emacs/mps/code/protix.c

/* protix.c: PROTECTION FOR UNIX
 *
 *  $Id$
 *  Copyright (c) 2001-2020 Ravenbrook Limited.  See end of file for license.
 *
 *  Somewhat generic across different Unix systems.  Shared between
 *  macOS, FreeBSD, and Linux.
 *
 *
 *  SOURCES
 *
 *  [SUSV2MPROTECT] Single UNIX Specification, Version 2, mprotect
 *  <https://pubs.opengroup.org/onlinepubs/007908799/xsh/mprotect.html>
 *
 *  ASSUMPTIONS
 *
 *  .assume.mprotect.base: We assume that the first argument to mprotect can
 *    be safely passed as a void *. Single UNIX Specification Version 2 (aka
 *    X/OPEN XSH5) says that the parameter is a void *. Some Unix-likes may
 *    declare this parameter as a caddr_t. FreeBSD used to do this (on the now
 *    very obsolete FreeBSD 2.2.x series), as did macOS, but both now implement
 *    it correctly as void *. caddr_t is usually char *.
 *
 *  .assume.write-only:  More of an anti-assumption really.  We
 *    assume that asking the OS for a write-only page (that is, flags =
 *    PROT_WRITE) does not work.  What actually happens on all the
 *    Unix-like OSes that we've seen is that asking for write-permission
 *    (flags = PROT_WRITE) grants read-permission as well.  That is why
 *    when the MPS requires that a page be read-protected (mode ==
 *    AccessREAD) we must ensure that writes are also not allowed.
 *    The portable guarantees of mprotect (see [SUSV2MPROTECT]) are that
 *    writes are not permitted where PROT_WRITE is not used and no access
 *    is permitted when PROT_NONE alone is used.
 */

#include "mpm.h"

#if !defined(MPS_OS_FR) && !defined(MPS_OS_LI) && !defined(MPS_OS_XC)
#error "protix.c is specific to MPS_OS_FR, MPS_OS_LI or MPS_OS_XC"
#endif

#include "vm.h"

#include <errno.h>
#include <limits.h>
#include <signal.h> /* sig_atomic_t */
#include <stddef.h>
#include <sys/mman.h>
#include <sys/types.h>

SRCID(protix, "$Id$");


/* Value for memory protection corresponding to AccessSetEMPTY.
 * See .convert.access for an explanation of the conversion.
 * We use a global variable and not a constant so that we can clear
 * the executable flag from future requests if Apple Hardened Runtime
 * is detected. See <design/prot#impl.xc.prot.exec> for details. */

static sig_atomic_t prot_all = PROT_READ | PROT_WRITE | PROT_EXEC;


/* ProtSet -- set protection
 *
 * This is just a thin veneer on top of mprotect(2).
 */

void ProtSet(Addr base, Addr limit, AccessSet mode)
{
  int flags, result;

  AVER(sizeof(size_t) == sizeof(Addr));
  AVER(base < limit);
  AVER(base != 0);
  AVER(AddrOffset(base, limit) <= INT_MAX);     /* should be redundant */
  AVERT(AccessSet, mode);

  /* .convert.access: Convert between MPS AccessSet and UNIX PROT thingies.
     In this function, AccessREAD means protect against read accesses
     (disallow them).  PROT_READ means allow read accesses.  Notice that
     this follows a difference in contract as well as style.  AccessREAD
     means that no reads should be permitted (all reads should go via
     the signal handler), possibly other operations (write) also go via
     the signal handler; PROT_WRITE means that all writes should be
     allowed, possibly that means other operations (read) are also
     allowed.
   */
  switch(mode) {
  case AccessWRITE | AccessREAD:
  case AccessREAD:      /* forbids writes as well, see .assume.write-only */
    flags = PROT_NONE;
    break;
  case AccessWRITE:
    flags = PROT_READ | PROT_EXEC;
    break;
  case AccessSetEMPTY:
    flags = (int)prot_all; /* potential narrowing cast, but safe */
    break;
  default:
    NOTREACHED;
    flags = PROT_NONE;
  }

  /* .assume.mprotect.base */
  result = mprotect((void *)base, (size_t)AddrOffset(base, limit), flags);
  if (MAYBE_HARDENED_RUNTIME && result != 0 && errno == EACCES
      && (flags & PROT_WRITE) && (flags & PROT_EXEC))
  {
    /* Apple Hardened Runtime is enabled, so that we cannot have
     * memory that is simultaneously writable and executable. Handle
     * this by dropping the executable part of the request. See
     * <design/prot#impl.xc.prot.exec> for details. */
    prot_all = PROT_READ | PROT_WRITE;
    result = mprotect((void *)base, (size_t)AddrOffset(base, limit), flags & prot_all);
  }
  if (result != 0)
    NOTREACHED;
}


/* ProtSync -- synchronize protection settings with hardware
 *
 * This does nothing under Posix.  See protan.c.
 */

void ProtSync(Arena arena)
{
  UNUSED(arena);
  NOOP;
}


/* ProtGranularity -- return the granularity of protection */

Size ProtGranularity(void)
{
  /* Individual pages can be protected. */
  return PageSize();
}


/* C. COPYRIGHT AND LICENSE
 *
 * Copyright (C) 2001-2020 Ravenbrook Limited <https://www.ravenbrook.com/>.
 *
 * 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 COPYRIGHT HOLDERS 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 COPYRIGHT
 * HOLDER 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.
 */