src/basic/capability-util.c

   1 /***
   2   This file is part of systemd.
   3
   4   Copyright 2010 Lennart Poettering
   5
   6   systemd is free software; you can redistribute it and/or modify it
   7   under the terms of the GNU Lesser General Public License as published by
   8   the Free Software Foundation; either version 2.1 of the License, or
   9   (at your option) any later version.
  10
  11   systemd is distributed in the hope that it will be useful, but
  12   WITHOUT ANY WARRANTY; without even the implied warranty of
  13   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  14   Lesser General Public License for more details.
  15
  16   You should have received a copy of the GNU Lesser General Public License
  17   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  18 ***/
  19
  20 #include <errno.h>
  21 #include <grp.h>
  22 #include <stdio.h>
  23 #include <stdlib.h>
  24 #include <sys/capability.h>
  25 #include <sys/prctl.h>
  26 #include <unistd.h>
  27
  28 #include "alloc-util.h"
  29 #include "capability-util.h"
  30 #include "fileio.h"
  31 #include "log.h"
  32 #include "macro.h"
  33 #include "parse-util.h"
  34 #include "util.h"
  35
  36 int have_effective_cap(int value) {
  37         _cleanup_cap_free_ cap_t cap;
  38         cap_flag_value_t fv;
  39
  40         cap = cap_get_proc();
  41         if (!cap)
  42                 return -errno;
  43
  44         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  45                 return -errno;
  46         else
  47                 return fv == CAP_SET;
  48 }
  49
  50 unsigned long cap_last_cap(void) {
  51         static thread_local unsigned long saved;
  52         static thread_local bool valid = false;
  53         _cleanup_free_ char *content = NULL;
  54         unsigned long p = 0;
  55         int r;
  56
  57         if (valid)
  58                 return saved;
  59
  60         /* available since linux-3.2 */
  61         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  62         if (r >= 0) {
  63                 r = safe_atolu(content, &p);
  64                 if (r >= 0) {
  65                         saved = p;
  66                         valid = true;
  67                         return p;
  68                 }
  69         }
  70
  71         /* fall back to syscall-probing for pre linux-3.2 */
  72         p = (unsigned long) CAP_LAST_CAP;
  73
  74         if (prctl(PR_CAPBSET_READ, p) < 0) {
  75
  76                 /* Hmm, look downwards, until we find one that
  77                  * works */
  78                 for (p--; p > 0; p --)
  79                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  80                                 break;
  81
  82         } else {
  83
  84                 /* Hmm, look upwards, until we find one that doesn't
  85                  * work */
  86                 for (;; p++)
  87                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  88                                 break;
  89         }
  90
  91         saved = p;
  92         valid = true;
  93
  94         return p;
  95 }
  96
  97 int capability_update_inherited_set(cap_t caps, uint64_t set) {
  98         unsigned long i;
  99
 100         /* Add capabilities in the set to the inherited caps. Do not apply
 101          * them yet. */
 102
 103         for (i = 0; i < cap_last_cap(); i++) {
 104
 105                 if (set & (UINT64_C(1) << i)) {
 106                         cap_value_t v;
 107
 108                         v = (cap_value_t) i;
 109
 110                         /* Make the capability inheritable. */
 111                         if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, CAP_SET) < 0)
 112                                 return -errno;
 113                 }
 114         }
 115
 116         return 0;
 117 }
 118
 119 int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
 120         unsigned long i;
 121         _cleanup_cap_free_ cap_t caps = NULL;
 122
 123         /* Add the capabilities to the ambient set. */
 124
 125         if (also_inherit) {
 126                 int r;
 127                 caps = cap_get_proc();
 128                 if (!caps)
 129                         return -errno;
 130
 131                 r = capability_update_inherited_set(caps, set);
 132                 if (r < 0)
 133                         return -errno;
 134
 135                 if (cap_set_proc(caps) < 0)
 136                         return -errno;
 137         }
 138
 139         for (i = 0; i < cap_last_cap(); i++) {
 140
 141                 if (set & (UINT64_C(1) << i)) {
 142
 143                         /* Add the capability to the ambient set. */
 144                         if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
 145                                 return -errno;
 146                 }
 147         }
 148
 149         return 0;
 150 }
 151
 152 int capability_bounding_set_drop(uint64_t keep, bool right_now) {
 153         _cleanup_cap_free_ cap_t after_cap = NULL;
 154         cap_flag_value_t fv;
 155         unsigned long i;
 156         int r;
 157
 158         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 159          * in the effective set (yes, the kernel drops that when
 160          * executing init!), so get it back temporarily so that we can
 161          * call PR_CAPBSET_DROP. */
 162
 163         after_cap = cap_get_proc();
 164         if (!after_cap)
 165                 return -errno;
 166
 167         if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 168                 return -errno;
 169
 170         if (fv != CAP_SET) {
 171                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 172                 static const cap_value_t v = CAP_SETPCAP;
 173
 174                 temp_cap = cap_dup(after_cap);
 175                 if (!temp_cap) {
 176                         r = -errno;
 177                         goto finish;
 178                 }
 179
 180                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
 181                         r = -errno;
 182                         goto finish;
 183                 }
 184
 185                 if (cap_set_proc(temp_cap) < 0) {
 186                         r = -errno;
 187                         goto finish;
 188                 }
 189         }
 190
 191         for (i = 0; i <= cap_last_cap(); i++) {
 192
 193                 if (!(keep & (UINT64_C(1) << i))) {
 194                         cap_value_t v;
 195
 196                         /* Drop it from the bounding set */
 197                         if (prctl(PR_CAPBSET_DROP, i) < 0) {
 198                                 r = -errno;
 199                                 goto finish;
 200                         }
 201                         v = (cap_value_t) i;
 202
 203                         /* Also drop it from the inheritable set, so
 204                          * that anything we exec() loses the
 205                          * capability for good. */
 206                         if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 207                                 r = -errno;
 208                                 goto finish;
 209                         }
 210
 211                         /* If we shall apply this right now drop it
 212                          * also from our own capability sets. */
 213                         if (right_now) {
 214                                 if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 215                                     cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 216                                         r = -errno;
 217                                         goto finish;
 218                                 }
 219                         }
 220                 }
 221         }
 222
 223         r = 0;
 224
 225 finish:
 226         if (cap_set_proc(after_cap) < 0)
 227                 return -errno;
 228
 229         return r;
 230 }
 231
 232 static int drop_from_file(const char *fn, uint64_t keep) {
 233         int r, k;
 234         uint32_t hi, lo;
 235         uint64_t current, after;
 236         char *p;
 237
 238         r = read_one_line_file(fn, &p);
 239         if (r < 0)
 240                 return r;
 241
 242         assert_cc(sizeof(hi) == sizeof(unsigned));
 243         assert_cc(sizeof(lo) == sizeof(unsigned));
 244
 245         k = sscanf(p, "%u %u", &lo, &hi);
 246         free(p);
 247
 248         if (k != 2)
 249                 return -EIO;
 250
 251         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 252         after = current & keep;
 253
 254         if (current == after)
 255                 return 0;
 256
 257         lo = (unsigned) (after & 0xFFFFFFFFULL);
 258         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 259
 260         if (asprintf(&p, "%u %u", lo, hi) < 0)
 261                 return -ENOMEM;
 262
 263         r = write_string_file(fn, p, WRITE_STRING_FILE_CREATE);
 264         free(p);
 265
 266         return r;
 267 }
 268
 269 int capability_bounding_set_drop_usermode(uint64_t keep) {
 270         int r;
 271
 272         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 273         if (r < 0)
 274                 return r;
 275
 276         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 277         if (r < 0)
 278                 return r;
 279
 280         return r;
 281 }
 282
 283 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 284         _cleanup_cap_free_ cap_t d = NULL;
 285         unsigned i, j = 0;
 286         int r;
 287
 288         /* Unfortunately we cannot leave privilege dropping to PID 1
 289          * here, since we want to run as user but want to keep some
 290          * capabilities. Since file capabilities have been introduced
 291          * this cannot be done across exec() anymore, unless our
 292          * binary has the capability configured in the file system,
 293          * which we want to avoid. */
 294
 295         if (setresgid(gid, gid, gid) < 0)
 296                 return log_error_errno(errno, "Failed to change group ID: %m");
 297
 298         if (setgroups(0, NULL) < 0)
 299                 return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 300
 301         /* Ensure we keep the permitted caps across the setresuid() */
 302         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 303                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 304
 305         r = setresuid(uid, uid, uid);
 306         if (r < 0)
 307                 return log_error_errno(errno, "Failed to change user ID: %m");
 308
 309         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 310                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 311
 312         /* Drop all caps from the bounding set, except the ones we want */
 313         r = capability_bounding_set_drop(keep_capabilities, true);
 314         if (r < 0)
 315                 return log_error_errno(r, "Failed to drop capabilities: %m");
 316
 317         /* Now upgrade the permitted caps we still kept to effective caps */
 318         d = cap_init();
 319         if (!d)
 320                 return log_oom();
 321
 322         if (keep_capabilities) {
 323                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 324
 325                 for (i = 0; i < ELEMENTSOF(bits); i++)
 326                         if (keep_capabilities & (1ULL << i))
 327                                 bits[j++] = i;
 328
 329                 /* use enough bits */
 330                 assert(i == 64 || (keep_capabilities >> i) == 0);
 331                 /* don't use too many bits */
 332                 assert(keep_capabilities & (1ULL << (i - 1)));
 333
 334                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 335                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
 336                         return log_error_errno(errno, "Failed to enable capabilities bits: %m");
 337
 338                 if (cap_set_proc(d) < 0)
 339                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 340         }
 341
 342         return 0;
 343 }
 344
 345 int drop_capability(cap_value_t cv) {
 346         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 347
 348         tmp_cap = cap_get_proc();
 349         if (!tmp_cap)
 350                 return -errno;
 351
 352         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 353             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 354             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 355                 return -errno;
 356
 357         if (cap_set_proc(tmp_cap) < 0)
 358                 return -errno;
 359
 360         return 0;
 361 }