librunas/runas.c

   1 /*
   2  * Copyright (C) 2011 - David Goulet <david.goulet@polymtl.ca>
   3  *                      Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
   4  *
   5  * This program is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License as published by the Free
   7  * Software Foundation; only version 2 of the License.
   8  *
   9  * This program is distributed in the hope that it will be useful, but WITHOUT
  10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License for
  12  * more details.
  13  *
  14  * You should have received a copy of the GNU General Public License along with
  15  * this program; if not, write to the Free Software Foundation, Inc., 59 Temple
  16  * Place - Suite 330, Boston, MA  02111-1307, USA.
  17  */
  18
  19 #define _GNU_SOURCE
  20 #include <errno.h>
  21 #include <limits.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <string.h>
  25 #include <sys/wait.h>
  26 #include <sys/types.h>
  27 #include <sys/stat.h>
  28 #include <unistd.h>
  29 #include <fcntl.h>
  30 #include <sched.h>
  31 #include <sys/mman.h>
  32
  33 #include <lttngerr.h>
  34
  35 #define CHILD_STACK_SIZE        10485760
  36
  37 struct run_as_data {
  38         int (*cmd)(void *data);
  39         void *data;
  40         uid_t uid;
  41         gid_t gid;
  42         int retval_pipe;
  43 };
  44
  45 struct mkdir_data {
  46         const char *path;
  47         mode_t mode;
  48 };
  49
  50 struct open_data {
  51         const char *path;
  52         int flags;
  53         mode_t mode;
  54 };
  55
  56 /*
  57  * Create recursively directory using the FULL path.
  58  */
  59 static
  60 int _mkdir_recursive(void *_data)
  61 {
  62         struct mkdir_data *data = _data;
  63         const char *path;
  64         char *p, tmp[PATH_MAX];
  65         struct stat statbuf;
  66         mode_t mode;
  67         size_t len;
  68         int ret;
  69
  70         path = data->path;
  71         mode = data->mode;
  72
  73         ret = snprintf(tmp, sizeof(tmp), "%s", path);
  74         if (ret < 0) {
  75                 PERROR("snprintf mkdir");
  76                 goto error;
  77         }
  78
  79         len = ret;
  80         if (tmp[len - 1] == '/') {
  81                 tmp[len - 1] = 0;
  82         }
  83
  84         for (p = tmp + 1; *p; p++) {
  85                 if (*p == '/') {
  86                         *p = 0;
  87                         ret = stat(tmp, &statbuf);
  88                         if (ret < 0) {
  89                                 ret = mkdir(tmp, mode);
  90                                 if (ret < 0) {
  91                                         if (!(errno == EEXIST)) {
  92                                                 PERROR("mkdir recursive");
  93                                                 ret = -errno;
  94                                                 goto error;
  95                                         }
  96                                 }
  97                         }
  98                         *p = '/';
  99                 }
 100         }
 101
 102         ret = mkdir(tmp, mode);
 103         if (ret < 0) {
 104                 if (!(errno == EEXIST)) {
 105                         PERROR("mkdir recursive last piece");
 106                         ret = -errno;
 107                 } else {
 108                         ret = 0;
 109                 }
 110         }
 111
 112 error:
 113         return ret;
 114 }
 115
 116 static
 117 int _mkdir(void *_data)
 118 {
 119         struct mkdir_data *data = _data;
 120         return mkdir(data->path, data->mode);
 121 }
 122
 123 static
 124 int _open(void *_data)
 125 {
 126         struct open_data *data = _data;
 127         return open(data->path, data->flags, data->mode);
 128 }
 129
 130 static
 131 int child_run_as(void *_data)
 132 {
 133         struct run_as_data *data = _data;
 134         size_t writelen, writeleft, index;
 135         union {
 136                 int i;
 137                 char c[sizeof(int)];
 138         } sendret;
 139         int ret;
 140
 141         /*
 142          * Child: it is safe to drop egid and euid while sharing the
 143          * file descriptors with the parent process, since we do not
 144          * drop "uid": therefore, the user we are dropping egid/euid to
 145          * cannot attach to this process with, e.g. ptrace, nor map this
 146          * process memory.
 147          */
 148         if (data->gid != getegid()) {
 149                 ret = setegid(data->gid);
 150                 if (ret < 0) {
 151                         perror("setegid");
 152                         exit(EXIT_FAILURE);
 153                 }
 154         }
 155         if (data->uid != geteuid()) {
 156                 ret = seteuid(data->uid);
 157                 if (ret < 0) {
 158                         perror("seteuid");
 159                         exit(EXIT_FAILURE);
 160                 }
 161         }
 162         /*
 163          * Also set umask to 0 for mkdir executable bit.
 164          */
 165         umask(0);
 166         sendret.i = (*data->cmd)(data->data);
 167         /* send back return value */
 168         writeleft = sizeof(sendret);
 169         index = 0;
 170         do {
 171                 writelen = write(data->retval_pipe, &sendret.c[index],
 172                                 writeleft);
 173                 if (writelen < 0) {
 174                         perror("write");
 175                         exit(EXIT_FAILURE);
 176                 }
 177                 writeleft -= writelen;
 178                 index += writelen;
 179         } while (writeleft > 0);
 180
 181         exit(EXIT_SUCCESS);
 182 }
 183
 184 static
 185 int run_as(int (*cmd)(void *data), void *data, uid_t uid, gid_t gid)
 186 {
 187         struct run_as_data run_as_data;
 188         int ret = 0;
 189         int status;
 190         pid_t pid;
 191         int retval_pipe[2];
 192         ssize_t readlen, readleft, index;
 193         void *child_stack;
 194         union {
 195                 int i;
 196                 char c[sizeof(int)];
 197         } retval;
 198
 199         /*
 200          * If we are non-root, we can only deal with our own uid.
 201          */
 202         if (geteuid() != 0) {
 203                 if (uid != geteuid()) {
 204                         ERR("Client (%d)/Server (%d) UID mismatch (and sessiond is not root)",
 205                                 uid, geteuid());
 206                         return -EPERM;
 207                 }
 208         }
 209
 210         ret = pipe(retval_pipe);
 211         if (ret < 0) {
 212                 perror("pipe");
 213                 goto end;
 214         }
 215         run_as_data.data = data;
 216         run_as_data.cmd = cmd;
 217         run_as_data.uid = uid;
 218         run_as_data.gid = gid;
 219         run_as_data.retval_pipe = retval_pipe[1];       /* write end */
 220         child_stack = mmap(NULL, CHILD_STACK_SIZE,
 221                 PROT_WRITE | PROT_READ,
 222                 MAP_PRIVATE | MAP_GROWSDOWN | MAP_ANONYMOUS | MAP_STACK,
 223                 -1, 0);
 224         if (child_stack == MAP_FAILED) {
 225                 perror("mmap");
 226                 ret = -ENOMEM;
 227                 goto close_pipe;
 228         }
 229         /*
 230          * Pointing to the middle of the stack to support architectures
 231          * where the stack grows up (HPPA).
 232          */
 233         pid = clone(child_run_as, child_stack + (CHILD_STACK_SIZE / 2),
 234                 CLONE_FILES | SIGCHLD,
 235                 &run_as_data, NULL);
 236         if (pid < 0) {
 237                 perror("clone");
 238                 ret = pid;
 239                 goto unmap_stack;
 240         }
 241         /* receive return value */
 242         readleft = sizeof(retval);
 243         index = 0;
 244         do {
 245                 readlen = read(retval_pipe[0], &retval.c[index], readleft);
 246                 if (readlen < 0) {
 247                         perror("read");
 248                         ret = -1;
 249                         break;
 250                 }
 251                 readleft -= readlen;
 252                 index += readlen;
 253         } while (readleft > 0);
 254
 255         /*
 256          * Parent: wait for child to return, in which case the
 257          * shared memory map will have been created.
 258          */
 259         pid = wait(&status);
 260         if (pid < 0 || !WIFEXITED(status) || WEXITSTATUS(status) != 0) {
 261                 perror("wait");
 262                 ret = -1;
 263         }
 264 unmap_stack:
 265         ret = munmap(child_stack, CHILD_STACK_SIZE);
 266         if (ret < 0) {
 267                 perror("munmap");
 268         }
 269 close_pipe:
 270         close(retval_pipe[0]);
 271         close(retval_pipe[1]);
 272 end:
 273         return retval.i;
 274 }
 275
 276 int mkdir_recursive_run_as(const char *path, mode_t mode, uid_t uid, gid_t gid)
 277 {
 278         struct mkdir_data data;
 279
 280         DBG3("mkdir() recursive %s with mode %d for uid %d and gid %d",
 281                         path, mode, uid, gid);
 282         data.path = path;
 283         data.mode = mode;
 284         return run_as(_mkdir_recursive, &data, uid, gid);
 285 }
 286
 287 int mkdir_run_as(const char *path, mode_t mode, uid_t uid, gid_t gid)
 288 {
 289         struct mkdir_data data;
 290
 291         DBG3("mkdir() %s with mode %d for uid %d and gid %d",
 292                         path, mode, uid, gid);
 293         data.path = path;
 294         data.mode = mode;
 295         return run_as(_mkdir, &data, uid, gid);
 296 }
 297
 298 /*
 299  * Note: open_run_as is currently not working. We'd need to pass the fd
 300  * opened in the child to the parent.
 301  */
 302 int open_run_as(const char *path, int flags, mode_t mode, uid_t uid, gid_t gid)
 303 {
 304         struct open_data data;
 305
 306         data.path = path;
 307         data.flags = flags;
 308         data.mode = mode;
 309         return run_as(_open, &data, uid, gid);
 310 }