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 "alloc-util.h"
  30 #include "capability-util.h"
  31 #include "fileio.h"
  32 #include "log.h"
  33 #include "macro.h"
  34 #include "parse-util.h"
  35 #include "util.h"
  36
  37 int have_effective_cap(int value) {
  38         _cleanup_cap_free_ cap_t cap;
  39         cap_flag_value_t fv;
  40
  41         cap = cap_get_proc();
  42         if (!cap)
  43                 return -errno;
  44
  45         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  46                 return -errno;
  47         else
  48                 return fv == CAP_SET;
  49 }
  50
  51 unsigned long cap_last_cap(void) {
  52         static thread_local unsigned long saved;
  53         static thread_local bool valid = false;
  54         _cleanup_free_ char *content = NULL;
  55         unsigned long p = 0;
  56         int r;
  57
  58         if (valid)
  59                 return saved;
  60
  61         /* available since linux-3.2 */
  62         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  63         if (r >= 0) {
  64                 r = safe_atolu(content, &p);
  65                 if (r >= 0) {
  66                         saved = p;
  67                         valid = true;
  68                         return p;
  69                 }
  70         }
  71
  72         /* fall back to syscall-probing for pre linux-3.2 */
  73         p = (unsigned long) CAP_LAST_CAP;
  74
  75         if (prctl(PR_CAPBSET_READ, p) < 0) {
  76
  77                 /* Hmm, look downwards, until we find one that
  78                  * works */
  79                 for (p--; p > 0; p --)
  80                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  81                                 break;
  82
  83         } else {
  84
  85                 /* Hmm, look upwards, until we find one that doesn't
  86                  * work */
  87                 for (;; p++)
  88                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  89                                 break;
  90         }
  91
  92         saved = p;
  93         valid = true;
  94
  95         return p;
  96 }
  97
  98 int capability_bounding_set_drop(uint64_t drop, bool right_now) {
  99         _cleanup_cap_free_ cap_t after_cap = NULL;
 100         cap_flag_value_t fv;
 101         unsigned long i;
 102         int r;
 103
 104         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 105          * in the effective set (yes, the kernel drops that when
 106          * executing init!), so get it back temporarily so that we can
 107          * call PR_CAPBSET_DROP. */
 108
 109         after_cap = cap_get_proc();
 110         if (!after_cap)
 111                 return -errno;
 112
 113         if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 114                 return -errno;
 115
 116         if (fv != CAP_SET) {
 117                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 118                 static const cap_value_t v = CAP_SETPCAP;
 119
 120                 temp_cap = cap_dup(after_cap);
 121                 if (!temp_cap) {
 122                         r = -errno;
 123                         goto finish;
 124                 }
 125
 126                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
 127                         r = -errno;
 128                         goto finish;
 129                 }
 130
 131                 if (cap_set_proc(temp_cap) < 0) {
 132                         r = -errno;
 133                         goto finish;
 134                 }
 135         }
 136
 137         for (i = 0; i <= cap_last_cap(); i++) {
 138
 139                 if (drop & ((uint64_t) 1ULL << (uint64_t) i)) {
 140                         cap_value_t v;
 141
 142                         /* Drop it from the bounding set */
 143                         if (prctl(PR_CAPBSET_DROP, i) < 0) {
 144                                 r = -errno;
 145                                 goto finish;
 146                         }
 147                         v = (cap_value_t) i;
 148
 149                         /* Also drop it from the inheritable set, so
 150                          * that anything we exec() loses the
 151                          * capability for good. */
 152                         if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 153                                 r = -errno;
 154                                 goto finish;
 155                         }
 156
 157                         /* If we shall apply this right now drop it
 158                          * also from our own capability sets. */
 159                         if (right_now) {
 160                                 if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 161                                     cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 162                                         r = -errno;
 163                                         goto finish;
 164                                 }
 165                         }
 166                 }
 167         }
 168
 169         r = 0;
 170
 171 finish:
 172         if (cap_set_proc(after_cap) < 0)
 173                 return -errno;
 174
 175         return r;
 176 }
 177
 178 static int drop_from_file(const char *fn, uint64_t drop) {
 179         int r, k;
 180         uint32_t hi, lo;
 181         uint64_t current, after;
 182         char *p;
 183
 184         r = read_one_line_file(fn, &p);
 185         if (r < 0)
 186                 return r;
 187
 188         assert_cc(sizeof(hi) == sizeof(unsigned));
 189         assert_cc(sizeof(lo) == sizeof(unsigned));
 190
 191         k = sscanf(p, "%u %u", &lo, &hi);
 192         free(p);
 193
 194         if (k != 2)
 195                 return -EIO;
 196
 197         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 198         after = current & ~drop;
 199
 200         if (current == after)
 201                 return 0;
 202
 203         lo = (unsigned) (after & 0xFFFFFFFFULL);
 204         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 205
 206         if (asprintf(&p, "%u %u", lo, hi) < 0)
 207                 return -ENOMEM;
 208
 209         r = write_string_file(fn, p, WRITE_STRING_FILE_CREATE);
 210         free(p);
 211
 212         return r;
 213 }
 214
 215 int capability_bounding_set_drop_usermode(uint64_t drop) {
 216         int r;
 217
 218         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", drop);
 219         if (r < 0)
 220                 return r;
 221
 222         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", drop);
 223         if (r < 0)
 224                 return r;
 225
 226         return r;
 227 }
 228
 229 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 230         _cleanup_cap_free_ cap_t d = NULL;
 231         unsigned i, j = 0;
 232         int r;
 233
 234         /* Unfortunately we cannot leave privilege dropping to PID 1
 235          * here, since we want to run as user but want to keep some
 236          * capabilities. Since file capabilities have been introduced
 237          * this cannot be done across exec() anymore, unless our
 238          * binary has the capability configured in the file system,
 239          * which we want to avoid. */
 240
 241         if (setresgid(gid, gid, gid) < 0)
 242                 return log_error_errno(errno, "Failed to change group ID: %m");
 243
 244         if (setgroups(0, NULL) < 0)
 245                 return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 246
 247         /* Ensure we keep the permitted caps across the setresuid() */
 248         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 249                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 250
 251         r = setresuid(uid, uid, uid);
 252         if (r < 0)
 253                 return log_error_errno(errno, "Failed to change user ID: %m");
 254
 255         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 256                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 257
 258         /* Drop all caps from the bounding set, except the ones we want */
 259         r = capability_bounding_set_drop(~keep_capabilities, true);
 260         if (r < 0)
 261                 return log_error_errno(r, "Failed to drop capabilities: %m");
 262
 263         /* Now upgrade the permitted caps we still kept to effective caps */
 264         d = cap_init();
 265         if (!d)
 266                 return log_oom();
 267
 268         if (keep_capabilities) {
 269                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 270
 271                 for (i = 0; i < ELEMENTSOF(bits); i++)
 272                         if (keep_capabilities & (1ULL << i))
 273                                 bits[j++] = i;
 274
 275                 /* use enough bits */
 276                 assert(i == 64 || (keep_capabilities >> i) == 0);
 277                 /* don't use too many bits */
 278                 assert(keep_capabilities & (1ULL << (i - 1)));
 279
 280                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 281                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
 282                         log_error_errno(errno, "Failed to enable capabilities bits: %m");
 283                         return -errno;
 284                 }
 285
 286                 if (cap_set_proc(d) < 0)
 287                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 288         }
 289
 290         return 0;
 291 }
 292
 293 int drop_capability(cap_value_t cv) {
 294         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 295
 296         tmp_cap = cap_get_proc();
 297         if (!tmp_cap)
 298                 return -errno;
 299
 300         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 301             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 302             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 303                 return -errno;
 304
 305         if (cap_set_proc(tmp_cap) < 0)
 306                 return -errno;
 307
 308         return 0;
 309 }