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