summaryrefslogtreecommitdiff
path: root/src/shared/capability.c
blob: 57f392a4da887228c616577f63c8b37ca8ad03b5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
/*-*- Mode: C; c-basic-offset: 8; indent-tabs-mode: nil -*-*/

/***
  This file is part of systemd.

  Copyright 2010 Lennart Poettering

  systemd is free software; you can redistribute it and/or modify it
  under the terms of the GNU Lesser General Public License as published by
  the Free Software Foundation; either version 2.1 of the License, or
  (at your option) any later version.

  systemd is distributed in the hope that it will be useful, but
  WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public License
  along with systemd; If not, see <http://www.gnu.org/licenses/>.
***/

#include <assert.h>
#include <string.h>
#include <unistd.h>
#include <errno.h>
#include <stdio.h>
#include <sys/types.h>
#include <stdarg.h>
#include <ctype.h>
#include <sys/capability.h>
#include <sys/prctl.h>
#include "grp.h"

#include "macro.h"
#include "util.h"
#include "log.h"
#include "fileio.h"
#include "capability.h"

int have_effective_cap(int value) {
        _cleanup_cap_free_ cap_t cap;
        cap_flag_value_t fv;

        cap = cap_get_proc();
        if (!cap)
                return -errno;

        if (cap_get_flag(cap, value, CAP_EFFECTIVE, &fv) < 0)
                return -errno;
        else
                return fv == CAP_SET;
}

unsigned long cap_last_cap(void) {
        static thread_local unsigned long saved;
        static thread_local bool valid = false;
        _cleanup_free_ char *content = NULL;
        unsigned long p;
        int r;

        if (valid)
                return saved;

        /* available since linux-3.2 */
        r = read_one_line_file("/proc/sys/kernel/cap_last_cap", &content);
        if (r >= 0) {
                r = safe_atolu(content, &p);
                if (r >= 0) {
                        saved = p;
                        valid = true;
                        return p;
                }
        }

        /* fall back to syscall-probing for pre linux-3.2 */
        p = (unsigned long) CAP_LAST_CAP;

        if (prctl(PR_CAPBSET_READ, p) < 0) {

                /* Hmm, look downwards, until we find one that
                 * works */
                for (p--; p > 0; p --)
                        if (prctl(PR_CAPBSET_READ, p) >= 0)
                                break;

        } else {

                /* Hmm, look upwards, until we find one that doesn't
                 * work */
                for (;; p++)
                        if (prctl(PR_CAPBSET_READ, p+1) < 0)
                                break;
        }

        saved = p;
        valid = true;

        return p;
}

int capability_bounding_set_drop(uint64_t drop, bool right_now) {
        _cleanup_cap_free_ cap_t after_cap = NULL;
        cap_flag_value_t fv;
        unsigned long i;
        int r;

        /* If we are run as PID 1 we will lack CAP_SETPCAP by default
         * in the effective set (yes, the kernel drops that when
         * executing init!), so get it back temporarily so that we can
         * call PR_CAPBSET_DROP. */

        after_cap = cap_get_proc();
        if (!after_cap)
                return -errno;

        if (cap_get_flag(after_cap, CAP_SETPCAP, CAP_EFFECTIVE, &fv) < 0)
                return -errno;

        if (fv != CAP_SET) {
                _cleanup_cap_free_ cap_t temp_cap = NULL;
                static const cap_value_t v = CAP_SETPCAP;

                temp_cap = cap_dup(after_cap);
                if (!temp_cap) {
                        r = -errno;
                        goto finish;
                }

                if (cap_set_flag(temp_cap, CAP_EFFECTIVE, 1, &v, CAP_SET) < 0) {
                        r = -errno;
                        goto finish;
                }

                if (cap_set_proc(temp_cap) < 0) {
                        r = -errno;
                        goto finish;
                }
        }

        for (i = 0; i <= cap_last_cap(); i++) {

                if (drop & ((uint64_t) 1ULL << (uint64_t) i)) {
                        cap_value_t v;

                        /* Drop it from the bounding set */
                        if (prctl(PR_CAPBSET_DROP, i) < 0) {
                                r = -errno;
                                goto finish;
                        }
                        v = (cap_value_t) i;

                        /* Also drop it from the inheritable set, so
                         * that anything we exec() loses the
                         * capability for good. */
                        if (cap_set_flag(after_cap, CAP_INHERITABLE, 1, &v, CAP_CLEAR) < 0) {
                                r = -errno;
                                goto finish;
                        }

                        /* If we shall apply this right now drop it
                         * also from our own capability sets. */
                        if (right_now) {
                                if (cap_set_flag(after_cap, CAP_PERMITTED, 1, &v, CAP_CLEAR) < 0 ||
                                    cap_set_flag(after_cap, CAP_EFFECTIVE, 1, &v, CAP_CLEAR) < 0) {
                                        r = -errno;
                                        goto finish;
                                }
                        }
                }
        }

        r = 0;

finish:
        if (cap_set_proc(after_cap) < 0)
                return -errno;

        return r;
}

static int drop_from_file(const char *fn, uint64_t drop) {
        int r, k;
        uint32_t hi, lo;
        uint64_t current, after;
        char *p;

        r = read_one_line_file(fn, &p);
        if (r < 0)
                return r;

        assert_cc(sizeof(hi) == sizeof(unsigned));
        assert_cc(sizeof(lo) == sizeof(unsigned));

        k = sscanf(p, "%u %u", &lo, &hi);
        free(p);

        if (k != 2)
                return -EIO;

        current = (uint64_t) lo | ((uint64_t) hi << 32ULL);
        after = current & ~drop;

        if (current == after)
                return 0;

        lo = (unsigned) (after & 0xFFFFFFFFULL);
        hi = (unsigned) ((after >> 32ULL) & 0xFFFFFFFFULL);

        if (asprintf(&p, "%u %u", lo, hi) < 0)
                return -ENOMEM;

        r = write_string_file(fn, p);
        free(p);

        return r;
}

int capability_bounding_set_drop_usermode(uint64_t drop) {
        int r;

        r = drop_from_file("/proc/sys/kernel/usermodehelper/inheritable", drop);
        if (r < 0)
                return r;

        r = drop_from_file("/proc/sys/kernel/usermodehelper/bset", drop);
        if (r < 0)
                return r;

        return r;
}

int drop_privileges(uid_t uid, gid_t gid, uint64_t keep_capabilities) {
        _cleanup_cap_free_ cap_t d = NULL;
        unsigned i, j = 0;
        int r;

        /* Unfortunately we cannot leave privilege dropping to PID 1
         * here, since we want to run as user but want to keep some
         * capabilities. Since file capabilities have been introduced
         * this cannot be done across exec() anymore, unless our
         * binary has the capability configured in the file system,
         * which we want to avoid. */

        if (setresgid(gid, gid, gid) < 0)
                return log_error_errno(errno, "Failed to change group ID: %m");

        if (setgroups(0, NULL) < 0)
                return log_error_errno(errno, "Failed to drop auxiliary groups list: %m");

        /* Ensure we keep the permitted caps across the setresuid() */
        if (prctl(PR_SET_KEEPCAPS, 1) < 0)
                return log_error_errno(errno, "Failed to enable keep capabilities flag: %m");

        r = setresuid(uid, uid, uid);
        if (r < 0)
                return log_error_errno(errno, "Failed to change user ID: %m");

        if (prctl(PR_SET_KEEPCAPS, 0) < 0)
                return log_error_errno(errno, "Failed to disable keep capabilities flag: %m");

        /* Drop all caps from the bounding set, except the ones we want */
        r = capability_bounding_set_drop(~keep_capabilities, true);
        if (r < 0)
                return log_error_errno(r, "Failed to drop capabilities: %m");

        /* Now upgrade the permitted caps we still kept to effective caps */
        d = cap_init();
        if (!d)
                return log_oom();

        if (keep_capabilities) {
                cap_value_t bits[u64log2(keep_capabilities) + 1];

                for (i = 0; i < ELEMENTSOF(bits); i++)
                        if (keep_capabilities & (1ULL << i))
                                bits[j++] = i;

                /* don't keep too many bits */
                assert((keep_capabilities & (~1ULL << i)) == 0);
                /* don't throw away too many bits */
                assert(((keep_capabilities >> i) & (~1ULL >> i)) == 0);

                if (cap_set_flag(d, CAP_EFFECTIVE, j, bits, CAP_SET) < 0 ||
                    cap_set_flag(d, CAP_PERMITTED, j, bits, CAP_SET) < 0) {
                        log_error_errno(errno, "Failed to enable capabilities bits: %m");
                        return -errno;
                }

                if (cap_set_proc(d) < 0)
                        return log_error_errno(errno, "Failed to increase capabilities: %m");
        }

        return 0;
}

int drop_capability(cap_value_t cv) {
        _cleanup_cap_free_ cap_t tmp_cap = NULL;

        tmp_cap = cap_get_proc();
        if (!tmp_cap)
                return -errno;

        if ((cap_set_flag(tmp_cap, CAP_INHERITABLE, 1, &cv, CAP_CLEAR) < 0) ||
            (cap_set_flag(tmp_cap, CAP_PERMITTED, 1, &cv, CAP_CLEAR) < 0) ||
            (cap_set_flag(tmp_cap, CAP_EFFECTIVE, 1, &cv, CAP_CLEAR) < 0))
                return -errno;

        if (cap_set_proc(tmp_cap) < 0)
                return -errno;

        return 0;
}