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

     /*-
      * Copyright (c) 2006 nCircle Network Security, Inc.
      * Copyright (c) 2009 Robert N. M. Watson
      * All rights reserved.
      *
      * This software was developed by Robert N. M. Watson for the TrustedBSD
      * Project under contract to nCircle Network Security, Inc.
      *
      * Redistribution and use in source and binary forms, with or without
     * modification, are permitted provided that the following conditions
     * are met:
     * 1. Redistributions of source code must retain the above copyright
     *    notice, this list of conditions and the following disclaimer.
     * 2. Redistributions in binary form must reproduce the above copyright
     *    notice, this list of conditions and the following disclaimer in the
     *    documentation and/or other materials provided with the distribution.
     *
     * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
     * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
     * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR, NCIRCLE NETWORK SECURITY,
     * INC., OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED
     * TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
     * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
     * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
     * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
     * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     */
    
    #include <sys/cdefs.h>
    __FBSDID("$FreeBSD$");
    
    #include <sys/param.h>
    #include <sys/jail.h>
    #include <sys/kernel.h>
    #include <sys/priv.h>
    #include <sys/proc.h>
    #include <sys/sdt.h>
    #include <sys/sysctl.h>
    #include <sys/systm.h>
    
    #include <security/mac/mac_framework.h>
    
    /*
     * `suser_enabled' (which can be set by the security.bsd.suser_enabled
     * sysctl) determines whether the system 'super-user' policy is in effect.  If
     * it is nonzero, an effective uid of 0 connotes special privilege,
     * overriding many mandatory and discretionary protections.  If it is zero,
     * uid 0 is offered no special privilege in the kernel security policy.
     * Setting it to zero may seriously impact the functionality of many existing
     * userland programs, and should not be done without careful consideration of
     * the consequences.
     */
    static int      suser_enabled = 1;
    SYSCTL_INT(_security_bsd, OID_AUTO, suser_enabled, CTLFLAG_RWTUN,
        &suser_enabled, 0, "processes with uid 0 have privilege");
    
    static int      unprivileged_mlock = 1;
    SYSCTL_INT(_security_bsd, OID_AUTO, unprivileged_mlock, CTLFLAG_RWTUN,
        &unprivileged_mlock, 0, "Allow non-root users to call mlock(2)");
    
    static int      unprivileged_read_msgbuf = 1;
    SYSCTL_INT(_security_bsd, OID_AUTO, unprivileged_read_msgbuf,
        CTLFLAG_RW, &unprivileged_read_msgbuf, 0,
        "Unprivileged processes may read the kernel message buffer");
    
    SDT_PROVIDER_DEFINE(priv);
    SDT_PROBE_DEFINE1(priv, kernel, priv_check, priv__ok, "int");
    SDT_PROBE_DEFINE1(priv, kernel, priv_check, priv__err, "int");
    
    /*
     * Check a credential for privilege.  Lots of good reasons to deny privilege;
     * only a few to grant it.
     */
    int
    priv_check_cred(struct ucred *cred, int priv, int flags)
    {
            int error;
    
            KASSERT(PRIV_VALID(priv), ("priv_check_cred: invalid privilege %d",
                priv));
    
            /*
             * We first evaluate policies that may deny the granting of
             * privilege unilaterally.
             */
    #ifdef MAC
            error = mac_priv_check(cred, priv);
            if (error)
                    goto out;
    #endif
    
            /*
             * Jail policy will restrict certain privileges that may otherwise be
             * be granted.
             */
            error = prison_priv_check(cred, priv);
            if (error)
                   goto out;
   
           if (unprivileged_mlock) {
                   /*
                    * Allow unprivileged users to call mlock(2)/munlock(2) and
                    * mlockall(2)/munlockall(2).
                    */
                   switch (priv) {
                   case PRIV_VM_MLOCK:
                   case PRIV_VM_MUNLOCK:
                           error = 0;
                           goto out;
                   }
           }
   
           if (unprivileged_read_msgbuf) {
                   /*
                    * Allow an unprivileged user to read the kernel message
                    * buffer.
                    */
                   if (priv == PRIV_MSGBUF) {
                           error = 0;
                           goto out;
                   }
           }
   
           /*
            * Having determined if privilege is restricted by various policies,
            * now determine if privilege is granted.  At this point, any policy
            * may grant privilege.  For now, we allow short-circuit boolean
            * evaluation, so may not call all policies.  Perhaps we should.
            *
            * Superuser policy grants privilege based on the effective (or in
            * the case of specific privileges, real) uid being 0.  We allow the
            * superuser policy to be globally disabled, although this is
            * currenty of limited utility.
            */
           if (suser_enabled) {
                   switch (priv) {
                   case PRIV_MAXFILES:
                   case PRIV_MAXPROC:
                   case PRIV_PROC_LIMIT:
                           if (cred->cr_ruid == 0) {
                                   error = 0;
                                   goto out;
                           }
                           break;
                   default:
                           if (cred->cr_uid == 0) {
                                   error = 0;
                                   goto out;
                           }
                           break;
                   }
           }
   
           /*
            * Writes to kernel/physical memory are a typical root-only operation,
            * but non-root users are expected to be able to read it (provided they
            * have permission to access /dev/[k]mem).
            */
           if (priv == PRIV_KMEM_READ) {
                   error = 0;
                   goto out;
           }
   
           /*
            * Now check with MAC, if enabled, to see if a policy module grants
            * privilege.
            */
   #ifdef MAC
           if (mac_priv_grant(cred, priv) == 0) {
                   error = 0;
                   goto out;
           }
   #endif
   
           /*
            * The default is deny, so if no policies have granted it, reject
            * with a privilege error here.
            */
           error = EPERM;
   out:
           if (error)
                   SDT_PROBE1(priv, kernel, priv_check, priv__err, priv);
           else
                   SDT_PROBE1(priv, kernel, priv_check, priv__ok, priv);
           return (error);
   }
   
   int
   priv_check(struct thread *td, int priv)
   {
   
           KASSERT(td == curthread, ("priv_check: td != curthread"));
   
           return (priv_check_cred(td->td_ucred, priv, 0));
   }

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