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
  32 #include <lttngerr.h>
  33
  34 /*
  35  * We allocate a stack of twice this size and point in the middle of it
  36  * to support archs with stack growing up and down.
  37  */
  38 #define CHILD_STACK_SIZE 2048
  39
  40 struct run_as_data {
  41         int (*cmd)(void *data);
  42         void *data;
  43         uid_t uid;
  44         gid_t gid;
  45         int retval_pipe;
  46 };
  47
  48 struct mkdir_data {
  49         const char *path;
  50         mode_t mode;
  51 };
  52
  53 struct open_data {
  54         const char *path;
  55         int flags;
  56         mode_t mode;
  57 };
  58
  59 /*
  60  * Create recursively directory using the FULL path.
  61  */
  62 static
  63 int _mkdir_recursive(void *_data)
  64 {
  65         struct mkdir_data *data = _data;
  66         const char *path;
  67         char *p, tmp[PATH_MAX];
  68         struct stat statbuf;
  69         mode_t mode;
  70         size_t len;
  71         int ret;
  72
  73         path = data->path;
  74         mode = data->mode;
  75
  76         ret = snprintf(tmp, sizeof(tmp), "%s", path);
  77         if (ret < 0) {
  78                 PERROR("snprintf mkdir");
  79                 goto error;
  80         }
  81
  82         len = ret;
  83         if (tmp[len - 1] == '/') {
  84                 tmp[len - 1] = 0;
  85         }
  86
  87         for (p = tmp + 1; *p; p++) {
  88                 if (*p == '/') {
  89                         *p = 0;
  90                         ret = stat(tmp, &statbuf);
  91                         if (ret < 0) {
  92                                 ret = mkdir(tmp, mode);
  93                                 if (ret < 0) {
  94                                         if (!(errno == EEXIST)) {
  95                                                 PERROR("mkdir recursive");
  96                                                 ret = -errno;
  97                                                 goto error;
  98                                         }
  99                                 }
 100                         }
 101                         *p = '/';
 102                 }
 103         }
 104
 105         ret = mkdir(tmp, mode);
 106         if (ret < 0) {
 107                 if (!(errno == EEXIST)) {
 108                         PERROR("mkdir recursive last piece");
 109                         ret = -errno;
 110                 } else {
 111                         ret = 0;
 112                 }
 113         }
 114
 115 error:
 116         return ret;
 117 }
 118
 119 static
 120 int _mkdir(void *_data)
 121 {
 122         struct mkdir_data *data = _data;
 123         return mkdir(data->path, data->mode);
 124 }
 125
 126 static
 127 int _open(void *_data)
 128 {
 129         struct open_data *data = _data;
 130         return open(data->path, data->flags, data->mode);
 131 }
 132
 133 static
 134 int child_run_as(void *_data)
 135 {
 136         struct run_as_data *data = _data;
 137         size_t writelen, writeleft, index;
 138         union {
 139                 int i;
 140                 char c[sizeof(int)];
 141         } sendret;
 142         int ret;
 143
 144         /*
 145          * Child: it is safe to drop egid and euid while sharing the
 146          * file descriptors with the parent process, since we do not
 147          * drop "uid": therefore, the user we are dropping egid/euid to
 148          * cannot attach to this process with, e.g. ptrace, nor map this
 149          * process memory.
 150          */
 151         ret = setegid(data->gid);
 152         if (ret < 0) {
 153                 perror("setegid");
 154                 exit(EXIT_FAILURE);
 155         }
 156         ret = seteuid(data->uid);
 157         if (ret < 0) {
 158                 perror("seteuid");
 159                 exit(EXIT_FAILURE);
 160         }
 161         /*
 162          * Also set umask to 0 for mkdir executable bit.
 163          */
 164         umask(0);
 165         sendret.i = (*data->cmd)(data->data);
 166         /* send back return value */
 167         writeleft = sizeof(sendret);
 168         index = 0;
 169         do {
 170                 writelen = write(data->retval_pipe, &sendret.c[index],
 171                                 writeleft);
 172                 if (writelen < 0) {
 173                         perror("write");
 174                         exit(EXIT_FAILURE);
 175                 }
 176                 writeleft -= writelen;
 177                 index += writelen;
 178         } while (writeleft > 0);
 179
 180         exit(EXIT_SUCCESS);
 181 }
 182
 183 static
 184 int run_as(int (*cmd)(void *data), void *data, uid_t uid, gid_t gid)
 185 {
 186         struct run_as_data run_as_data;
 187         int ret = 0;
 188         int status;
 189         pid_t pid;
 190         char *child_stack;
 191         int retval_pipe[2];
 192         ssize_t readlen, readleft, index;
 193         union {
 194                 int i;
 195                 char c[sizeof(int)];
 196         } retval;
 197
 198         /*
 199          * If we are non-root, we can only deal with our own uid.
 200          */
 201         if (geteuid() != 0) {
 202                 if (uid != geteuid()) {
 203                         ERR("Client (%d)/Server (%d) UID mismatch (and sessiond is not root)",
 204                                 uid, geteuid());
 205                         return -EPERM;
 206                 }
 207         }
 208
 209         child_stack = malloc(CHILD_STACK_SIZE * 2);
 210         if (!child_stack) {
 211                 return -ENOMEM;
 212         }
 213         ret = pipe(retval_pipe);
 214         if (ret < 0) {
 215                 perror("pipe");
 216                 goto end;
 217         }
 218         run_as_data.data = data;
 219         run_as_data.cmd = cmd;
 220         run_as_data.uid = uid;
 221         run_as_data.gid = gid;
 222         run_as_data.retval_pipe = retval_pipe[1];       /* write end */
 223
 224         pid = clone(child_run_as, child_stack + CHILD_STACK_SIZE,
 225                 CLONE_FILES | SIGCHLD, &run_as_data, NULL);
 226         if (pid < 0) {
 227                 perror("clone");
 228                 ret = pid;
 229                 goto close_pipe;
 230         }
 231         /* receive return value */
 232         readleft = sizeof(retval);
 233         index = 0;
 234         do {
 235                 readlen = read(retval_pipe[0], &retval.c[index], readleft);
 236                 if (readlen < 0) {
 237                         perror("read");
 238                         ret = -1;
 239                         break;
 240                 }
 241                 readleft -= readlen;
 242                 index += readlen;
 243         } while (readleft > 0);
 244
 245         /*
 246          * Parent: wait for child to return, in which case the
 247          * shared memory map will have been created.
 248          */
 249         pid = wait(&status);
 250         if (pid < 0 || !WIFEXITED(status) || WEXITSTATUS(status) != 0) {
 251                 perror("wait");
 252                 ret = -1;
 253         }
 254 close_pipe:
 255         close(retval_pipe[0]);
 256         close(retval_pipe[1]);
 257 end:
 258         free(child_stack);
 259         return retval.i;
 260 }
 261
 262 int mkdir_recursive_run_as(const char *path, mode_t mode, uid_t uid, gid_t gid)
 263 {
 264         struct mkdir_data data;
 265
 266         DBG3("mkdir() recursive %s with mode %d for uid %d and gid %d",
 267                         path, mode, uid, gid);
 268         data.path = path;
 269         data.mode = mode;
 270         return run_as(_mkdir_recursive, &data, uid, gid);
 271 }
 272
 273 int mkdir_run_as(const char *path, mode_t mode, uid_t uid, gid_t gid)
 274 {
 275         struct mkdir_data data;
 276
 277         DBG3("mkdir() %s with mode %d for uid %d and gid %d",
 278                         path, mode, uid, gid);
 279         data.path = path;
 280         data.mode = mode;
 281         return run_as(_mkdir, &data, uid, gid);
 282 }
 283
 284 /*
 285  * Note: open_run_as is currently not working. We'd need to pass the fd
 286  * opened in the child to the parent.
 287  */
 288 int open_run_as(const char *path, int flags, mode_t mode, uid_t uid, gid_t gid)
 289 {
 290         struct open_data data;
 291
 292         data.path = path;
 293         data.flags = flags;
 294         data.mode = mode;
 295         return run_as(_open, &data, uid, gid);
 296 }