crypto/openssh/uidswap.c

   1 /* $OpenBSD: uidswap.c,v 1.42 2019/06/28 13:35:04 deraadt Exp $ */
   2 /*
   3  * Author: Tatu Ylonen <ylo@cs.hut.fi>
   4  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
   5  *                    All rights reserved
   6  * Code for uid-swapping.
   7  *
   8  * As far as I am concerned, the code I have written for this software
   9  * can be used freely for any purpose.  Any derived versions of this
  10  * software must be clearly marked as such, and if the derived work is
  11  * incompatible with the protocol description in the RFC file, it must be
  12  * called by a name other than "ssh" or "Secure Shell".
  13  */
  14
  15 #include "includes.h"
  16
  17 #include <errno.h>
  18 #include <pwd.h>
  19 #include <string.h>
  20 #include <unistd.h>
  21 #include <limits.h>
  22 #include <stdarg.h>
  23 #include <stdlib.h>
  24
  25 #include <grp.h>
  26
  27 #include "log.h"
  28 #include "uidswap.h"
  29 #include "xmalloc.h"
  30
  31 /*
  32  * Note: all these functions must work in all of the following cases:
  33  *    1. euid=0, ruid=0
  34  *    2. euid=0, ruid!=0
  35  *    3. euid!=0, ruid!=0
  36  * Additionally, they must work regardless of whether the system has
  37  * POSIX saved uids or not.
  38  */
  39
  40 #if defined(_POSIX_SAVED_IDS) && !defined(BROKEN_SAVED_UIDS)
  41 /* Lets assume that posix saved ids also work with seteuid, even though that
  42    is not part of the posix specification. */
  43 #define SAVED_IDS_WORK_WITH_SETEUID
  44 /* Saved effective uid. */
  45 static uid_t    saved_euid = 0;
  46 static gid_t    saved_egid = 0;
  47 #endif
  48
  49 /* Saved effective uid. */
  50 static int      privileged = 0;
  51 static int      temporarily_use_uid_effective = 0;
  52 static uid_t    user_groups_uid;
  53 static gid_t    *saved_egroups = NULL, *user_groups = NULL;
  54 static int      saved_egroupslen = -1, user_groupslen = -1;
  55
  56 /*
  57  * Temporarily changes to the given uid.  If the effective user
  58  * id is not root, this does nothing.  This call cannot be nested.
  59  */
  60 void
  61 temporarily_use_uid(struct passwd *pw)
  62 {
  63         /* Save the current euid, and egroups. */
  64 #ifdef SAVED_IDS_WORK_WITH_SETEUID
  65         saved_euid = geteuid();
  66         saved_egid = getegid();
  67         debug("temporarily_use_uid: %u/%u (e=%u/%u)",
  68             (u_int)pw->pw_uid, (u_int)pw->pw_gid,
  69             (u_int)saved_euid, (u_int)saved_egid);
  70 #ifndef HAVE_CYGWIN
  71         if (saved_euid != 0) {
  72                 privileged = 0;
  73                 return;
  74         }
  75 #endif
  76 #else
  77         if (geteuid() != 0) {
  78                 privileged = 0;
  79                 return;
  80         }
  81 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
  82
  83         privileged = 1;
  84         temporarily_use_uid_effective = 1;
  85
  86         saved_egroupslen = getgroups(0, NULL);
  87         if (saved_egroupslen == -1)
  88                 fatal("getgroups: %.100s", strerror(errno));
  89         if (saved_egroupslen > 0) {
  90                 saved_egroups = xreallocarray(saved_egroups,
  91                     saved_egroupslen, sizeof(gid_t));
  92                 if (getgroups(saved_egroupslen, saved_egroups) == -1)
  93                         fatal("getgroups: %.100s", strerror(errno));
  94         } else { /* saved_egroupslen == 0 */
  95                 free(saved_egroups);
  96                 saved_egroups = NULL;
  97         }
  98
  99         /* set and save the user's groups */
 100         if (user_groupslen == -1 || user_groups_uid != pw->pw_uid) {
 101                 if (initgroups(pw->pw_name, pw->pw_gid) == -1)
 102                         fatal("initgroups: %s: %.100s", pw->pw_name,
 103                             strerror(errno));
 104
 105                 user_groupslen = getgroups(0, NULL);
 106                 if (user_groupslen == -1)
 107                         fatal("getgroups: %.100s", strerror(errno));
 108                 if (user_groupslen > 0) {
 109                         user_groups = xreallocarray(user_groups,
 110                             user_groupslen, sizeof(gid_t));
 111                         if (getgroups(user_groupslen, user_groups) == -1)
 112                                 fatal("getgroups: %.100s", strerror(errno));
 113                 } else { /* user_groupslen == 0 */
 114                         free(user_groups);
 115                         user_groups = NULL;
 116                 }
 117                 user_groups_uid = pw->pw_uid;
 118         }
 119         /* Set the effective uid to the given (unprivileged) uid. */
 120         if (setgroups(user_groupslen, user_groups) == -1)
 121                 fatal("setgroups: %.100s", strerror(errno));
 122 #ifndef SAVED_IDS_WORK_WITH_SETEUID
 123         /* Propagate the privileged gid to all of our gids. */
 124         if (setgid(getegid()) == -1)
 125                 debug("setgid %u: %.100s", (u_int) getegid(), strerror(errno));
 126         /* Propagate the privileged uid to all of our uids. */
 127         if (setuid(geteuid()) == -1)
 128                 debug("setuid %u: %.100s", (u_int) geteuid(), strerror(errno));
 129 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
 130         if (setegid(pw->pw_gid) == -1)
 131                 fatal("setegid %u: %.100s", (u_int)pw->pw_gid,
 132                     strerror(errno));
 133         if (seteuid(pw->pw_uid) == -1)
 134                 fatal("seteuid %u: %.100s", (u_int)pw->pw_uid,
 135                     strerror(errno));
 136 }
 137
 138 /*
 139  * Restores to the original (privileged) uid.
 140  */
 141 void
 142 restore_uid(void)
 143 {
 144         /* it's a no-op unless privileged */
 145         if (!privileged) {
 146                 debug("restore_uid: (unprivileged)");
 147                 return;
 148         }
 149         if (!temporarily_use_uid_effective)
 150                 fatal("restore_uid: temporarily_use_uid not effective");
 151
 152 #ifdef SAVED_IDS_WORK_WITH_SETEUID
 153         debug("restore_uid: %u/%u", (u_int)saved_euid, (u_int)saved_egid);
 154         /* Set the effective uid back to the saved privileged uid. */
 155         if (seteuid(saved_euid) == -1)
 156                 fatal("seteuid %u: %.100s", (u_int)saved_euid, strerror(errno));
 157         if (setegid(saved_egid) == -1)
 158                 fatal("setegid %u: %.100s", (u_int)saved_egid, strerror(errno));
 159 #else /* SAVED_IDS_WORK_WITH_SETEUID */
 160         /*
 161          * We are unable to restore the real uid to its unprivileged value.
 162          * Propagate the real uid (usually more privileged) to effective uid
 163          * as well.
 164          */
 165         if (setuid(getuid()) == -1)
 166                 fatal("%s: setuid failed: %s", __func__, strerror(errno));
 167         if (setgid(getgid()) == -1)
 168                 fatal("%s: setgid failed: %s", __func__, strerror(errno));
 169 #endif /* SAVED_IDS_WORK_WITH_SETEUID */
 170
 171         if (setgroups(saved_egroupslen, saved_egroups) == -1)
 172                 fatal("setgroups: %.100s", strerror(errno));
 173         temporarily_use_uid_effective = 0;
 174 }
 175
 176 /*
 177  * Permanently sets all uids to the given uid.  This cannot be
 178  * called while temporarily_use_uid is effective.
 179  */
 180 void
 181 permanently_set_uid(struct passwd *pw)
 182 {
 183 #ifndef NO_UID_RESTORATION_TEST
 184         uid_t old_uid = getuid();
 185         gid_t old_gid = getgid();
 186 #endif
 187
 188         if (pw == NULL)
 189                 fatal("permanently_set_uid: no user given");
 190         if (temporarily_use_uid_effective)
 191                 fatal("permanently_set_uid: temporarily_use_uid effective");
 192         debug("permanently_set_uid: %u/%u", (u_int)pw->pw_uid,
 193             (u_int)pw->pw_gid);
 194
 195         if (setresgid(pw->pw_gid, pw->pw_gid, pw->pw_gid) == -1)
 196                 fatal("setresgid %u: %.100s", (u_int)pw->pw_gid, strerror(errno));
 197
 198 #ifdef __APPLE__
 199         /*
 200          * OS X requires initgroups after setgid to opt back into
 201          * memberd support for >16 supplemental groups.
 202          */
 203         if (initgroups(pw->pw_name, pw->pw_gid) == -1)
 204                 fatal("initgroups %.100s %u: %.100s",
 205                     pw->pw_name, (u_int)pw->pw_gid, strerror(errno));
 206 #endif
 207
 208         if (setresuid(pw->pw_uid, pw->pw_uid, pw->pw_uid) == -1)
 209                 fatal("setresuid %u: %.100s", (u_int)pw->pw_uid, strerror(errno));
 210
 211 #ifndef NO_UID_RESTORATION_TEST
 212         /* Try restoration of GID if changed (test clearing of saved gid) */
 213         if (old_gid != pw->pw_gid && pw->pw_uid != 0 &&
 214             (setgid(old_gid) != -1 || setegid(old_gid) != -1))
 215                 fatal("%s: was able to restore old [e]gid", __func__);
 216 #endif
 217
 218         /* Verify GID drop was successful */
 219         if (getgid() != pw->pw_gid || getegid() != pw->pw_gid) {
 220                 fatal("%s: egid incorrect gid:%u egid:%u (should be %u)",
 221                     __func__, (u_int)getgid(), (u_int)getegid(),
 222                     (u_int)pw->pw_gid);
 223         }
 224
 225 #ifndef NO_UID_RESTORATION_TEST
 226         /* Try restoration of UID if changed (test clearing of saved uid) */
 227         if (old_uid != pw->pw_uid &&
 228             (setuid(old_uid) != -1 || seteuid(old_uid) != -1))
 229                 fatal("%s: was able to restore old [e]uid", __func__);
 230 #endif
 231
 232         /* Verify UID drop was successful */
 233         if (getuid() != pw->pw_uid || geteuid() != pw->pw_uid) {
 234                 fatal("%s: euid incorrect uid:%u euid:%u (should be %u)",
 235                     __func__, (u_int)getuid(), (u_int)geteuid(),
 236                     (u_int)pw->pw_uid);
 237         }
 238 }