src/basic/capability-util.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 <errno.h>
  23 #include <grp.h>
  24 #include <stdio.h>
  25 #include <stdlib.h>
  26 #include <sys/capability.h>
  27 #include <sys/prctl.h>
  28 #include <unistd.h>
  29
  30 #include "alloc-util.h"
  31 #include "capability-util.h"
  32 #include "fileio.h"
  33 #include "log.h"
  34 #include "macro.h"
  35 #include "parse-util.h"
  36 #include "util.h"
  37
  38 int have_effective_cap(int value) {
  39         _cleanup_cap_free_ cap_t cap;
  40         cap_flag_value_t fv;
  41
  42         cap = cap_get_proc();
  43         if (!cap)
  44                 return -errno;
  45
  46         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  47                 return -errno;
  48         else
  49                 return fv == CAP_SET;
  50 }
  51
  52 unsigned long cap_last_cap(void) {
  53         static thread_local unsigned long saved;
  54         static thread_local bool valid = false;
  55         _cleanup_free_ char *content = NULL;
  56         unsigned long p = 0;
  57         int r;
  58
  59         if (valid)
  60                 return saved;
  61
  62         /* available since linux-3.2 */
  63         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  64         if (r >= 0) {
  65                 r = safe_atolu(content, &p);
  66                 if (r >= 0) {
  67                         saved = p;
  68                         valid = true;
  69                         return p;
  70                 }
  71         }
  72
  73         /* fall back to syscall-probing for pre linux-3.2 */
  74         p = (unsigned long) CAP_LAST_CAP;
  75
  76         if (prctl(PR_CAPBSET_READ, p) < 0) {
  77
  78                 /* Hmm, look downwards, until we find one that
  79                  * works */
  80                 for (p--; p > 0; p --)
  81                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  82                                 break;
  83
  84         } else {
  85
  86                 /* Hmm, look upwards, until we find one that doesn't
  87                  * work */
  88                 for (;; p++)
  89                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  90                                 break;
  91         }
  92
  93         saved = p;
  94         valid = true;
  95
  96         return p;
  97 }
  98
  99 int capability_update_inherited_set(cap_t caps, uint64_t set) {
 100         unsigned long i;
 101
 102         /* Add capabilities in the set to the inherited caps. Do not apply
 103          * them yet. */
 104
 105         for (i = 0; i < cap_last_cap(); i++) {
 106
 107                 if (set & (UINT64_C(1) << i)) {
 108                         cap_value_t v;
 109
 110                         v = (cap_value_t) i;
 111
 112                         /* Make the capability inheritable. */
 113                         if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, CAP_SET) < 0)
 114                                 return -errno;
 115                 }
 116         }
 117
 118         return 0;
 119 }
 120
 121 int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
 122         unsigned long i;
 123         _cleanup_cap_free_ cap_t caps = NULL;
 124
 125         /* Add the capabilities to the ambient set. */
 126
 127         if (also_inherit) {
 128                 int r;
 129                 caps = cap_get_proc();
 130                 if (!caps)
 131                         return -errno;
 132
 133                 r = capability_update_inherited_set(caps, set);
 134                 if (r < 0)
 135                         return -errno;
 136
 137                 if (cap_set_proc(caps) < 0)
 138                         return -errno;
 139         }
 140
 141         for (i = 0; i < cap_last_cap(); i++) {
 142
 143                 if (set & (UINT64_C(1) << i)) {
 144
 145                         /* Add the capability to the ambient set. */
 146                         if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
 147                                 return -errno;
 148                 }
 149         }
 150
 151         return 0;
 152 }
 153
 154 int capability_bounding_set_drop(uint64_t keep, bool right_now) {
 155         _cleanup_cap_free_ cap_t after_cap = NULL;
 156         cap_flag_value_t fv;
 157         unsigned long i;
 158         int r;
 159
 160         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 161          * in the effective set (yes, the kernel drops that when
 162          * executing init!), so get it back temporarily so that we can
 163          * call PR_CAPBSET_DROP. */
 164
 165         after_cap = cap_get_proc();
 166         if (!after_cap)
 167                 return -errno;
 168
 169         if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 170                 return -errno;
 171
 172         if (fv != CAP_SET) {
 173                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 174                 static const cap_value_t v = CAP_SETPCAP;
 175
 176                 temp_cap = cap_dup(after_cap);
 177                 if (!temp_cap) {
 178                         r = -errno;
 179                         goto finish;
 180                 }
 181
 182                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
 183                         r = -errno;
 184                         goto finish;
 185                 }
 186
 187                 if (cap_set_proc(temp_cap) < 0) {
 188                         r = -errno;
 189                         goto finish;
 190                 }
 191         }
 192
 193         for (i = 0; i <= cap_last_cap(); i++) {
 194
 195                 if (!(keep & (UINT64_C(1) << i))) {
 196                         cap_value_t v;
 197
 198                         /* Drop it from the bounding set */
 199                         if (prctl(PR_CAPBSET_DROP, i) < 0) {
 200                                 r = -errno;
 201                                 goto finish;
 202                         }
 203                         v = (cap_value_t) i;
 204
 205                         /* Also drop it from the inheritable set, so
 206                          * that anything we exec() loses the
 207                          * capability for good. */
 208                         if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 209                                 r = -errno;
 210                                 goto finish;
 211                         }
 212
 213                         /* If we shall apply this right now drop it
 214                          * also from our own capability sets. */
 215                         if (right_now) {
 216                                 if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 217                                     cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 218                                         r = -errno;
 219                                         goto finish;
 220                                 }
 221                         }
 222                 }
 223         }
 224
 225         r = 0;
 226
 227 finish:
 228         if (cap_set_proc(after_cap) < 0)
 229                 return -errno;
 230
 231         return r;
 232 }
 233
 234 static int drop_from_file(const char *fn, uint64_t keep) {
 235         int r, k;
 236         uint32_t hi, lo;
 237         uint64_t current, after;
 238         char *p;
 239
 240         r = read_one_line_file(fn, &p);
 241         if (r < 0)
 242                 return r;
 243
 244         assert_cc(sizeof(hi) == sizeof(unsigned));
 245         assert_cc(sizeof(lo) == sizeof(unsigned));
 246
 247         k = sscanf(p, "%u %u", &lo, &hi);
 248         free(p);
 249
 250         if (k != 2)
 251                 return -EIO;
 252
 253         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 254         after = current & keep;
 255
 256         if (current == after)
 257                 return 0;
 258
 259         lo = (unsigned) (after & 0xFFFFFFFFULL);
 260         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 261
 262         if (asprintf(&p, "%u %u", lo, hi) < 0)
 263                 return -ENOMEM;
 264
 265         r = write_string_file(fn, p, WRITE_STRING_FILE_CREATE);
 266         free(p);
 267
 268         return r;
 269 }
 270
 271 int capability_bounding_set_drop_usermode(uint64_t keep) {
 272         int r;
 273
 274         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 275         if (r < 0)
 276                 return r;
 277
 278         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 279         if (r < 0)
 280                 return r;
 281
 282         return r;
 283 }
 284
 285 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 286         _cleanup_cap_free_ cap_t d = NULL;
 287         unsigned i, j = 0;
 288         int r;
 289
 290         /* Unfortunately we cannot leave privilege dropping to PID 1
 291          * here, since we want to run as user but want to keep some
 292          * capabilities. Since file capabilities have been introduced
 293          * this cannot be done across exec() anymore, unless our
 294          * binary has the capability configured in the file system,
 295          * which we want to avoid. */
 296
 297         if (setresgid(gid, gid, gid) < 0)
 298                 return log_error_errno(errno, "Failed to change group ID: %m");
 299
 300         if (setgroups(0, NULL) < 0)
 301                 return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 302
 303         /* Ensure we keep the permitted caps across the setresuid() */
 304         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 305                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 306
 307         r = setresuid(uid, uid, uid);
 308         if (r < 0)
 309                 return log_error_errno(errno, "Failed to change user ID: %m");
 310
 311         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 312                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 313
 314         /* Drop all caps from the bounding set, except the ones we want */
 315         r = capability_bounding_set_drop(keep_capabilities, true);
 316         if (r < 0)
 317                 return log_error_errno(r, "Failed to drop capabilities: %m");
 318
 319         /* Now upgrade the permitted caps we still kept to effective caps */
 320         d = cap_init();
 321         if (!d)
 322                 return log_oom();
 323
 324         if (keep_capabilities) {
 325                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 326
 327                 for (i = 0; i < ELEMENTSOF(bits); i++)
 328                         if (keep_capabilities & (1ULL << i))
 329                                 bits[j++] = i;
 330
 331                 /* use enough bits */
 332                 assert(i == 64 || (keep_capabilities >> i) == 0);
 333                 /* don't use too many bits */
 334                 assert(keep_capabilities & (1ULL << (i - 1)));
 335
 336                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 337                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
 338                         return log_error_errno(errno, "Failed to enable capabilities bits: %m");
 339
 340                 if (cap_set_proc(d) < 0)
 341                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 342         }
 343
 344         return 0;
 345 }
 346
 347 int drop_capability(cap_value_t cv) {
 348         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 349
 350         tmp_cap = cap_get_proc();
 351         if (!tmp_cap)
 352                 return -errno;
 353
 354         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 355             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 356             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 357                 return -errno;
 358
 359         if (cap_set_proc(tmp_cap) < 0)
 360                 return -errno;
 361
 362         return 0;
 363 }