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