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 before_cap = NULL, 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         before_cap = cap_get_proc();
 165         if (!before_cap)
 166                 return -errno;
 167
 168         if (cap_get_flag(before_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(before_cap);
 176                 if (!temp_cap)
 177                         return -errno;
 178
 179                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0)
 180                         return -errno;
 181
 182                 if (cap_set_proc(temp_cap) < 0)
 183                         log_debug_errno(errno, "Can't acquire effective CAP_SETPCAP bit, ignoring: %m");
 184
 185                 /* If we didn't manage to acquire the CAP_SETPCAP bit, we continue anyway, after all this just means
 186                  * we'll fail later, when we actually intend to drop some capabilities. */
 187         }
 188
 189         after_cap = cap_dup(before_cap);
 190         if (!after_cap)
 191                 return -errno;
 192
 193         for (i = 0; i <= cap_last_cap(); i++) {
 194                 cap_value_t v;
 195
 196                 if ((keep & (UINT64_C(1) << i)))
 197                         continue;
 198
 199                 /* Drop it from the bounding set */
 200                 if (prctl(PR_CAPBSET_DROP, i) < 0) {
 201                         r = -errno;
 202
 203                         /* If dropping the capability failed, let's see if we didn't have it in the first place. If so,
 204                          * continue anyway, as dropping a capability we didn't have in the first place doesn't really
 205                          * matter anyway. */
 206                         if (prctl(PR_CAPBSET_READ, i) != 0)
 207                                 goto finish;
 208                 }
 209                 v = (cap_value_t) i;
 210
 211                 /* Also drop it from the inheritable set, so
 212                  * that anything we exec() loses the
 213                  * capability for good. */
 214                 if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 215                         r = -errno;
 216                         goto finish;
 217                 }
 218
 219                 /* If we shall apply this right now drop it
 220                  * also from our own capability sets. */
 221                 if (right_now) {
 222                         if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 223                             cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 224                                 r = -errno;
 225                                 goto finish;
 226                         }
 227                 }
 228         }
 229
 230         r = 0;
 231
 232 finish:
 233         if (cap_set_proc(after_cap) < 0) {
 234                 /* If there are no actual changes anyway then let's ignore this error. */
 235                 if (cap_compare(before_cap, after_cap) != 0)
 236                         r = -errno;
 237         }
 238
 239         return r;
 240 }
 241
 242 static int drop_from_file(const char *fn, uint64_t keep) {
 243         int r, k;
 244         uint32_t hi, lo;
 245         uint64_t current, after;
 246         char *p;
 247
 248         r = read_one_line_file(fn, &p);
 249         if (r < 0)
 250                 return r;
 251
 252         assert_cc(sizeof(hi) == sizeof(unsigned));
 253         assert_cc(sizeof(lo) == sizeof(unsigned));
 254
 255         k = sscanf(p, "%u %u", &lo, &hi);
 256         free(p);
 257
 258         if (k != 2)
 259                 return -EIO;
 260
 261         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 262         after = current & keep;
 263
 264         if (current == after)
 265                 return 0;
 266
 267         lo = (unsigned) (after & 0xFFFFFFFFULL);
 268         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 269
 270         if (asprintf(&p, "%u %u", lo, hi) < 0)
 271                 return -ENOMEM;
 272
 273         r = write_string_file(fn, p, WRITE_STRING_FILE_CREATE);
 274         free(p);
 275
 276         return r;
 277 }
 278
 279 int capability_bounding_set_drop_usermode(uint64_t keep) {
 280         int r;
 281
 282         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 283         if (r < 0)
 284                 return r;
 285
 286         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 287         if (r < 0)
 288                 return r;
 289
 290         return r;
 291 }
 292
 293 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 294         _cleanup_cap_free_ cap_t d = NULL;
 295         unsigned i, j = 0;
 296         int r;
 297
 298         /* Unfortunately we cannot leave privilege dropping to PID 1
 299          * here, since we want to run as user but want to keep some
 300          * capabilities. Since file capabilities have been introduced
 301          * this cannot be done across exec() anymore, unless our
 302          * binary has the capability configured in the file system,
 303          * which we want to avoid. */
 304
 305         if (setresgid(gid, gid, gid) < 0)
 306                 return log_error_errno(errno, "Failed to change group ID: %m");
 307
 308         r = maybe_setgroups(0, NULL);
 309         if (r < 0)
 310                 return log_error_errno(r, "Failed to drop auxiliary groups list: %m");
 311
 312         /* Ensure we keep the permitted caps across the setresuid() */
 313         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 314                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 315
 316         r = setresuid(uid, uid, uid);
 317         if (r < 0)
 318                 return log_error_errno(errno, "Failed to change user ID: %m");
 319
 320         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 321                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 322
 323         /* Drop all caps from the bounding set, except the ones we want */
 324         r = capability_bounding_set_drop(keep_capabilities, true);
 325         if (r < 0)
 326                 return log_error_errno(r, "Failed to drop capabilities: %m");
 327
 328         /* Now upgrade the permitted caps we still kept to effective caps */
 329         d = cap_init();
 330         if (!d)
 331                 return log_oom();
 332
 333         if (keep_capabilities) {
 334                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 335
 336                 for (i = 0; i < ELEMENTSOF(bits); i++)
 337                         if (keep_capabilities & (1ULL << i))
 338                                 bits[j++] = i;
 339
 340                 /* use enough bits */
 341                 assert(i == 64 || (keep_capabilities >> i) == 0);
 342                 /* don't use too many bits */
 343                 assert(keep_capabilities & (1ULL << (i - 1)));
 344
 345                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 346                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
 347                         return log_error_errno(errno, "Failed to enable capabilities bits: %m");
 348
 349                 if (cap_set_proc(d) < 0)
 350                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 351         }
 352
 353         return 0;
 354 }
 355
 356 int drop_capability(cap_value_t cv) {
 357         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 358
 359         tmp_cap = cap_get_proc();
 360         if (!tmp_cap)
 361                 return -errno;
 362
 363         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 364             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 365             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 366                 return -errno;
 367
 368         if (cap_set_proc(tmp_cap) < 0)
 369                 return -errno;
 370
 371         return 0;
 372 }
 373
 374 bool ambient_capabilities_supported(void) {
 375         static int cache = -1;
 376
 377         if (cache >= 0)
 378                 return cache;
 379
 380         /* If PR_CAP_AMBIENT returns something valid, or an unexpected error code we assume that ambient caps are
 381          * available. */
 382
 383         cache = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_KILL, 0, 0) >= 0 ||
 384                 !IN_SET(errno, EINVAL, EOPNOTSUPP, ENOSYS);
 385
 386         return cache;
 387 }