src/basic/capability-util.c

   1 /*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/
   2
   3 /***
   4   This file is part of systemd.
   5
   6   Copyright 2010 Lennart Poettering
   7
   8   systemd is free software; you can redistribute it and/or modify it
   9   under the terms of the GNU Lesser General Public License as published by
  10   the Free Software Foundation; either version 2.1 of the License, or
  11   (at your option) any later version.
  12
  13   systemd is distributed in the hope that it will be useful, but
  14   WITHOUT ANY WARRANTY; without even the implied warranty of
  15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  16   Lesser General Public License for more details.
  17
  18   You should have received a copy of the GNU Lesser General Public License
  19   along with systemd; If not, see <http://www.gnu.org/licenses/>.
  20 ***/
  21
  22 #include <errno.h>
  23 #include <grp.h>
  24 #include <stdio.h>
  25 #include <sys/capability.h>
  26 #include <sys/prctl.h>
  27 #include <unistd.h>
  28
  29 #include "capability-util.h"
  30 #include "fileio.h"
  31 #include "log.h"
  32 #include "macro.h"
  33 #include "parse-util.h"
  34 #include "util.h"
  35
  36 int have_effective_cap(int value) {
  37         _cleanup_cap_free_ cap_t cap;
  38         cap_flag_value_t fv;
  39
  40         cap = cap_get_proc();
  41         if (!cap)
  42                 return -errno;
  43
  44         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  45                 return -errno;
  46         else
  47                 return fv == CAP_SET;
  48 }
  49
  50 unsigned long cap_last_cap(void) {
  51         static thread_local unsigned long saved;
  52         static thread_local bool valid = false;
  53         _cleanup_free_ char *content = NULL;
  54         unsigned long p = 0;
  55         int r;
  56
  57         if (valid)
  58                 return saved;
  59
  60         /* available since linux-3.2 */
  61         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  62         if (r >= 0) {
  63                 r = safe_atolu(content, &p);
  64                 if (r >= 0) {
  65                         saved = p;
  66                         valid = true;
  67                         return p;
  68                 }
  69         }
  70
  71         /* fall back to syscall-probing for pre linux-3.2 */
  72         p = (unsigned long) CAP_LAST_CAP;
  73
  74         if (prctl(PR_CAPBSET_READ, p) < 0) {
  75
  76                 /* Hmm, look downwards, until we find one that
  77                  * works */
  78                 for (p--; p > 0; p --)
  79                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  80                                 break;
  81
  82         } else {
  83
  84                 /* Hmm, look upwards, until we find one that doesn't
  85                  * work */
  86                 for (;; p++)
  87                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  88                                 break;
  89         }
  90
  91         saved = p;
  92         valid = true;
  93
  94         return p;
  95 }
  96
  97 int capability_bounding_set_drop(uint64_t drop, bool right_now) {
  98         _cleanup_cap_free_ cap_t after_cap = NULL;
  99         cap_flag_value_t fv;
 100         unsigned long i;
 101         int r;
 102
 103         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 104          * in the effective set (yes, the kernel drops that when
 105          * executing init!), so get it back temporarily so that we can
 106          * call PR_CAPBSET_DROP. */
 107
 108         after_cap = cap_get_proc();
 109         if (!after_cap)
 110                 return -errno;
 111
 112         if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 113                 return -errno;
 114
 115         if (fv != CAP_SET) {
 116                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 117                 static const cap_value_t v = CAP_SETPCAP;
 118
 119                 temp_cap = cap_dup(after_cap);
 120                 if (!temp_cap) {
 121                         r = -errno;
 122                         goto finish;
 123                 }
 124
 125                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
 126                         r = -errno;
 127                         goto finish;
 128                 }
 129
 130                 if (cap_set_proc(temp_cap) < 0) {
 131                         r = -errno;
 132                         goto finish;
 133                 }
 134         }
 135
 136         for (i = 0; i <= cap_last_cap(); i++) {
 137
 138                 if (drop & ((uint64_t) 1ULL << (uint64_t) i)) {
 139                         cap_value_t v;
 140
 141                         /* Drop it from the bounding set */
 142                         if (prctl(PR_CAPBSET_DROP, i) < 0) {
 143                                 r = -errno;
 144                                 goto finish;
 145                         }
 146                         v = (cap_value_t) i;
 147
 148                         /* Also drop it from the inheritable set, so
 149                          * that anything we exec() loses the
 150                          * capability for good. */
 151                         if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 152                                 r = -errno;
 153                                 goto finish;
 154                         }
 155
 156                         /* If we shall apply this right now drop it
 157                          * also from our own capability sets. */
 158                         if (right_now) {
 159                                 if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 160                                     cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 161                                         r = -errno;
 162                                         goto finish;
 163                                 }
 164                         }
 165                 }
 166         }
 167
 168         r = 0;
 169
 170 finish:
 171         if (cap_set_proc(after_cap) < 0)
 172                 return -errno;
 173
 174         return r;
 175 }
 176
 177 static int drop_from_file(const char *fn, uint64_t drop) {
 178         int r, k;
 179         uint32_t hi, lo;
 180         uint64_t current, after;
 181         char *p;
 182
 183         r = read_one_line_file(fn, &p);
 184         if (r < 0)
 185                 return r;
 186
 187         assert_cc(sizeof(hi) == sizeof(unsigned));
 188         assert_cc(sizeof(lo) == sizeof(unsigned));
 189
 190         k = sscanf(p, "%u %u", &lo, &hi);
 191         free(p);
 192
 193         if (k != 2)
 194                 return -EIO;
 195
 196         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 197         after = current & ~drop;
 198
 199         if (current == after)
 200                 return 0;
 201
 202         lo = (unsigned) (after & 0xFFFFFFFFULL);
 203         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 204
 205         if (asprintf(&p, "%u %u", lo, hi) < 0)
 206                 return -ENOMEM;
 207
 208         r = write_string_file(fn, p, WRITE_STRING_FILE_CREATE);
 209         free(p);
 210
 211         return r;
 212 }
 213
 214 int capability_bounding_set_drop_usermode(uint64_t drop) {
 215         int r;
 216
 217         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", drop);
 218         if (r < 0)
 219                 return r;
 220
 221         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", drop);
 222         if (r < 0)
 223                 return r;
 224
 225         return r;
 226 }
 227
 228 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 229         _cleanup_cap_free_ cap_t d = NULL;
 230         unsigned i, j = 0;
 231         int r;
 232
 233         /* Unfortunately we cannot leave privilege dropping to PID 1
 234          * here, since we want to run as user but want to keep some
 235          * capabilities. Since file capabilities have been introduced
 236          * this cannot be done across exec() anymore, unless our
 237          * binary has the capability configured in the file system,
 238          * which we want to avoid. */
 239
 240         if (setresgid(gid, gid, gid) < 0)
 241                 return log_error_errno(errno, "Failed to change group ID: %m");
 242
 243         if (setgroups(0, NULL) < 0)
 244                 return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 245
 246         /* Ensure we keep the permitted caps across the setresuid() */
 247         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 248                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 249
 250         r = setresuid(uid, uid, uid);
 251         if (r < 0)
 252                 return log_error_errno(errno, "Failed to change user ID: %m");
 253
 254         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 255                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 256
 257         /* Drop all caps from the bounding set, except the ones we want */
 258         r = capability_bounding_set_drop(~keep_capabilities, true);
 259         if (r < 0)
 260                 return log_error_errno(r, "Failed to drop capabilities: %m");
 261
 262         /* Now upgrade the permitted caps we still kept to effective caps */
 263         d = cap_init();
 264         if (!d)
 265                 return log_oom();
 266
 267         if (keep_capabilities) {
 268                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 269
 270                 for (i = 0; i < ELEMENTSOF(bits); i++)
 271                         if (keep_capabilities & (1ULL << i))
 272                                 bits[j++] = i;
 273
 274                 /* use enough bits */
 275                 assert(i == 64 || (keep_capabilities >> i) == 0);
 276                 /* don't use too many bits */
 277                 assert(keep_capabilities & (1ULL << (i - 1)));
 278
 279                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 280                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
 281                         log_error_errno(errno, "Failed to enable capabilities bits: %m");
 282                         return -errno;
 283                 }
 284
 285                 if (cap_set_proc(d) < 0)
 286                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 287         }
 288
 289         return 0;
 290 }
 291
 292 int drop_capability(cap_value_t cv) {
 293         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 294
 295         tmp_cap = cap_get_proc();
 296         if (!tmp_cap)
 297                 return -errno;
 298
 299         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 300             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 301             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 302                 return -errno;
 303
 304         if (cap_set_proc(tmp_cap) < 0)
 305                 return -errno;
 306
 307         return 0;
 308 }