src/basic/capability-util.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2
   3 #include <errno.h>
   4 #include <grp.h>
   5 #include <stdio.h>
   6 #include <stdlib.h>
   7 #include <sys/capability.h>
   8 #include <sys/prctl.h>
   9 #include <unistd.h>
  10
  11 #include "alloc-util.h"
  12 #include "capability-util.h"
  13 #include "fileio.h"
  14 #include "log.h"
  15 #include "macro.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         else
  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                         saved = p;
  50                         valid = true;
  51                         return p;
  52                 }
  53         }
  54
  55         /* fall back to syscall-probing for pre linux-3.2 */
  56         p = (unsigned long) CAP_LAST_CAP;
  57
  58         if (prctl(PR_CAPBSET_READ, p) < 0) {
  59
  60                 /* Hmm, look downwards, until we find one that
  61                  * works */
  62                 for (p--; p > 0; p --)
  63                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  64                                 break;
  65
  66         } else {
  67
  68                 /* Hmm, look upwards, until we find one that doesn't
  69                  * work */
  70                 for (;; p++)
  71                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  72                                 break;
  73         }
  74
  75         saved = p;
  76         valid = true;
  77
  78         return p;
  79 }
  80
  81 int capability_update_inherited_set(cap_t caps, uint64_t set) {
  82         unsigned long i;
  83
  84         /* Add capabilities in the set to the inherited caps. Do not apply
  85          * them yet. */
  86
  87         for (i = 0; i < cap_last_cap(); i++) {
  88
  89                 if (set & (UINT64_C(1) << i)) {
  90                         cap_value_t v;
  91
  92                         v = (cap_value_t) i;
  93
  94                         /* Make the capability inheritable. */
  95                         if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, CAP_SET) < 0)
  96                                 return -errno;
  97                 }
  98         }
  99
 100         return 0;
 101 }
 102
 103 int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
 104         unsigned long i;
 105         _cleanup_cap_free_ cap_t caps = NULL;
 106
 107         /* Add the capabilities to the ambient set. */
 108
 109         if (also_inherit) {
 110                 int r;
 111                 caps = cap_get_proc();
 112                 if (!caps)
 113                         return -errno;
 114
 115                 r = capability_update_inherited_set(caps, set);
 116                 if (r < 0)
 117                         return -errno;
 118
 119                 if (cap_set_proc(caps) < 0)
 120                         return -errno;
 121         }
 122
 123         for (i = 0; i < cap_last_cap(); i++) {
 124
 125                 if (set & (UINT64_C(1) << i)) {
 126
 127                         /* Add the capability to the ambient set. */
 128                         if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
 129                                 return -errno;
 130                 }
 131         }
 132
 133         return 0;
 134 }
 135
 136 int capability_bounding_set_drop(uint64_t keep, bool right_now) {
 137         _cleanup_cap_free_ cap_t before_cap = NULL, after_cap = NULL;
 138         cap_flag_value_t fv;
 139         unsigned long i;
 140         int r;
 141
 142         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 143          * in the effective set (yes, the kernel drops that when
 144          * executing init!), so get it back temporarily so that we can
 145          * call PR_CAPBSET_DROP. */
 146
 147         before_cap = cap_get_proc();
 148         if (!before_cap)
 149                 return -errno;
 150
 151         if (cap_get_flag(before_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 152                 return -errno;
 153
 154         if (fv != CAP_SET) {
 155                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 156                 static const cap_value_t v = CAP_SETPCAP;
 157
 158                 temp_cap = cap_dup(before_cap);
 159                 if (!temp_cap)
 160                         return -errno;
 161
 162                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0)
 163                         return -errno;
 164
 165                 if (cap_set_proc(temp_cap) < 0)
 166                         log_debug_errno(errno, "Can't acquire effective CAP_SETPCAP bit, ignoring: %m");
 167
 168                 /* If we didn't manage to acquire the CAP_SETPCAP bit, we continue anyway, after all this just means
 169                  * we'll fail later, when we actually intend to drop some capabilities. */
 170         }
 171
 172         after_cap = cap_dup(before_cap);
 173         if (!after_cap)
 174                 return -errno;
 175
 176         for (i = 0; i <= cap_last_cap(); i++) {
 177                 cap_value_t v;
 178
 179                 if ((keep & (UINT64_C(1) << i)))
 180                         continue;
 181
 182                 /* Drop it from the bounding set */
 183                 if (prctl(PR_CAPBSET_DROP, i) < 0) {
 184                         r = -errno;
 185
 186                         /* If dropping the capability failed, let's see if we didn't have it in the first place. If so,
 187                          * continue anyway, as dropping a capability we didn't have in the first place doesn't really
 188                          * matter anyway. */
 189                         if (prctl(PR_CAPBSET_READ, i) != 0)
 190                                 goto finish;
 191                 }
 192                 v = (cap_value_t) i;
 193
 194                 /* Also drop it from the inheritable set, so
 195                  * that anything we exec() loses the
 196                  * capability for good. */
 197                 if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 198                         r = -errno;
 199                         goto finish;
 200                 }
 201
 202                 /* If we shall apply this right now drop it
 203                  * also from our own capability sets. */
 204                 if (right_now) {
 205                         if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 206                             cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 207                                 r = -errno;
 208                                 goto finish;
 209                         }
 210                 }
 211         }
 212
 213         r = 0;
 214
 215 finish:
 216         if (cap_set_proc(after_cap) < 0) {
 217                 /* If there are no actual changes anyway then let's ignore this error. */
 218                 if (cap_compare(before_cap, after_cap) != 0)
 219                         r = -errno;
 220         }
 221
 222         return r;
 223 }
 224
 225 static int drop_from_file(const char *fn, uint64_t keep) {
 226         _cleanup_free_ char *p = NULL;
 227         uint64_t current, after;
 228         uint32_t hi, lo;
 229         int r, k;
 230
 231         r = read_one_line_file(fn, &p);
 232         if (r < 0)
 233                 return r;
 234
 235         assert_cc(sizeof(hi) == sizeof(unsigned));
 236         assert_cc(sizeof(lo) == sizeof(unsigned));
 237
 238         k = sscanf(p, "%u %u", &lo, &hi);
 239         if (k != 2)
 240                 return -EIO;
 241
 242         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 243         after = current & keep;
 244
 245         if (current == after)
 246                 return 0;
 247
 248         lo = (unsigned) (after & 0xFFFFFFFFULL);
 249         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 250
 251         return write_string_filef(fn, WRITE_STRING_FILE_CREATE, "%u %u", lo, hi);
 252 }
 253
 254 int capability_bounding_set_drop_usermode(uint64_t keep) {
 255         int r;
 256
 257         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 258         if (r < 0)
 259                 return r;
 260
 261         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 262         if (r < 0)
 263                 return r;
 264
 265         return r;
 266 }
 267
 268 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 269         _cleanup_cap_free_ cap_t d = NULL;
 270         unsigned i, j = 0;
 271         int r;
 272
 273         /* Unfortunately we cannot leave privilege dropping to PID 1
 274          * here, since we want to run as user but want to keep some
 275          * capabilities. Since file capabilities have been introduced
 276          * this cannot be done across exec() anymore, unless our
 277          * binary has the capability configured in the file system,
 278          * which we want to avoid. */
 279
 280         if (setresgid(gid, gid, gid) < 0)
 281                 return log_error_errno(errno, "Failed to change group ID: %m");
 282
 283         r = maybe_setgroups(0, NULL);
 284         if (r < 0)
 285                 return log_error_errno(r, "Failed to drop auxiliary groups list: %m");
 286
 287         /* Ensure we keep the permitted caps across the setresuid() */
 288         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 289                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 290
 291         if (setresuid(uid, uid, uid) < 0)
 292                 return log_error_errno(errno, "Failed to change user ID: %m");
 293
 294         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 295                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 296
 297         /* Drop all caps from the bounding set, except the ones we want */
 298         r = capability_bounding_set_drop(keep_capabilities, true);
 299         if (r < 0)
 300                 return log_error_errno(r, "Failed to drop capabilities: %m");
 301
 302         /* Now upgrade the permitted caps we still kept to effective caps */
 303         d = cap_init();
 304         if (!d)
 305                 return log_oom();
 306
 307         if (keep_capabilities) {
 308                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 309
 310                 for (i = 0; i < ELEMENTSOF(bits); i++)
 311                         if (keep_capabilities & (1ULL << i))
 312                                 bits[j++] = i;
 313
 314                 /* use enough bits */
 315                 assert(i == 64 || (keep_capabilities >> i) == 0);
 316                 /* don't use too many bits */
 317                 assert(keep_capabilities & (1ULL << (i - 1)));
 318
 319                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 320                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
 321                         return log_error_errno(errno, "Failed to enable capabilities bits: %m");
 322
 323                 if (cap_set_proc(d) < 0)
 324                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 325         }
 326
 327         return 0;
 328 }
 329
 330 int drop_capability(cap_value_t cv) {
 331         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 332
 333         tmp_cap = cap_get_proc();
 334         if (!tmp_cap)
 335                 return -errno;
 336
 337         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 338             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 339             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 340                 return -errno;
 341
 342         if (cap_set_proc(tmp_cap) < 0)
 343                 return -errno;
 344
 345         return 0;
 346 }
 347
 348 bool ambient_capabilities_supported(void) {
 349         static int cache = -1;
 350
 351         if (cache >= 0)
 352                 return cache;
 353
 354         /* If PR_CAP_AMBIENT returns something valid, or an unexpected error code we assume that ambient caps are
 355          * available. */
 356
 357         cache = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_KILL, 0, 0) >= 0 ||
 358                 !IN_SET(errno, EINVAL, EOPNOTSUPP, ENOSYS);
 359
 360         return cache;
 361 }
 362
 363 int capability_quintet_enforce(const CapabilityQuintet *q) {
 364         _cleanup_cap_free_ cap_t c = NULL;
 365         int r;
 366
 367         if (q->ambient != (uint64_t) -1) {
 368                 unsigned long i;
 369                 bool changed = false;
 370
 371                 c = cap_get_proc();
 372                 if (!c)
 373                         return -errno;
 374
 375                 /* In order to raise the ambient caps set we first need to raise the matching inheritable + permitted
 376                  * cap */
 377                 for (i = 0; i <= cap_last_cap(); i++) {
 378                         uint64_t m = UINT64_C(1) << i;
 379                         cap_value_t cv = (cap_value_t) i;
 380                         cap_flag_value_t old_value_inheritable, old_value_permitted;
 381
 382                         if ((q->ambient & m) == 0)
 383                                 continue;
 384
 385                         if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value_inheritable) < 0)
 386                                 return -errno;
 387                         if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value_permitted) < 0)
 388                                 return -errno;
 389
 390                         if (old_value_inheritable == CAP_SET && old_value_permitted == CAP_SET)
 391                                 continue;
 392
 393                         if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, CAP_SET) < 0)
 394                                 return -errno;
 395
 396                         if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, CAP_SET) < 0)
 397                                 return -errno;
 398
 399                         changed = true;
 400                 }
 401
 402                 if (changed)
 403                         if (cap_set_proc(c) < 0)
 404                                 return -errno;
 405
 406                 r = capability_ambient_set_apply(q->ambient, false);
 407                 if (r < 0)
 408                         return r;
 409         }
 410
 411         if (q->inheritable != (uint64_t) -1 || q->permitted != (uint64_t) -1 || q->effective != (uint64_t) -1) {
 412                 bool changed = false;
 413                 unsigned long i;
 414
 415                 if (!c) {
 416                         c = cap_get_proc();
 417                         if (!c)
 418                                 return -errno;
 419                 }
 420
 421                 for (i = 0; i <= cap_last_cap(); i++) {
 422                         uint64_t m = UINT64_C(1) << i;
 423                         cap_value_t cv = (cap_value_t) i;
 424
 425                         if (q->inheritable != (uint64_t) -1) {
 426                                 cap_flag_value_t old_value, new_value;
 427
 428                                 if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value) < 0)
 429                                         return -errno;
 430
 431                                 new_value = (q->inheritable & m) ? CAP_SET : CAP_CLEAR;
 432
 433                                 if (old_value != new_value) {
 434                                         changed = true;
 435
 436                                         if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, new_value) < 0)
 437                                                 return -errno;
 438                                 }
 439                         }
 440
 441                         if (q->permitted != (uint64_t) -1) {
 442                                 cap_flag_value_t old_value, new_value;
 443
 444                                 if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value) < 0)
 445                                         return -errno;
 446
 447                                 new_value = (q->permitted & m) ? CAP_SET : CAP_CLEAR;
 448
 449                                 if (old_value != new_value) {
 450                                         changed = true;
 451
 452                                         if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, new_value) < 0)
 453                                                 return -errno;
 454                                 }
 455                         }
 456
 457                         if (q->effective != (uint64_t) -1) {
 458                                 cap_flag_value_t old_value, new_value;
 459
 460                                 if (cap_get_flag(c, cv, CAP_EFFECTIVE, &old_value) < 0)
 461                                         return -errno;
 462
 463                                 new_value = (q->effective & m) ? CAP_SET : CAP_CLEAR;
 464
 465                                 if (old_value != new_value) {
 466                                         changed = true;
 467
 468                                         if (cap_set_flag(c, CAP_EFFECTIVE, 1, &cv, new_value) < 0)
 469                                                 return -errno;
 470                                 }
 471                         }
 472                 }
 473
 474                 if (changed)
 475                         if (cap_set_proc(c) < 0)
 476                                 return -errno;
 477         }
 478
 479         if (q->bounding != (uint64_t) -1) {
 480                 r = capability_bounding_set_drop(q->bounding, false);
 481                 if (r < 0)
 482                         return r;
 483         }
 484
 485         return 0;
 486 }