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 #include "dotlock.h"
  30
  31 static void _ign_signal(int signum);
  32
  33 static void
  34 _ign_signal(int signum)
  35 {
  36    struct sigaction nact, oact;
  37
  38    nact.sa_handler = SIG_IGN;
  39    sigemptyset(&nact.sa_mask);
  40    nact.sa_flags = 0;
  41    sigaction(signum, &nact, &oact);
  42 }
  43
  44 int
  45 main(int argc, char **argv)
  46 {
  47    struct dotlock_info di;
  48    struct stat stb;
  49    sigset_t nset, oset;
  50    enum dotlock_state dls;
  51
  52    /* We're a dumb helper, ensure as much as we can noone else uses us */
  53    if (argc != 12 ||
  54          strcmp(argv[ 0], PRIVSEP) ||
  55          (argv[1][0] != 'r' && argv[1][0] != 'w') ||
  56          strcmp(argv[ 1] + 1, "dotlock") ||
  57          strcmp(argv[ 2], "mailbox") ||
  58          strcmp(argv[ 4], "name") ||
  59          strcmp(argv[ 6], "hostname") ||
  60          strcmp(argv[ 8], "randstr") ||
  61          strcmp(argv[10], "pollmsecs") ||
  62          fstat(STDIN_FILENO, &stb) == -1 || !S_ISFIFO(stb.st_mode) ||
  63          fstat(STDOUT_FILENO, &stb) == -1 || !S_ISFIFO(stb.st_mode)) {
  64 jeuse:
  65       fprintf(stderr,
  66          "This is a helper program of \"" UAGENT "\" (in " BINDIR ").\n"
  67          "  It is capable of gaining more privileges than \"" UAGENT "\"\n"
  68          "  and will be used to create lock files.\n"
  69          "  It's sole purpose is outsourcing of high privileges into\n"
  70          "  fewest lines of code in order to reduce attack surface.\n"
  71          "  It cannot be run by itself.\n");
  72       exit(EXIT_USE);
  73    }
  74
  75    di.di_file_name = argv[3];
  76    di.di_lock_name = argv[5];
  77    di.di_hostname = argv[7];
  78    di.di_randstr = argv[9];
  79    di.di_pollmsecs = (size_t)strtoul(argv[11], NULL, 10);
  80    {
  81       size_t i = strlen(di.di_file_name);
  82
  83       if (i == 0 || strncmp(di.di_file_name, di.di_lock_name, i) ||
  84             di.di_lock_name[i] == '\0' || strcmp(di.di_lock_name + i, ".lock"))
  85          goto jeuse;
  86    }
  87
  88    close(STDERR_FILENO);
  89
  90    /* In order to prevent stale lock files at all cost block any signals until
  91     * we have unlinked the lock file.
  92     * It is still not safe because we may be SIGKILLed and may linger around
  93     * because we have been SIGSTOPped, but unfortunately the standard doesn't
  94     * give any option, e.g. atcrash() or open(O_TEMPORARY_KEEP_NAME) or so, ---
  95     * and then again we should not unlink(2) the lock file unless our parent
  96     * has finalized the synchronization!  While at it, let me rant about the
  97     * default action of realtime signals, program termination */
  98    _ign_signal(SIGPIPE); /* (Inherited, though) */
  99    sigfillset(&nset);
 100    sigdelset(&nset, SIGCONT); /* (Rather redundant, though) */
 101    sigprocmask(SIG_BLOCK, &nset, &oset);
 102
 103    dls = DLS_NOPERM | DLS_ABANDON;
 104
 105    /* First of all: we only dotlock when the executing user has the necessary
 106     * rights to access the mailbox */
 107    if (access(di.di_file_name, (argv[1][0] == 'r' ? R_OK : R_OK | W_OK)))
 108       goto jmsg;
 109
 110    /* We need UID and GID information about the mailbox to lock */
 111    if (stat(di.di_file_name, di.di_stb = &stb) == -1)
 112       goto jmsg;
 113
 114    dls = DLS_PRIVFAILED | DLS_ABANDON;
 115
 116    /* This privsep helper only gets executed when needed, it thus doesn't make
 117     * sense to try to continue with initial privileges */
 118    if (setuid(geteuid()))
 119       goto jmsg;
 120
 121    dls = _dotlock_create(&di);
 122
 123    /* Finally: notify our parent about the actual lock state.. */
 124 jmsg:
 125    write(STDOUT_FILENO, &dls, sizeof dls);
 126    close(STDOUT_FILENO);
 127
 128    /* ..then eventually wait until we shall remove the lock again, which will
 129     * be notified via the read returning */
 130    if (dls == DLS_NONE) {
 131       read(STDIN_FILENO, &dls, sizeof dls);
 132
 133       unlink(di.di_lock_name);
 134    }
 135
 136    sigprocmask(SIG_SETMASK, &oset, NULL);
 137    return (dls == DLS_NONE ? EXIT_OK : EXIT_ERR);
 138 }
 139
 140 /* s-it-mode */