src/shared/capability.c

   1 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
   2
   3 /***
   4   This file is part of systemd.
   5
   6   Copyright 2010 Lennart Poettering
   7
   8   systemd is free software; you can redistribute it and/or modify it
   9   under the terms of the GNU Lesser General Public License as published by
  10   the Free Software Foundation; either version 2.1 of the License, or
  11   (at your option) any later version.
  12
  13   systemd is distributed in the hope that it will be useful, but
  14   WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16   Lesser General Public License for more details.
  17
  18   You should have received a copy of the GNU Lesser General Public License
  19   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  20 ***/
  21
  22 #include <assert.h>
  23 #include <string.h>
  24 #include <unistd.h>
  25 #include <errno.h>
  26 #include <stdio.h>
  27 #include <sys/types.h>
  28 #include <stdarg.h>
  29 #include <ctype.h>
  30 #include <sys/capability.h>
  31 #include <sys/prctl.h>
  32 #include "grp.h"
  33
  34 #include "macro.h"
  35 #include "util.h"
  36 #include "log.h"
  37 #include "fileio.h"
  38 #include "capability.h"
  39
  40 int have_effective_cap(int value) {
  41         _cleanup_cap_free_ cap_t cap;
  42         cap_flag_value_t fv;
  43
  44         cap = cap_get_proc();
  45         if (!cap)
  46                 return -errno;
  47
  48         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  49                 return -errno;
  50         else
  51                 return fv == CAP_SET;
  52 }
  53
  54 unsigned long cap_last_cap(void) {
  55         static thread_local unsigned long saved;
  56         static thread_local bool valid = false;
  57         unsigned long p;
  58
  59         if (valid)
  60                 return saved;
  61
  62         p = (unsigned long) CAP_LAST_CAP;
  63
  64         if (prctl(PR_CAPBSET_READ, p) < 0) {
  65
  66                 /* Hmm, look downwards, until we find one that
  67                  * works */
  68                 for (p--; p > 0; p --)
  69                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  70                                 break;
  71
  72         } else {
  73
  74                 /* Hmm, look upwards, until we find one that doesn't
  75                  * work */
  76                 for (;; p++)
  77                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  78                                 break;
  79         }
  80
  81         saved = p;
  82         valid = true;
  83
  84         return p;
  85 }
  86
  87 int capability_bounding_set_drop(uint64_t drop, bool right_now) {
  88         _cleanup_cap_free_ cap_t after_cap = NULL;
  89         cap_flag_value_t fv;
  90         unsigned long i;
  91         int r;
  92
  93         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
  94          * in the effective set (yes, the kernel drops that when
  95          * executing init!), so get it back temporarily so that we can
  96          * call PR_CAPBSET_DROP. */
  97
  98         after_cap = cap_get_proc();
  99         if (!after_cap)
 100                 return -errno;
 101
 102         if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 103                 return -errno;
 104
 105         if (fv != CAP_SET) {
 106                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 107                 static const cap_value_t v = CAP_SETPCAP;
 108
 109                 temp_cap = cap_dup(after_cap);
 110                 if (!temp_cap) {
 111                         r = -errno;
 112                         goto finish;
 113                 }
 114
 115                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
 116                         r = -errno;
 117                         goto finish;
 118                 }
 119
 120                 if (cap_set_proc(temp_cap) < 0) {
 121                         r = -errno;
 122                         goto finish;
 123                 }
 124         }
 125
 126         for (i = 0; i <= cap_last_cap(); i++) {
 127
 128                 if (drop & ((uint64_t) 1ULL << (uint64_t) i)) {
 129                         cap_value_t v;
 130
 131                         /* Drop it from the bounding set */
 132                         if (prctl(PR_CAPBSET_DROP, i) < 0) {
 133                                 r = -errno;
 134                                 goto finish;
 135                         }
 136                         v = (cap_value_t) i;
 137
 138                         /* Also drop it from the inheritable set, so
 139                          * that anything we exec() loses the
 140                          * capability for good. */
 141                         if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 142                                 r = -errno;
 143                                 goto finish;
 144                         }
 145
 146                         /* If we shall apply this right now drop it
 147                          * also from our own capability sets. */
 148                         if (right_now) {
 149                                 if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 150                                     cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 151                                         r = -errno;
 152                                         goto finish;
 153                                 }
 154                         }
 155                 }
 156         }
 157
 158         r = 0;
 159
 160 finish:
 161         if (cap_set_proc(after_cap) < 0)
 162                 return -errno;
 163
 164         return r;
 165 }
 166
 167 static int drop_from_file(const char *fn, uint64_t drop) {
 168         int r, k;
 169         uint32_t hi, lo;
 170         uint64_t current, after;
 171         char *p;
 172
 173         r = read_one_line_file(fn, &p);
 174         if (r < 0)
 175                 return r;
 176
 177         assert_cc(sizeof(hi) == sizeof(unsigned));
 178         assert_cc(sizeof(lo) == sizeof(unsigned));
 179
 180         k = sscanf(p, "%u %u", &lo, &hi);
 181         free(p);
 182
 183         if (k != 2)
 184                 return -EIO;
 185
 186         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 187         after = current & ~drop;
 188
 189         if (current == after)
 190                 return 0;
 191
 192         lo = (unsigned) (after & 0xFFFFFFFFULL);
 193         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 194
 195         if (asprintf(&p, "%u %u", lo, hi) < 0)
 196                 return -ENOMEM;
 197
 198         r = write_string_file(fn, p);
 199         free(p);
 200
 201         return r;
 202 }
 203
 204 int capability_bounding_set_drop_usermode(uint64_t drop) {
 205         int r;
 206
 207         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", drop);
 208         if (r < 0)
 209                 return r;
 210
 211         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", drop);
 212         if (r < 0)
 213                 return r;
 214
 215         return r;
 216 }
 217
 218 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 219
 220         _cleanup_cap_free_ cap_t d = NULL;
 221         int r;
 222
 223         /* Unfortunately we cannot leave privilege dropping to PID 1
 224          * here, since we want to run as user but want to keep some
 225          * capabilities. Since file capabilities have been introduced
 226          * this cannot be done across exec() anymore, unless our
 227          * binary has the capability configured in the file system,
 228          * which we want to avoid. */
 229
 230         if (setresgid(gid, gid, gid) < 0) {
 231                 log_error("Failed to change group ID: %m");
 232                 return -errno;
 233         }
 234
 235         if (setgroups(0, NULL) < 0) {
 236                 log_error("Failed to drop auxiliary groups list: %m");
 237                 return -errno;
 238         }
 239
 240         if (prctl(PR_SET_KEEPCAPS, 1) < 0) {
 241                 log_error("Failed to enable keep capabilities flag: %m");
 242                 return -errno;
 243         }
 244
 245         r = setresuid(uid, uid, uid);
 246         if (r < 0) {
 247                 log_error("Failed to change user ID: %m");
 248                 return -errno;
 249         }
 250
 251         if (prctl(PR_SET_KEEPCAPS, 0) < 0) {
 252                 log_error("Failed to disable keep capabilities flag: %m");
 253                 return -errno;
 254         }
 255
 256         r = capability_bounding_set_drop(~keep_capabilities, true);
 257         if (r < 0) {
 258                 log_error("Failed to drop capabilities: %s", strerror(-r));
 259                 return r;
 260         }
 261
 262         d = cap_init();
 263         if (!d)
 264                 return log_oom();
 265
 266         if (keep_capabilities) {
 267                 cap_value_t bits[sizeof(keep_capabilities)*8];
 268                 unsigned i, j = 0;
 269
 270                 for (i = 0; i < sizeof(keep_capabilities)*8; i++)
 271                         if (keep_capabilities & (1ULL << i))
 272                                 bits[j++] = i;
 273
 274                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 275                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
 276                         log_error("Failed to enable capabilities bits: %m");
 277                         return -errno;
 278                 }
 279         }
 280
 281         if (cap_set_proc(d) < 0) {
 282                 log_error("Failed to increase capabilities: %m");
 283                 return -errno;
 284         }
 285
 286         return 0;
 287 }