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 "missing_prctl.h"
  17 #include "parse-util.h"
  18 #include "user-util.h"
  19 #include "util.h"
  20
  21 int have_effective_cap(int value) {
  22         _cleanup_cap_free_ cap_t cap;
  23         cap_flag_value_t fv;
  24
  25         cap = cap_get_proc();
  26         if (!cap)
  27                 return -errno;
  28
  29         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  30                 return -errno;
  31         else
  32                 return fv == CAP_SET;
  33 }
  34
  35 unsigned long cap_last_cap(void) {
  36         static thread_local unsigned long saved;
  37         static thread_local bool valid = false;
  38         _cleanup_free_ char *content = NULL;
  39         unsigned long p = 0;
  40         int r;
  41
  42         if (valid)
  43                 return saved;
  44
  45         /* available since linux-3.2 */
  46         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  47         if (r >= 0) {
  48                 r = safe_atolu(content, &p);
  49                 if (r >= 0) {
  50                         saved = p;
  51                         valid = true;
  52                         return p;
  53                 }
  54         }
  55
  56         /* fall back to syscall-probing for pre linux-3.2 */
  57         p = (unsigned long) CAP_LAST_CAP;
  58
  59         if (prctl(PR_CAPBSET_READ, p) < 0) {
  60
  61                 /* Hmm, look downwards, until we find one that
  62                  * works */
  63                 for (p--; p > 0; p --)
  64                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  65                                 break;
  66
  67         } else {
  68
  69                 /* Hmm, look upwards, until we find one that doesn't
  70                  * work */
  71                 for (;; p++)
  72                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  73                                 break;
  74         }
  75
  76         saved = p;
  77         valid = true;
  78
  79         return p;
  80 }
  81
  82 int capability_update_inherited_set(cap_t caps, uint64_t set) {
  83         unsigned long i;
  84
  85         /* Add capabilities in the set to the inherited caps. Do not apply
  86          * them yet. */
  87
  88         for (i = 0; i < cap_last_cap(); i++) {
  89
  90                 if (set & (UINT64_C(1) << i)) {
  91                         cap_value_t v;
  92
  93                         v = (cap_value_t) i;
  94
  95                         /* Make the capability inheritable. */
  96                         if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, CAP_SET) < 0)
  97                                 return -errno;
  98                 }
  99         }
 100
 101         return 0;
 102 }
 103
 104 int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
 105         unsigned long i;
 106         _cleanup_cap_free_ cap_t caps = NULL;
 107
 108         /* Add the capabilities to the ambient set. */
 109
 110         if (also_inherit) {
 111                 int r;
 112                 caps = cap_get_proc();
 113                 if (!caps)
 114                         return -errno;
 115
 116                 r = capability_update_inherited_set(caps, set);
 117                 if (r < 0)
 118                         return -errno;
 119
 120                 if (cap_set_proc(caps) < 0)
 121                         return -errno;
 122         }
 123
 124         for (i = 0; i < cap_last_cap(); i++) {
 125
 126                 if (set & (UINT64_C(1) << i)) {
 127
 128                         /* Add the capability to the ambient set. */
 129                         if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
 130                                 return -errno;
 131                 }
 132         }
 133
 134         return 0;
 135 }
 136
 137 int capability_bounding_set_drop(uint64_t keep, bool right_now) {
 138         _cleanup_cap_free_ cap_t before_cap = NULL, after_cap = NULL;
 139         cap_flag_value_t fv;
 140         unsigned long i;
 141         int r;
 142
 143         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 144          * in the effective set (yes, the kernel drops that when
 145          * executing init!), so get it back temporarily so that we can
 146          * call PR_CAPBSET_DROP. */
 147
 148         before_cap = cap_get_proc();
 149         if (!before_cap)
 150                 return -errno;
 151
 152         if (cap_get_flag(before_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 153                 return -errno;
 154
 155         if (fv != CAP_SET) {
 156                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 157                 static const cap_value_t v = CAP_SETPCAP;
 158
 159                 temp_cap = cap_dup(before_cap);
 160                 if (!temp_cap)
 161                         return -errno;
 162
 163                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0)
 164                         return -errno;
 165
 166                 if (cap_set_proc(temp_cap) < 0)
 167                         log_debug_errno(errno, "Can't acquire effective CAP_SETPCAP bit, ignoring: %m");
 168
 169                 /* If we didn't manage to acquire the CAP_SETPCAP bit, we continue anyway, after all this just means
 170                  * we'll fail later, when we actually intend to drop some capabilities. */
 171         }
 172
 173         after_cap = cap_dup(before_cap);
 174         if (!after_cap)
 175                 return -errno;
 176
 177         for (i = 0; i <= cap_last_cap(); i++) {
 178                 cap_value_t v;
 179
 180                 if ((keep & (UINT64_C(1) << i)))
 181                         continue;
 182
 183                 /* Drop it from the bounding set */
 184                 if (prctl(PR_CAPBSET_DROP, i) < 0) {
 185                         r = -errno;
 186
 187                         /* If dropping the capability failed, let's see if we didn't have it in the first place. If so,
 188                          * continue anyway, as dropping a capability we didn't have in the first place doesn't really
 189                          * matter anyway. */
 190                         if (prctl(PR_CAPBSET_READ, i) != 0)
 191                                 goto finish;
 192                 }
 193                 v = (cap_value_t) i;
 194
 195                 /* Also drop it from the inheritable set, so
 196                  * that anything we exec() loses the
 197                  * capability for good. */
 198                 if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 199                         r = -errno;
 200                         goto finish;
 201                 }
 202
 203                 /* If we shall apply this right now drop it
 204                  * also from our own capability sets. */
 205                 if (right_now) {
 206                         if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 207                             cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 208                                 r = -errno;
 209                                 goto finish;
 210                         }
 211                 }
 212         }
 213
 214         r = 0;
 215
 216 finish:
 217         if (cap_set_proc(after_cap) < 0) {
 218                 /* If there are no actual changes anyway then let's ignore this error. */
 219                 if (cap_compare(before_cap, after_cap) != 0)
 220                         r = -errno;
 221         }
 222
 223         return r;
 224 }
 225
 226 static int drop_from_file(const char *fn, uint64_t keep) {
 227         _cleanup_free_ char *p = NULL;
 228         uint64_t current, after;
 229         uint32_t hi, lo;
 230         int r, k;
 231
 232         r = read_one_line_file(fn, &p);
 233         if (r < 0)
 234                 return r;
 235
 236         assert_cc(sizeof(hi) == sizeof(unsigned));
 237         assert_cc(sizeof(lo) == sizeof(unsigned));
 238
 239         k = sscanf(p, "%u %u", &lo, &hi);
 240         if (k != 2)
 241                 return -EIO;
 242
 243         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 244         after = current & keep;
 245
 246         if (current == after)
 247                 return 0;
 248
 249         lo = (unsigned) (after & 0xFFFFFFFFULL);
 250         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 251
 252         return write_string_filef(fn, WRITE_STRING_FILE_CREATE, "%u %u", lo, hi);
 253 }
 254
 255 int capability_bounding_set_drop_usermode(uint64_t keep) {
 256         int r;
 257
 258         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 259         if (r < 0)
 260                 return r;
 261
 262         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 263         if (r < 0)
 264                 return r;
 265
 266         return r;
 267 }
 268
 269 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 270         _cleanup_cap_free_ cap_t d = NULL;
 271         unsigned i, j = 0;
 272         int r;
 273
 274         /* Unfortunately we cannot leave privilege dropping to PID 1
 275          * here, since we want to run as user but want to keep some
 276          * capabilities. Since file capabilities have been introduced
 277          * this cannot be done across exec() anymore, unless our
 278          * binary has the capability configured in the file system,
 279          * which we want to avoid. */
 280
 281         if (setresgid(gid, gid, gid) < 0)
 282                 return log_error_errno(errno, "Failed to change group ID: %m");
 283
 284         r = maybe_setgroups(0, NULL);
 285         if (r < 0)
 286                 return log_error_errno(r, "Failed to drop auxiliary groups list: %m");
 287
 288         /* Ensure we keep the permitted caps across the setresuid() */
 289         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 290                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 291
 292         if (setresuid(uid, uid, uid) < 0)
 293                 return log_error_errno(errno, "Failed to change user ID: %m");
 294
 295         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 296                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 297
 298         /* Drop all caps from the bounding set, except the ones we want */
 299         r = capability_bounding_set_drop(keep_capabilities, true);
 300         if (r < 0)
 301                 return log_error_errno(r, "Failed to drop capabilities: %m");
 302
 303         /* Now upgrade the permitted caps we still kept to effective caps */
 304         d = cap_init();
 305         if (!d)
 306                 return log_oom();
 307
 308         if (keep_capabilities) {
 309                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 310
 311                 for (i = 0; i < ELEMENTSOF(bits); i++)
 312                         if (keep_capabilities & (1ULL << i))
 313                                 bits[j++] = i;
 314
 315                 /* use enough bits */
 316                 assert(i == 64 || (keep_capabilities >> i) == 0);
 317                 /* don't use too many bits */
 318                 assert(keep_capabilities & (1ULL << (i - 1)));
 319
 320                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 321                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
 322                         return log_error_errno(errno, "Failed to enable capabilities bits: %m");
 323
 324                 if (cap_set_proc(d) < 0)
 325                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 326         }
 327
 328         return 0;
 329 }
 330
 331 int drop_capability(cap_value_t cv) {
 332         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 333
 334         tmp_cap = cap_get_proc();
 335         if (!tmp_cap)
 336                 return -errno;
 337
 338         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 339             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 340             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 341                 return -errno;
 342
 343         if (cap_set_proc(tmp_cap) < 0)
 344                 return -errno;
 345
 346         return 0;
 347 }
 348
 349 bool ambient_capabilities_supported(void) {
 350         static int cache = -1;
 351
 352         if (cache >= 0)
 353                 return cache;
 354
 355         /* If PR_CAP_AMBIENT returns something valid, or an unexpected error code we assume that ambient caps are
 356          * available. */
 357
 358         cache = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_KILL, 0, 0) >= 0 ||
 359                 !IN_SET(errno, EINVAL, EOPNOTSUPP, ENOSYS);
 360
 361         return cache;
 362 }
 363
 364 int capability_quintet_enforce(const CapabilityQuintet *q) {
 365         _cleanup_cap_free_ cap_t c = NULL;
 366         int r;
 367
 368         if (q->ambient != (uint64_t) -1) {
 369                 unsigned long i;
 370                 bool changed = false;
 371
 372                 c = cap_get_proc();
 373                 if (!c)
 374                         return -errno;
 375
 376                 /* In order to raise the ambient caps set we first need to raise the matching inheritable + permitted
 377                  * cap */
 378                 for (i = 0; i <= cap_last_cap(); i++) {
 379                         uint64_t m = UINT64_C(1) << i;
 380                         cap_value_t cv = (cap_value_t) i;
 381                         cap_flag_value_t old_value_inheritable, old_value_permitted;
 382
 383                         if ((q->ambient & m) == 0)
 384                                 continue;
 385
 386                         if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value_inheritable) < 0)
 387                                 return -errno;
 388                         if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value_permitted) < 0)
 389                                 return -errno;
 390
 391                         if (old_value_inheritable == CAP_SET && old_value_permitted == CAP_SET)
 392                                 continue;
 393
 394                         if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, CAP_SET) < 0)
 395                                 return -errno;
 396
 397                         if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, CAP_SET) < 0)
 398                                 return -errno;
 399
 400                         changed = true;
 401                 }
 402
 403                 if (changed)
 404                         if (cap_set_proc(c) < 0)
 405                                 return -errno;
 406
 407                 r = capability_ambient_set_apply(q->ambient, false);
 408                 if (r < 0)
 409                         return r;
 410         }
 411
 412         if (q->inheritable != (uint64_t) -1 || q->permitted != (uint64_t) -1 || q->effective != (uint64_t) -1) {
 413                 bool changed = false;
 414                 unsigned long i;
 415
 416                 if (!c) {
 417                         c = cap_get_proc();
 418                         if (!c)
 419                                 return -errno;
 420                 }
 421
 422                 for (i = 0; i <= cap_last_cap(); i++) {
 423                         uint64_t m = UINT64_C(1) << i;
 424                         cap_value_t cv = (cap_value_t) i;
 425
 426                         if (q->inheritable != (uint64_t) -1) {
 427                                 cap_flag_value_t old_value, new_value;
 428
 429                                 if (cap_get_flag(c, cv, CAP_INHERITABLE, &old_value) < 0)
 430                                         return -errno;
 431
 432                                 new_value = (q->inheritable & m) ? CAP_SET : CAP_CLEAR;
 433
 434                                 if (old_value != new_value) {
 435                                         changed = true;
 436
 437                                         if (cap_set_flag(c, CAP_INHERITABLE, 1, &cv, new_value) < 0)
 438                                                 return -errno;
 439                                 }
 440                         }
 441
 442                         if (q->permitted != (uint64_t) -1) {
 443                                 cap_flag_value_t old_value, new_value;
 444
 445                                 if (cap_get_flag(c, cv, CAP_PERMITTED, &old_value) < 0)
 446                                         return -errno;
 447
 448                                 new_value = (q->permitted & m) ? CAP_SET : CAP_CLEAR;
 449
 450                                 if (old_value != new_value) {
 451                                         changed = true;
 452
 453                                         if (cap_set_flag(c, CAP_PERMITTED, 1, &cv, new_value) < 0)
 454                                                 return -errno;
 455                                 }
 456                         }
 457
 458                         if (q->effective != (uint64_t) -1) {
 459                                 cap_flag_value_t old_value, new_value;
 460
 461                                 if (cap_get_flag(c, cv, CAP_EFFECTIVE, &old_value) < 0)
 462                                         return -errno;
 463
 464                                 new_value = (q->effective & m) ? CAP_SET : CAP_CLEAR;
 465
 466                                 if (old_value != new_value) {
 467                                         changed = true;
 468
 469                                         if (cap_set_flag(c, CAP_EFFECTIVE, 1, &cv, new_value) < 0)
 470                                                 return -errno;
 471                                 }
 472                         }
 473                 }
 474
 475                 if (changed)
 476                         if (cap_set_proc(c) < 0)
 477                                 return -errno;
 478         }
 479
 480         if (q->bounding != (uint64_t) -1) {
 481                 r = capability_bounding_set_drop(q->bounding, false);
 482                 if (r < 0)
 483                         return r;
 484         }
 485
 486         return 0;
 487 }