The Design and Implementation of the FreeBSD Operating System, Second Edition
Now available: The Design and Implementation of the FreeBSD Operating System (Second Edition)


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FreeBSD/Linux Kernel Cross Reference
sys/kern/kern_priv.c

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    1 /*-
    2  * SPDX-License-Identifier: BSD-2-Clause-FreeBSD
    3  *
    4  * Copyright (c) 2006 nCircle Network Security, Inc.
    5  * Copyright (c) 2009 Robert N. M. Watson
    6  * All rights reserved.
    7  *
    8  * This software was developed by Robert N. M. Watson for the TrustedBSD
    9  * Project under contract to nCircle Network Security, Inc.
   10  *
   11  * Redistribution and use in source and binary forms, with or without
   12  * modification, are permitted provided that the following conditions
   13  * are met:
   14  * 1. Redistributions of source code must retain the above copyright
   15  *    notice, this list of conditions and the following disclaimer.
   16  * 2. Redistributions in binary form must reproduce the above copyright
   17  *    notice, this list of conditions and the following disclaimer in the
   18  *    documentation and/or other materials provided with the distribution.
   19  *
   20  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR, NCIRCLE NETWORK SECURITY,
   24  * INC., OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
   25  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
   26  * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
   27  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
   28  * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
   29  * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
   30  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
   31  */
   32 
   33 #include <sys/cdefs.h>
   34 __FBSDID("$FreeBSD: releng/12.0/sys/kern/kern_priv.c 339409 2018-10-17 16:11:43Z jamie $");
   35 
   36 #include <sys/param.h>
   37 #include <sys/jail.h>
   38 #include <sys/kernel.h>
   39 #include <sys/priv.h>
   40 #include <sys/proc.h>
   41 #include <sys/sdt.h>
   42 #include <sys/sysctl.h>
   43 #include <sys/systm.h>
   44 
   45 #include <security/mac/mac_framework.h>
   46 
   47 /*
   48  * `suser_enabled' (which can be set by the security.bsd.suser_enabled
   49  * sysctl) determines whether the system 'super-user' policy is in effect.  If
   50  * it is nonzero, an effective uid of 0 connotes special privilege,
   51  * overriding many mandatory and discretionary protections.  If it is zero,
   52  * uid 0 is offered no special privilege in the kernel security policy.
   53  * Setting it to zero may seriously impact the functionality of many existing
   54  * userland programs, and should not be done without careful consideration of
   55  * the consequences.
   56  */
   57 static int      suser_enabled = 1;
   58 SYSCTL_INT(_security_bsd, OID_AUTO, suser_enabled, CTLFLAG_RWTUN,
   59     &suser_enabled, 0, "processes with uid 0 have privilege");
   60 
   61 static int      unprivileged_mlock = 1;
   62 SYSCTL_INT(_security_bsd, OID_AUTO, unprivileged_mlock, CTLFLAG_RWTUN,
   63     &unprivileged_mlock, 0, "Allow non-root users to call mlock(2)");
   64 
   65 static int      unprivileged_read_msgbuf = 1;
   66 SYSCTL_INT(_security_bsd, OID_AUTO, unprivileged_read_msgbuf,
   67     CTLFLAG_RW, &unprivileged_read_msgbuf, 0,
   68     "Unprivileged processes may read the kernel message buffer");
   69 
   70 SDT_PROVIDER_DEFINE(priv);
   71 SDT_PROBE_DEFINE1(priv, kernel, priv_check, priv__ok, "int");
   72 SDT_PROBE_DEFINE1(priv, kernel, priv_check, priv__err, "int");
   73 
   74 /*
   75  * Check a credential for privilege.  Lots of good reasons to deny privilege;
   76  * only a few to grant it.
   77  */
   78 int
   79 priv_check_cred(struct ucred *cred, int priv, int flags)
   80 {
   81         int error;
   82 
   83         KASSERT(PRIV_VALID(priv), ("priv_check_cred: invalid privilege %d",
   84             priv));
   85 
   86         /*
   87          * We first evaluate policies that may deny the granting of
   88          * privilege unilaterally.
   89          */
   90 #ifdef MAC
   91         error = mac_priv_check(cred, priv);
   92         if (error)
   93                 goto out;
   94 #endif
   95 
   96         /*
   97          * Jail policy will restrict certain privileges that may otherwise be
   98          * be granted.
   99          */
  100         error = prison_priv_check(cred, priv);
  101         if (error)
  102                 goto out;
  103 
  104         if (unprivileged_mlock) {
  105                 /*
  106                  * Allow unprivileged users to call mlock(2)/munlock(2) and
  107                  * mlockall(2)/munlockall(2).
  108                  */
  109                 switch (priv) {
  110                 case PRIV_VM_MLOCK:
  111                 case PRIV_VM_MUNLOCK:
  112                         error = 0;
  113                         goto out;
  114                 }
  115         }
  116 
  117         if (unprivileged_read_msgbuf) {
  118                 /*
  119                  * Allow an unprivileged user to read the kernel message
  120                  * buffer.
  121                  */
  122                 if (priv == PRIV_MSGBUF) {
  123                         error = 0;
  124                         goto out;
  125                 }
  126         }
  127 
  128         /*
  129          * Having determined if privilege is restricted by various policies,
  130          * now determine if privilege is granted.  At this point, any policy
  131          * may grant privilege.  For now, we allow short-circuit boolean
  132          * evaluation, so may not call all policies.  Perhaps we should.
  133          *
  134          * Superuser policy grants privilege based on the effective (or in
  135          * the case of specific privileges, real) uid being 0.  We allow the
  136          * superuser policy to be globally disabled, although this is
  137          * currenty of limited utility.
  138          */
  139         if (suser_enabled) {
  140                 switch (priv) {
  141                 case PRIV_MAXFILES:
  142                 case PRIV_MAXPROC:
  143                 case PRIV_PROC_LIMIT:
  144                         if (cred->cr_ruid == 0) {
  145                                 error = 0;
  146                                 goto out;
  147                         }
  148                         break;
  149                 default:
  150                         if (cred->cr_uid == 0) {
  151                                 error = 0;
  152                                 goto out;
  153                         }
  154                         break;
  155                 }
  156         }
  157 
  158         /*
  159          * Writes to kernel/physical memory are a typical root-only operation,
  160          * but non-root users are expected to be able to read it (provided they
  161          * have permission to access /dev/[k]mem).
  162          */
  163         if (priv == PRIV_KMEM_READ) {
  164                 error = 0;
  165                 goto out;
  166         }
  167 
  168         /*
  169          * Now check with MAC, if enabled, to see if a policy module grants
  170          * privilege.
  171          */
  172 #ifdef MAC
  173         if (mac_priv_grant(cred, priv) == 0) {
  174                 error = 0;
  175                 goto out;
  176         }
  177 #endif
  178 
  179         /*
  180          * The default is deny, so if no policies have granted it, reject
  181          * with a privilege error here.
  182          */
  183         error = EPERM;
  184 out:
  185         if (error)
  186                 SDT_PROBE1(priv, kernel, priv_check, priv__err, priv);
  187         else
  188                 SDT_PROBE1(priv, kernel, priv_check, priv__ok, priv);
  189         return (error);
  190 }
  191 
  192 int
  193 priv_check(struct thread *td, int priv)
  194 {
  195 
  196         KASSERT(td == curthread, ("priv_check: td != curthread"));
  197 
  198         return (priv_check_cred(td->td_ucred, priv, 0));
  199 }

Cache object: 99140421db00449d8ab2d34c05760b36


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