src/basic/capability-util.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <errno.h>
   4 #include <stdio.h>
   5 #include <stdlib.h>
   6 #include <sys/prctl.h>
   7 #include <unistd.h>
   8
   9 #include "alloc-util.h"
  10 #include "capability-util.h"
  11 #include "cap-list.h"
  12 #include "fileio.h"
  13 #include "log.h"
  14 #include "macro.h"
  15 #include "missing_prctl.h"
  16 #include "parse-util.h"
  17 #include "user-util.h"
  18 #include "util.h"
  19
  20 int have_effective_cap(int value) {
  21         _cleanup_cap_free_ cap_t cap;
  22         cap_flag_value_t fv;
  23
  24         cap = cap_get_proc();
  25         if (!cap)
  26                 return -errno;
  27
  28         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  29                 return -errno;
  30
  31         return fv == CAP_SET;
  32 }
  33
  34 unsigned long cap_last_cap(void) {
  35         static thread_local unsigned long saved;
  36         static thread_local bool valid = false;
  37         _cleanup_free_ char *content = NULL;
  38         unsigned long p = 0;
  39         int r;
  40
  41         if (valid)
  42                 return saved;
  43
  44         /* available since linux-3.2 */
  45         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  46         if (r >= 0) {
  47                 r = safe_atolu(content, &p);
  48                 if (r >= 0) {
  49
  50                         if (p > 63) /* Safety for the future: if one day the kernel learns more than 64 caps,
  51                                      * then we are in trouble (since we, as much userspace and kernel space
  52                                      * store capability masks in uint64_t types). Let's hence protect
  53                                      * ourselves against that and always cap at 63 for now. */
  54                                 p = 63;
  55
  56                         saved = p;
  57                         valid = true;
  58                         return p;
  59                 }
  60         }
  61
  62         /* fall back to syscall-probing for pre linux-3.2 */
  63         p = MIN((unsigned long) CAP_LAST_CAP, 63U);
  64
  65         if (prctl(PR_CAPBSET_READ, p) < 0) {
  66
  67                 /* Hmm, look downwards, until we find one that 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 work */
  75                 for (; p < 63; p++)
  76                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  77                                 break;
  78         }
  79
  80         saved = p;
  81         valid = true;
  82
  83         return p;
  84 }
  85
  86 int capability_update_inherited_set(cap_t caps, uint64_t set) {
  87         unsigned long i;
  88
  89         /* Add capabilities in the set to the inherited caps, drops capabilities not in the set.
  90          * Do not apply them yet. */
  91
  92         for (i = 0; i <= cap_last_cap(); i++) {
  93                 cap_flag_value_t flag = set & (UINT64_C(1) << i) ? CAP_SET : CAP_CLEAR;
  94                 cap_value_t v;
  95
  96                 v = (cap_value_t) i;
  97
  98                 if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, flag) < 0)
  99                         return -errno;
 100         }
 101
 102         return 0;
 103 }
 104
 105 int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
 106         _cleanup_cap_free_ cap_t caps = NULL;
 107         unsigned long i;
 108         int r;
 109
 110         /* Check that we can use PR_CAP_AMBIENT or quit early. */
 111         if (!ambient_capabilities_supported())
 112                 return 0;
 113
 114         /* Add the capabilities to the ambient set. */
 115
 116         if (also_inherit) {
 117                 caps = cap_get_proc();
 118                 if (!caps)
 119                         return -errno;
 120
 121                 r = capability_update_inherited_set(caps, set);
 122                 if (r < 0)
 123                         return -errno;
 124
 125                 if (cap_set_proc(caps) < 0)
 126                         return -errno;
 127         }
 128
 129         for (i = 0; i <= cap_last_cap(); i++) {
 130
 131                 if (set & (UINT64_C(1) << i)) {
 132
 133                         /* Add the capability to the ambient set. */
 134                         if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
 135                                 return -errno;
 136                 } else {
 137
 138                         /* Drop the capability so we don't inherit capabilities we didn't ask for. */
 139                         r = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, i, 0, 0);
 140                         if (r < 0)
 141                                 return -errno;
 142
 143                         if (r)
 144                                 if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_LOWER, i, 0, 0) < 0)
 145                                         return -errno;
 146
 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         _cleanup_free_ char *p = NULL;
 244         uint64_t current, after;
 245         uint32_t hi, lo;
 246         int r, k;
 247
 248         r = read_one_line_file(fn, &p);
 249         if (r < 0)
 250                 return r;
 251
 252         k = sscanf(p, "%" PRIu32 " %" PRIu32, &lo, &hi);
 253         if (k != 2)
 254                 return -EIO;
 255
 256         current = (uint64_t) lo | ((uint64_t) hi << 32);
 257         after = current & keep;
 258
 259         if (current == after)
 260                 return 0;
 261
 262         lo = after & UINT32_C(0xFFFFFFFF);
 263         hi = (after >> 32) & UINT32_C(0xFFFFFFFF);
 264
 265         return write_string_filef(fn, 0, "%" PRIu32 " %" PRIu32, lo, hi);
 266 }
 267
 268 int capability_bounding_set_drop_usermode(uint64_t keep) {
 269         int r;
 270
 271         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 272         if (r < 0)
 273                 return r;
 274
 275         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 276         if (r < 0)
 277                 return r;
 278
 279         return r;
 280 }
 281
 282 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 283         int r;
 284
 285         /* Unfortunately we cannot leave privilege dropping to PID 1 here, since we want to run as user but
 286          * want to keep some capabilities. Since file capabilities have been introduced this cannot be done
 287          * across exec() anymore, unless our binary has the capability configured in the file system, which
 288          * we want to avoid. */
 289
 290         if (setresgid(gid, gid, gid) < 0)
 291                 return log_error_errno(errno, "Failed to change group ID: %m");
 292
 293         r = maybe_setgroups(0, NULL);
 294         if (r < 0)
 295                 return log_error_errno(r, "Failed to drop auxiliary groups list: %m");
 296
 297         /* Ensure we keep the permitted caps across the setresuid(). Note that we do this even if we actually
 298          * don't want to keep any capabilities, since we want to be able to drop them from the bounding set
 299          * too, and we can only do that if we have capabilities. */
 300         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 301                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 302
 303         if (setresuid(uid, uid, uid) < 0)
 304                 return log_error_errno(errno, "Failed to change user ID: %m");
 305
 306         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 307                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 308
 309         /* Drop all caps from the bounding set (as well as the inheritable/permitted/effective sets), except
 310          * the ones we want to keep */
 311         r = capability_bounding_set_drop(keep_capabilities, true);
 312         if (r < 0)
 313                 return log_error_errno(r, "Failed to drop capabilities: %m");
 314
 315         /* Now upgrade the permitted caps we still kept to effective caps */
 316         if (keep_capabilities != 0) {
 317                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 318                 _cleanup_cap_free_ cap_t d = NULL;
 319                 unsigned i, j = 0;
 320
 321                 d = cap_init();
 322                 if (!d)
 323                         return log_oom();
 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 & (UINT64_C(1) << (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 }
 362
 363 bool ambient_capabilities_supported(void) {
 364         static int cache = -1;
 365
 366         if (cache >= 0)
 367                 return cache;
 368
 369         /* If PR_CAP_AMBIENT returns something valid, or an unexpected error code we assume that ambient caps are
 370          * available. */
 371
 372         cache = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_KILL, 0, 0) >= 0 ||
 373                 !IN_SET(errno, EINVAL, EOPNOTSUPP, ENOSYS);
 374
 375         return cache;
 376 }
 377
 378 bool capability_quintet_mangle(CapabilityQuintet *q) {
 379         unsigned long i;
 380         uint64_t combined, drop = 0;
 381         bool ambient_supported;
 382
 383         assert(q);
 384
 385         combined = q->effective | q->bounding | q->inheritable | q->permitted;
 386
 387         ambient_supported = q->ambient != (uint64_t) -1;
 388         if (ambient_supported)
 389                 combined |= q->ambient;
 390
 391         for (i = 0; i <= cap_last_cap(); i++) {
 392                 unsigned long bit = UINT64_C(1) << i;
 393                 if (!FLAGS_SET(combined, bit))
 394                         continue;
 395
 396                 if (prctl(PR_CAPBSET_READ, i) > 0)
 397                         continue;
 398
 399                 drop |= bit;
 400
 401                 log_debug("Not in the current bounding set: %s", capability_to_name(i));
 402         }
 403
 404         q->effective &= ~drop;
 405         q->bounding &= ~drop;
 406         q->inheritable &= ~drop;
 407         q->permitted &= ~drop;
 408
 409         if (ambient_supported)
 410                 q->ambient &= ~drop;
 411
 412         return drop != 0; /* Let the caller know we changed something */
 413 }
 414
 415 int capability_quintet_enforce(const CapabilityQuintet *q) {
 416         _cleanup_cap_free_ cap_t c = NULL, modified = NULL;
 417         int r;
 418
 419         if (q->ambient != (uint64_t) -1) {
 420                 unsigned long i;
 421                 bool changed = false;
 422
 423                 c = cap_get_proc();
 424                 if (!c)
 425                         return -errno;
 426
 427                 /* In order to raise the ambient caps set we first need to raise the matching inheritable + permitted
 428                  * cap */
 429                 for (i = 0; i <= cap_last_cap(); i++) {
 430                         uint64_t m = UINT64_C(1) << i;
 431                         cap_value_t cv = (cap_value_t) i;
 432                         cap_flag_value_t old_value_inheritable, old_value_permitted;
 433
 434                         if ((q->ambient & m) == 0)
 435                                 continue;
 436
 437                         if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value_inheritable) < 0)
 438                                 return -errno;
 439                         if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value_permitted) < 0)
 440                                 return -errno;
 441
 442                         if (old_value_inheritable == CAP_SET && old_value_permitted == CAP_SET)
 443                                 continue;
 444
 445                         if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, CAP_SET) < 0)
 446                                 return -errno;
 447                         if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, CAP_SET) < 0)
 448                                 return -errno;
 449
 450                         changed = true;
 451                 }
 452
 453                 if (changed)
 454                         if (cap_set_proc(c) < 0)
 455                                 return -errno;
 456
 457                 r = capability_ambient_set_apply(q->ambient, false);
 458                 if (r < 0)
 459                         return r;
 460         }
 461
 462         if (q->inheritable != (uint64_t) -1 || q->permitted != (uint64_t) -1 || q->effective != (uint64_t) -1) {
 463                 bool changed = false;
 464                 unsigned long i;
 465
 466                 if (!c) {
 467                         c = cap_get_proc();
 468                         if (!c)
 469                                 return -errno;
 470                 }
 471
 472                 for (i = 0; i <= cap_last_cap(); i++) {
 473                         uint64_t m = UINT64_C(1) << i;
 474                         cap_value_t cv = (cap_value_t) i;
 475
 476                         if (q->inheritable != (uint64_t) -1) {
 477                                 cap_flag_value_t old_value, new_value;
 478
 479                                 if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value) < 0) {
 480                                         if (errno == EINVAL) /* If the kernel knows more caps than this
 481                                                               * version of libcap, then this will return
 482                                                               * EINVAL. In that case, simply ignore it,
 483                                                               * pretend it doesn't exist. */
 484                                                 continue;
 485
 486                                         return -errno;
 487                                 }
 488
 489                                 new_value = (q->inheritable & m) ? CAP_SET : CAP_CLEAR;
 490
 491                                 if (old_value != new_value) {
 492                                         changed = true;
 493
 494                                         if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, new_value) < 0)
 495                                                 return -errno;
 496                                 }
 497                         }
 498
 499                         if (q->permitted != (uint64_t) -1) {
 500                                 cap_flag_value_t old_value, new_value;
 501
 502                                 if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value) < 0) {
 503                                         if (errno == EINVAL)
 504                                                 continue;
 505
 506                                         return -errno;
 507                                 }
 508
 509                                 new_value = (q->permitted & m) ? CAP_SET : CAP_CLEAR;
 510
 511                                 if (old_value != new_value) {
 512                                         changed = true;
 513
 514                                         if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, new_value) < 0)
 515                                                 return -errno;
 516                                 }
 517                         }
 518
 519                         if (q->effective != (uint64_t) -1) {
 520                                 cap_flag_value_t old_value, new_value;
 521
 522                                 if (cap_get_flag(c, cv, CAP_EFFECTIVE, &old_value) < 0) {
 523                                         if (errno == EINVAL)
 524                                                 continue;
 525
 526                                         return -errno;
 527                                 }
 528
 529                                 new_value = (q->effective & m) ? CAP_SET : CAP_CLEAR;
 530
 531                                 if (old_value != new_value) {
 532                                         changed = true;
 533
 534                                         if (cap_set_flag(c, CAP_EFFECTIVE, 1, &cv, new_value) < 0)
 535                                                 return -errno;
 536                                 }
 537                         }
 538                 }
 539
 540                 if (changed) {
 541                         /* In order to change the bounding caps, we need to keep CAP_SETPCAP for a bit
 542                          * longer. Let's add it to our list hence for now. */
 543                         if (q->bounding != (uint64_t) -1) {
 544                                 cap_value_t cv = CAP_SETPCAP;
 545
 546                                 modified = cap_dup(c);
 547                                 if (!modified)
 548                                         return -ENOMEM;
 549
 550                                 if (cap_set_flag(modified, CAP_PERMITTED, 1, &cv, CAP_SET) < 0)
 551                                         return -errno;
 552                                 if (cap_set_flag(modified, CAP_EFFECTIVE, 1, &cv, CAP_SET) < 0)
 553                                         return -errno;
 554
 555                                 if (cap_compare(modified, c) == 0) {
 556                                         /* No change? then drop this nonsense again */
 557                                         cap_free(modified);
 558                                         modified = NULL;
 559                                 }
 560                         }
 561
 562                         /* Now, let's enforce the caps for the first time. Note that this is where we acquire
 563                          * caps in any of the sets we currently don't have. We have to do this before
 564                          * dropping the bounding caps below, since at that point we can never acquire new
 565                          * caps in inherited/permitted/effective anymore, but only lose them. */
 566                         if (cap_set_proc(modified ?: c) < 0)
 567                                 return -errno;
 568                 }
 569         }
 570
 571         if (q->bounding != (uint64_t) -1) {
 572                 r = capability_bounding_set_drop(q->bounding, false);
 573                 if (r < 0)
 574                         return r;
 575         }
 576
 577         /* If needed, let's now set the caps again, this time in the final version, which differs from what
 578          * we have already set only in the CAP_SETPCAP bit, which we needed for dropping the bounding
 579          * bits. This call only undoes bits and doesn't acquire any which means the bounding caps don't
 580          * matter. */
 581         if (modified)
 582                 if (cap_set_proc(c) < 0)
 583                         return -errno;
 584
 585         return 0;
 586 }