct cred *old, kuid_t new_uid)
 101 {
 102         bool permitted;
 103
 104         /* If our old creds already had this UID in it, it's fine. */
 105         if (uid_eq(new_uid, old->uid) || uid_eq(new_uid, old->euid) ||
 106             uid_eq(new_uid, old->suid))
 107                 return true;
 108
 109         /*
 110          * Transitions to new UIDs require a check against the policy of the old
 111          * RUID.
 112          */
 113         permitted =
 114             setuid_policy_lookup(old->uid, new_uid) != SIDPOL_CONSTRAINED;
 115         if (!permitted) {
 116                 pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
 117                         __kuid_val(old->uid), __kuid_val(old->euid),
 118                         __kuid_val(old->suid), __kuid_val(new_uid));
 119         }
 120         return permitted;
 121 }
 122
 123 /*
 124  * Check whether there is either an exception for user under old cred struct to
 125  * set*uid to user under new cred struct, or the UID transition is allowed (by
 126  * Linux set*uid rules) even without CAP_SETUID.
 127  */
 128 static int safesetid_task_fix_setuid(struct cred *new,
 129                                      const struct cred *old,
 130                                      int flags)
 131 {
 132
 133         /* Do nothing if there are no setuid restrictions for our old RUID. */
 134         if (setuid_policy_lookup(old->uid, INVALID_UID) == SIDPOL_DEFAULT)
 135                 return 0;
 136
 137         if (uid_permitted_for_cred(old, new->uid) &&
 138             uid_permitted_for_cred(old, new->euid) &&
 139             uid_permitted_for_cred(old, new->suid) &&
 140             uid_permitted_for_cred(old, new->fsuid))
 141                 return 0;
 142
 143         /*
 144          * Kill this process to avoid potential security vulnerabilities
 145          * that could arise from a missing whitelist entry preventing a
 146          * privileged process from dropping to a lesser-privileged one.
 147          */
 148         force_sig(SIGKILL);
 149         return -EACCES;
 150 }
 151
 152 static struct security_hook_list safesetid_security_hooks[] = {
 153         LSM_HOOK_INIT(task_fix_setuid, safesetid_task_fix_setuid),
 154         LSM_HOOK_INIT(capable, safesetid_security_capable)
 155 };
 156
 157 static int __init safesetid_security_init(void)
 158 {
 159         security_add_hooks(safesetid_security_hooks,
 160                            ARRAY_SIZE(safesetid_security_hooks), "safesetid");
 161
 162         /* Report that SafeSetID successfully initialized */
 163         safesetid_initialized = 1;
 164
 165         return 0;
 166 }
 167
 168 DEFINE_LSM(safesetid_security_init) = {
 169         .init = safesetid_security_init,
 170         .name = "safesetid",
 171 };