uidswap.c

/*

uidswap.c

Author: Tatu Ylonen <ylo@cs.hut.fi>

Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
                   All rights reserved

Created: Sat Sep  9 01:56:14 1995 ylo

Code for uid-swapping.

*/

#include "includes.h"
RCSID("$Id: uidswap.c,v 1.1 1999/10/27 03:42:46 damien Exp $");

#include "ssh.h"
#include "uidswap.h"

/* Note: all these functions must work in all of the following cases:

   1. euid=0, ruid=0
   2. euid=0, ruid!=0
   3. euid!=0, ruid!=0

   Additionally, they must work regardless of whether the system has
   POSIX saved uids or not. */

#ifdef _POSIX_SAVED_IDS
/* Lets assume that posix saved ids also work with seteuid, even though that
   is not part of the posix specification. */
#define SAVED_IDS_WORK_WITH_SETEUID
#endif /* _POSIX_SAVED_IDS */

/* Saved effective uid. */
static uid_t saved_euid = 0;

/* Temporarily changes to the given uid.  If the effective user id is not
   root, this does nothing.  This call cannot be nested. */

void temporarily_use_uid(uid_t uid)
{
#ifdef SAVED_IDS_WORK_WITH_SETEUID

  /* Save the current euid. */
  saved_euid = geteuid();

  /* Set the effective uid to the given (unprivileged) uid. */
  if (seteuid(uid) == -1)
    debug("seteuid %d: %.100s", (int)uid, strerror(errno));

#else /* SAVED_IDS_WORK_WITH_SETUID */

  /* Propagate the privileged uid to all of our uids. */
  if (setuid(geteuid()) < 0)
    debug("setuid %d: %.100s", (int)geteuid(), strerror(errno));

  /* Set the effective uid to the given (unprivileged) uid. */
  if (seteuid(uid) == -1)
    debug("seteuid %d: %.100s", (int)uid, strerror(errno));

#endif /* SAVED_IDS_WORK_WITH_SETEUID */

}

/* Restores to the original uid. */

void restore_uid()
{
#ifdef SAVED_IDS_WORK_WITH_SETEUID

  /* Set the effective uid back to the saved uid. */
  if (seteuid(saved_euid) < 0)
    debug("seteuid %d: %.100s", (int)saved_euid, strerror(errno));

#else /* SAVED_IDS_WORK_WITH_SETEUID */

  /* We are unable to restore the real uid to its unprivileged value. */
  /* Propagate the real uid (usually more privileged) to effective uid
     as well. */
  setuid(getuid());

#endif /* SAVED_IDS_WORK_WITH_SETEUID */
}

/* Permanently sets all uids to the given uid.  This cannot be called while
   temporarily_use_uid is effective. */

void permanently_set_uid(uid_t uid)
{
  if (setuid(uid) < 0)
    debug("setuid %d: %.100s", (int)uid, strerror(errno));
}