src/basic/capability-util.c

   1 /* SPDX-License-Identifier: LGPL-2.1+ */
   2 /***
   3   This file is part of systemd.
   4
   5   Copyright 2010 Lennart Poettering
   6 ***/
   7
   8 #include <errno.h>
   9 #include <grp.h>
  10 #include <stdio.h>
  11 #include <stdlib.h>
  12 #include <sys/capability.h>
  13 #include <sys/prctl.h>
  14 #include <unistd.h>
  15
  16 #include "alloc-util.h"
  17 #include "capability-util.h"
  18 #include "fileio.h"
  19 #include "log.h"
  20 #include "macro.h"
  21 #include "parse-util.h"
  22 #include "user-util.h"
  23 #include "util.h"
  24
  25 int have_effective_cap(int value) {
  26         _cleanup_cap_free_ cap_t cap;
  27         cap_flag_value_t fv;
  28
  29         cap = cap_get_proc();
  30         if (!cap)
  31                 return -errno;
  32
  33         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  34                 return -errno;
  35         else
  36                 return fv == CAP_SET;
  37 }
  38
  39 unsigned long cap_last_cap(void) {
  40         static thread_local unsigned long saved;
  41         static thread_local bool valid = false;
  42         _cleanup_free_ char *content = NULL;
  43         unsigned long p = 0;
  44         int r;
  45
  46         if (valid)
  47                 return saved;
  48
  49         /* available since linux-3.2 */
  50         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  51         if (r >= 0) {
  52                 r = safe_atolu(content, &p);
  53                 if (r >= 0) {
  54                         saved = p;
  55                         valid = true;
  56                         return p;
  57                 }
  58         }
  59
  60         /* fall back to syscall-probing for pre linux-3.2 */
  61         p = (unsigned long) CAP_LAST_CAP;
  62
  63         if (prctl(PR_CAPBSET_READ, p) < 0) {
  64
  65                 /* Hmm, look downwards, until we find one that
  66                  * works */
  67                 for (p--; p > 0; p --)
  68                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  69                                 break;
  70
  71         } else {
  72
  73                 /* Hmm, look upwards, until we find one that doesn't
  74                  * work */
  75                 for (;; 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. Do not apply
  90          * them yet. */
  91
  92         for (i = 0; i < cap_last_cap(); i++) {
  93
  94                 if (set & (UINT64_C(1) << i)) {
  95                         cap_value_t v;
  96
  97                         v = (cap_value_t) i;
  98
  99                         /* Make the capability inheritable. */
 100                         if (cap_set_flag(caps, CAP_INHERITABLE, 1, &v, CAP_SET) < 0)
 101                                 return -errno;
 102                 }
 103         }
 104
 105         return 0;
 106 }
 107
 108 int capability_ambient_set_apply(uint64_t set, bool also_inherit) {
 109         unsigned long i;
 110         _cleanup_cap_free_ cap_t caps = NULL;
 111
 112         /* Add the capabilities to the ambient set. */
 113
 114         if (also_inherit) {
 115                 int r;
 116                 caps = cap_get_proc();
 117                 if (!caps)
 118                         return -errno;
 119
 120                 r = capability_update_inherited_set(caps, set);
 121                 if (r < 0)
 122                         return -errno;
 123
 124                 if (cap_set_proc(caps) < 0)
 125                         return -errno;
 126         }
 127
 128         for (i = 0; i < cap_last_cap(); i++) {
 129
 130                 if (set & (UINT64_C(1) << i)) {
 131
 132                         /* Add the capability to the ambient set. */
 133                         if (prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_RAISE, i, 0, 0) < 0)
 134                                 return -errno;
 135                 }
 136         }
 137
 138         return 0;
 139 }
 140
 141 int capability_bounding_set_drop(uint64_t keep, bool right_now) {
 142         _cleanup_cap_free_ cap_t before_cap = NULL, after_cap = NULL;
 143         cap_flag_value_t fv;
 144         unsigned long i;
 145         int r;
 146
 147         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 148          * in the effective set (yes, the kernel drops that when
 149          * executing init!), so get it back temporarily so that we can
 150          * call PR_CAPBSET_DROP. */
 151
 152         before_cap = cap_get_proc();
 153         if (!before_cap)
 154                 return -errno;
 155
 156         if (cap_get_flag(before_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 157                 return -errno;
 158
 159         if (fv != CAP_SET) {
 160                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 161                 static const cap_value_t v = CAP_SETPCAP;
 162
 163                 temp_cap = cap_dup(before_cap);
 164                 if (!temp_cap)
 165                         return -errno;
 166
 167                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0)
 168                         return -errno;
 169
 170                 if (cap_set_proc(temp_cap) < 0)
 171                         log_debug_errno(errno, "Can't acquire effective CAP_SETPCAP bit, ignoring: %m");
 172
 173                 /* If we didn't manage to acquire the CAP_SETPCAP bit, we continue anyway, after all this just means
 174                  * we'll fail later, when we actually intend to drop some capabilities. */
 175         }
 176
 177         after_cap = cap_dup(before_cap);
 178         if (!after_cap)
 179                 return -errno;
 180
 181         for (i = 0; i <= cap_last_cap(); i++) {
 182                 cap_value_t v;
 183
 184                 if ((keep & (UINT64_C(1) << i)))
 185                         continue;
 186
 187                 /* Drop it from the bounding set */
 188                 if (prctl(PR_CAPBSET_DROP, i) < 0) {
 189                         r = -errno;
 190
 191                         /* If dropping the capability failed, let's see if we didn't have it in the first place. If so,
 192                          * continue anyway, as dropping a capability we didn't have in the first place doesn't really
 193                          * matter anyway. */
 194                         if (prctl(PR_CAPBSET_READ, i) != 0)
 195                                 goto finish;
 196                 }
 197                 v = (cap_value_t) i;
 198
 199                 /* Also drop it from the inheritable set, so
 200                  * that anything we exec() loses the
 201                  * capability for good. */
 202                 if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 203                         r = -errno;
 204                         goto finish;
 205                 }
 206
 207                 /* If we shall apply this right now drop it
 208                  * also from our own capability sets. */
 209                 if (right_now) {
 210                         if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 211                             cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 212                                 r = -errno;
 213                                 goto finish;
 214                         }
 215                 }
 216         }
 217
 218         r = 0;
 219
 220 finish:
 221         if (cap_set_proc(after_cap) < 0) {
 222                 /* If there are no actual changes anyway then let's ignore this error. */
 223                 if (cap_compare(before_cap, after_cap) != 0)
 224                         r = -errno;
 225         }
 226
 227         return r;
 228 }
 229
 230 static int drop_from_file(const char *fn, uint64_t keep) {
 231         int r, k;
 232         uint32_t hi, lo;
 233         uint64_t current, after;
 234         char *p;
 235
 236         r = read_one_line_file(fn, &p);
 237         if (r < 0)
 238                 return r;
 239
 240         assert_cc(sizeof(hi) == sizeof(unsigned));
 241         assert_cc(sizeof(lo) == sizeof(unsigned));
 242
 243         k = sscanf(p, "%u %u", &lo, &hi);
 244         free(p);
 245
 246         if (k != 2)
 247                 return -EIO;
 248
 249         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 250         after = current & keep;
 251
 252         if (current == after)
 253                 return 0;
 254
 255         lo = (unsigned) (after & 0xFFFFFFFFULL);
 256         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 257
 258         if (asprintf(&p, "%u %u", lo, hi) < 0)
 259                 return -ENOMEM;
 260
 261         r = write_string_file(fn, p, WRITE_STRING_FILE_CREATE);
 262         free(p);
 263
 264         return r;
 265 }
 266
 267 int capability_bounding_set_drop_usermode(uint64_t keep) {
 268         int r;
 269
 270         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", keep);
 271         if (r < 0)
 272                 return r;
 273
 274         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", keep);
 275         if (r < 0)
 276                 return r;
 277
 278         return r;
 279 }
 280
 281 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 282         _cleanup_cap_free_ cap_t d = NULL;
 283         unsigned i, j = 0;
 284         int r;
 285
 286         /* Unfortunately we cannot leave privilege dropping to PID 1
 287          * here, since we want to run as user but want to keep some
 288          * capabilities. Since file capabilities have been introduced
 289          * this cannot be done across exec() anymore, unless our
 290          * binary has the capability configured in the file system,
 291          * which we want to avoid. */
 292
 293         if (setresgid(gid, gid, gid) < 0)
 294                 return log_error_errno(errno, "Failed to change group ID: %m");
 295
 296         r = maybe_setgroups(0, NULL);
 297         if (r < 0)
 298                 return log_error_errno(r, "Failed to drop auxiliary groups list: %m");
 299
 300         /* Ensure we keep the permitted caps across the setresuid() */
 301         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 302                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 303
 304         if (setresuid(uid, uid, uid) < 0)
 305                 return log_error_errno(errno, "Failed to change user ID: %m");
 306
 307         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 308                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 309
 310         /* Drop all caps from the bounding set, except the ones we want */
 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         d = cap_init();
 317         if (!d)
 318                 return log_oom();
 319
 320         if (keep_capabilities) {
 321                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 322
 323                 for (i = 0; i < ELEMENTSOF(bits); i++)
 324                         if (keep_capabilities & (1ULL << i))
 325                                 bits[j++] = i;
 326
 327                 /* use enough bits */
 328                 assert(i == 64 || (keep_capabilities >> i) == 0);
 329                 /* don't use too many bits */
 330                 assert(keep_capabilities & (1ULL << (i - 1)));
 331
 332                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 333                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0)
 334                         return log_error_errno(errno, "Failed to enable capabilities bits: %m");
 335
 336                 if (cap_set_proc(d) < 0)
 337                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 338         }
 339
 340         return 0;
 341 }
 342
 343 int drop_capability(cap_value_t cv) {
 344         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 345
 346         tmp_cap = cap_get_proc();
 347         if (!tmp_cap)
 348                 return -errno;
 349
 350         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 351             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 352             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 353                 return -errno;
 354
 355         if (cap_set_proc(tmp_cap) < 0)
 356                 return -errno;
 357
 358         return 0;
 359 }
 360
 361 bool ambient_capabilities_supported(void) {
 362         static int cache = -1;
 363
 364         if (cache >= 0)
 365                 return cache;
 366
 367         /* If PR_CAP_AMBIENT returns something valid, or an unexpected error code we assume that ambient caps are
 368          * available. */
 369
 370         cache = prctl(PR_CAP_AMBIENT, PR_CAP_AMBIENT_IS_SET, CAP_KILL, 0, 0) >= 0 ||
 371                 !IN_SET(errno, EINVAL, EOPNOTSUPP, ENOSYS);
 372
 373         return cache;
 374 }