src/shared/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 <assert.h>
  23 #include <string.h>
  24 #include <unistd.h>
  25 #include <errno.h>
  26 #include <stdio.h>
  27 #include <sys/types.h>
  28 #include <stdarg.h>
  29 #include <ctype.h>
  30 #include <sys/capability.h>
  31 #include <sys/prctl.h>
  32 #include "grp.h"
  33
  34 #include "macro.h"
  35 #include "util.h"
  36 #include "log.h"
  37 #include "fileio.h"
  38 #include "capability.h"
  39
  40 int have_effective_cap(int value) {
  41         _cleanup_cap_free_ cap_t cap;
  42         cap_flag_value_t fv;
  43
  44         cap = cap_get_proc();
  45         if (!cap)
  46                 return -errno;
  47
  48         if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
  49                 return -errno;
  50         else
  51                 return fv == CAP_SET;
  52 }
  53
  54 unsigned long cap_last_cap(void) {
  55         static thread_local unsigned long saved;
  56         static thread_local bool valid = false;
  57         _cleanup_free_ char *content = NULL;
  58         unsigned long p;
  59         int r;
  60
  61         if (valid)
  62                 return saved;
  63
  64         /* available since linux-3.2 */
  65         r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
  66         if (r >= 0) {
  67                 r = safe_atolu(content, &p);
  68                 if (r >= 0) {
  69                         saved = p;
  70                         valid = true;
  71                         return p;
  72                 }
  73         }
  74
  75         /* fall back to syscall-probing for pre linux-3.2 */
  76         p = (unsigned long) CAP_LAST_CAP;
  77
  78         if (prctl(PR_CAPBSET_READ, p) < 0) {
  79
  80                 /* Hmm, look downwards, until we find one that
  81                  * works */
  82                 for (p--; p > 0; p --)
  83                         if (prctl(PR_CAPBSET_READ, p) >= 0)
  84                                 break;
  85
  86         } else {
  87
  88                 /* Hmm, look upwards, until we find one that doesn't
  89                  * work */
  90                 for (;; p++)
  91                         if (prctl(PR_CAPBSET_READ, p+1) < 0)
  92                                 break;
  93         }
  94
  95         saved = p;
  96         valid = true;
  97
  98         return p;
  99 }
 100
 101 int capability_bounding_set_drop(uint64_t drop, bool right_now) {
 102         _cleanup_cap_free_ cap_t after_cap = NULL;
 103         cap_flag_value_t fv;
 104         unsigned long i;
 105         int r;
 106
 107         /* If we are run as PID 1 we will lack CAP_SETPCAP by default
 108          * in the effective set (yes, the kernel drops that when
 109          * executing init!), so get it back temporarily so that we can
 110          * call PR_CAPBSET_DROP. */
 111
 112         after_cap = cap_get_proc();
 113         if (!after_cap)
 114                 return -errno;
 115
 116         if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
 117                 return -errno;
 118
 119         if (fv != CAP_SET) {
 120                 _cleanup_cap_free_ cap_t temp_cap = NULL;
 121                 static const cap_value_t v = CAP_SETPCAP;
 122
 123                 temp_cap = cap_dup(after_cap);
 124                 if (!temp_cap) {
 125                         r = -errno;
 126                         goto finish;
 127                 }
 128
 129                 if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
 130                         r = -errno;
 131                         goto finish;
 132                 }
 133
 134                 if (cap_set_proc(temp_cap) < 0) {
 135                         r = -errno;
 136                         goto finish;
 137                 }
 138         }
 139
 140         for (i = 0; i <= cap_last_cap(); i++) {
 141
 142                 if (drop & ((uint64_t) 1ULL << (uint64_t) i)) {
 143                         cap_value_t v;
 144
 145                         /* Drop it from the bounding set */
 146                         if (prctl(PR_CAPBSET_DROP, i) < 0) {
 147                                 r = -errno;
 148                                 goto finish;
 149                         }
 150                         v = (cap_value_t) i;
 151
 152                         /* Also drop it from the inheritable set, so
 153                          * that anything we exec() loses the
 154                          * capability for good. */
 155                         if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
 156                                 r = -errno;
 157                                 goto finish;
 158                         }
 159
 160                         /* If we shall apply this right now drop it
 161                          * also from our own capability sets. */
 162                         if (right_now) {
 163                                 if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
 164                                     cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
 165                                         r = -errno;
 166                                         goto finish;
 167                                 }
 168                         }
 169                 }
 170         }
 171
 172         r = 0;
 173
 174 finish:
 175         if (cap_set_proc(after_cap) < 0)
 176                 return -errno;
 177
 178         return r;
 179 }
 180
 181 static int drop_from_file(const char *fn, uint64_t drop) {
 182         int r, k;
 183         uint32_t hi, lo;
 184         uint64_t current, after;
 185         char *p;
 186
 187         r = read_one_line_file(fn, &p);
 188         if (r < 0)
 189                 return r;
 190
 191         assert_cc(sizeof(hi) == sizeof(unsigned));
 192         assert_cc(sizeof(lo) == sizeof(unsigned));
 193
 194         k = sscanf(p, "%u %u", &lo, &hi);
 195         free(p);
 196
 197         if (k != 2)
 198                 return -EIO;
 199
 200         current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
 201         after = current & ~drop;
 202
 203         if (current == after)
 204                 return 0;
 205
 206         lo = (unsigned) (after & 0xFFFFFFFFULL);
 207         hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);
 208
 209         if (asprintf(&p, "%u %u", lo, hi) < 0)
 210                 return -ENOMEM;
 211
 212         r = write_string_file(fn, p);
 213         free(p);
 214
 215         return r;
 216 }
 217
 218 int capability_bounding_set_drop_usermode(uint64_t drop) {
 219         int r;
 220
 221         r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", drop);
 222         if (r < 0)
 223                 return r;
 224
 225         r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", drop);
 226         if (r < 0)
 227                 return r;
 228
 229         return r;
 230 }
 231
 232 int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
 233         _cleanup_cap_free_ cap_t d = NULL;
 234         unsigned i, j = 0;
 235         int r;
 236
 237         /* Unfortunately we cannot leave privilege dropping to PID 1
 238          * here, since we want to run as user but want to keep some
 239          * capabilities. Since file capabilities have been introduced
 240          * this cannot be done across exec() anymore, unless our
 241          * binary has the capability configured in the file system,
 242          * which we want to avoid. */
 243
 244         if (setresgid(gid, gid, gid) < 0)
 245                 return log_error_errno(errno, "Failed to change group ID: %m");
 246
 247         if (setgroups(0, NULL) < 0)
 248                 return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");
 249
 250         /* Ensure we keep the permitted caps across the setresuid() */
 251         if (prctl(PR_SET_KEEPCAPS, 1) < 0)
 252                 return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");
 253
 254         r = setresuid(uid, uid, uid);
 255         if (r < 0)
 256                 return log_error_errno(errno, "Failed to change user ID: %m");
 257
 258         if (prctl(PR_SET_KEEPCAPS, 0) < 0)
 259                 return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");
 260
 261         /* Drop all caps from the bounding set, except the ones we want */
 262         r = capability_bounding_set_drop(~keep_capabilities, true);
 263         if (r < 0)
 264                 return log_error_errno(r, "Failed to drop capabilities: %m");
 265
 266         /* Now upgrade the permitted caps we still kept to effective caps */
 267         d = cap_init();
 268         if (!d)
 269                 return log_oom();
 270
 271         if (keep_capabilities) {
 272                 cap_value_t bits[u64log2(keep_capabilities) + 1];
 273
 274                 for (i = 0; i < ELEMENTSOF(bits); i++)
 275                         if (keep_capabilities & (1ULL << i))
 276                                 bits[j++] = i;
 277
 278                 /* use enough bits */
 279                 assert(i == 64 || (keep_capabilities >> i) == 0);
 280                 /* don't use too many bits */
 281                 assert(keep_capabilities & (1ULL << (i - 1)));
 282
 283                 if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
 284                     cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
 285                         log_error_errno(errno, "Failed to enable capabilities bits: %m");
 286                         return -errno;
 287                 }
 288
 289                 if (cap_set_proc(d) < 0)
 290                         return log_error_errno(errno, "Failed to increase capabilities: %m");
 291         }
 292
 293         return 0;
 294 }
 295
 296 int drop_capability(cap_value_t cv) {
 297         _cleanup_cap_free_ cap_t tmp_cap = NULL;
 298
 299         tmp_cap = cap_get_proc();
 300         if (!tmp_cap)
 301                 return -errno;
 302
 303         if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
 304             (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
 305             (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
 306                 return -errno;
 307
 308         if (cap_set_proc(tmp_cap) < 0)
 309                 return -errno;
 310
 311         return 0;
 312 }