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