privsep.c

   1 /*@ S-nail - a mail user agent derived from Berkeley Mail.
   2  *@ Privilege-separated dot file lock program (WANT_DOTLOCK=yes)
   3  *@ that is capable of calling setuid(2) and change its user identity
   4  *@ to the configured PRIVSEP_USER (usually "root"), in order to create
   5  *@ a dotlock file with the same UID/GID as the mailbox to be locked.
   6  *@ It should be started when chdir(2)d to the lock file's directory,
   7  *@ and SIGPIPE should be ignored.
   8  *
   9  * Copyright (c) 2015 Steffen (Daode) Nurpmeso <sdaoden@users.sf.net>.
  10  *
  11  * Permission to use, copy, modify, and/or distribute this software for any
  12  * purpose with or without fee is hereby granted, provided that the above
  13  * copyright notice and this permission notice appear in all copies.
  14  *
  15  * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  16  * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  17  * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  18  * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  19  * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  20  * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  21  * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
  22  */
  23 #undef n_FILE
  24 #define n_FILE privsep
  25 #define n_PRIVSEP_SOURCE
  26
  27 #include "nail.h"
  28
  29 static void _ign_signal(int signum);
  30 static uiz_t n_msleep(uiz_t millis, bool_t ignint);
  31
  32 #include "dotlock.h"
  33
  34 static void
  35 _ign_signal(int signum)
  36 {
  37    struct sigaction nact, oact;
  38
  39    nact.sa_handler = SIG_IGN;
  40    sigemptyset(&nact.sa_mask);
  41    nact.sa_flags = 0;
  42    sigaction(signum, &nact, &oact);
  43 }
  44
  45 static uiz_t
  46 n_msleep(uiz_t millis, bool_t ignint){
  47    uiz_t rv;
  48
  49 #ifdef HAVE_NANOSLEEP
  50    /* C99 */{
  51       struct timespec ts, trem;
  52       int i;
  53
  54       ts.tv_sec = millis / 1000;
  55       ts.tv_nsec = (millis %= 1000) * 1000 * 1000;
  56
  57       while((i = nanosleep(&ts, &trem)) != 0 && ignint)
  58          ts = trem;
  59       rv = (i == 0) ? 0 : (trem.tv_sec * 1000) + (trem.tv_nsec / (1000 * 1000));
  60    }
  61
  62 #elif defined HAVE_SLEEP
  63    if((millis /= 1000) == 0)
  64       millis = 1;
  65    while((rv = sleep((unsigned int)millis)) != 0 && ignint)
  66       millis = rv;
  67 #else
  68 # error Configuration should have detected a function for sleeping.
  69 #endif
  70    return rv;
  71 }
  72
  73 int
  74 main(int argc, char **argv)
  75 {
  76    struct dotlock_info di;
  77    struct stat stb;
  78    sigset_t nset, oset;
  79    enum dotlock_state dls;
  80
  81    /* We're a dumb helper, ensure as much as we can noone else uses us */
  82    if (argc != 12 ||
  83          strcmp(argv[ 0], PRIVSEP) ||
  84          (argv[1][0] != 'r' && argv[1][0] != 'w') ||
  85          strcmp(argv[ 1] + 1, "dotlock") ||
  86          strcmp(argv[ 2], "mailbox") ||
  87          strcmp(argv[ 4], "name") ||
  88          strcmp(argv[ 6], "hostname") ||
  89          strcmp(argv[ 8], "randstr") ||
  90          strcmp(argv[10], "pollmsecs") ||
  91          fstat(STDIN_FILENO, &stb) == -1 || !S_ISFIFO(stb.st_mode) ||
  92          fstat(STDOUT_FILENO, &stb) == -1 || !S_ISFIFO(stb.st_mode)) {
  93 jeuse:
  94       fprintf(stderr,
  95          "This is a helper program of \"" UAGENT "\" (in " BINDIR ").\n"
  96          "  It is capable of gaining more privileges than \"" UAGENT "\"\n"
  97          "  and will be used to create lock files.\n"
  98          "  It's sole purpose is outsourcing of high privileges into\n"
  99          "  fewest lines of code in order to reduce attack surface.\n"
 100          "  It cannot be run by itself.\n");
 101       exit(EXIT_USE);
 102    }
 103
 104    di.di_file_name = argv[3];
 105    di.di_lock_name = argv[5];
 106    di.di_hostname = argv[7];
 107    di.di_randstr = argv[9];
 108    di.di_pollmsecs = (size_t)strtoul(argv[11], NULL, 10);
 109    {
 110       size_t i = strlen(di.di_file_name);
 111
 112       if (i == 0 || strncmp(di.di_file_name, di.di_lock_name, i) ||
 113             di.di_lock_name[i] == '\0' || strcmp(di.di_lock_name + i, ".lock"))
 114          goto jeuse;
 115    }
 116
 117    close(STDERR_FILENO);
 118
 119    /* In order to prevent stale lock files at all cost block any signals until
 120     * we have unlinked the lock file.
 121     * It is still not safe because we may be SIGKILLed and may linger around
 122     * because we have been SIGSTOPped, but unfortunately the standard doesn't
 123     * give any option, e.g. atcrash() or open(O_TEMPORARY_KEEP_NAME) or so, ---
 124     * and then again we should not unlink(2) the lock file unless our parent
 125     * has finalized the synchronization!  While at it, let me rant about the
 126     * default action of realtime signals, program termination */
 127    _ign_signal(SIGPIPE); /* (Inherited, though) */
 128    sigfillset(&nset);
 129    sigdelset(&nset, SIGCONT); /* (Rather redundant, though) */
 130    sigprocmask(SIG_BLOCK, &nset, &oset);
 131
 132    dls = DLS_NOPERM | DLS_ABANDON;
 133
 134    /* First of all: we only dotlock when the executing user has the necessary
 135     * rights to access the mailbox */
 136    if (access(di.di_file_name, (argv[1][0] == 'r' ? R_OK : R_OK | W_OK)))
 137       goto jmsg;
 138
 139    /* We need UID and GID information about the mailbox to lock */
 140    if (stat(di.di_file_name, di.di_stb = &stb) == -1)
 141       goto jmsg;
 142
 143    dls = DLS_PRIVFAILED | DLS_ABANDON;
 144
 145    /* This privsep helper only gets executed when needed, it thus doesn't make
 146     * sense to try to continue with initial privileges */
 147    if (setuid(geteuid()))
 148       goto jmsg;
 149
 150    dls = _dotlock_create(&di);
 151
 152    /* Finally: notify our parent about the actual lock state.. */
 153 jmsg:
 154    write(STDOUT_FILENO, &dls, sizeof dls);
 155    close(STDOUT_FILENO);
 156
 157    /* ..then eventually wait until we shall remove the lock again, which will
 158     * be notified via the read returning */
 159    if (dls == DLS_NONE) {
 160       read(STDIN_FILENO, &dls, sizeof dls);
 161
 162       unlink(di.di_lock_name);
 163    }
 164
 165    sigprocmask(SIG_SETMASK, &oset, NULL);
 166    return (dls == DLS_NONE ? EXIT_OK : EXIT_ERR);
 167 }
 168
 169 /* s-it-mode */