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
|
/*
* Copyright (c) 2012 Will Drewry <wad@dataspill.org>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/*
* Uncomment the SANDBOX_SECCOMP_FILTER_DEBUG macro below to help diagnose
* filter breakage during development. *Do not* use this in production,
* as it relies on making library calls that are unsafe in signal context.
*
* Instead, live systems the auditctl(8) may be used to monitor failures.
* E.g.
* auditctl -a task,always -F uid=<privsep uid>
*/
/* #define SANDBOX_SECCOMP_FILTER_DEBUG 1 */
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
/* Use the kernel headers in case of an older toolchain. */
# include <asm/siginfo.h>
# define __have_siginfo_t 1
# define __have_sigval_t 1
# define __have_sigevent_t 1
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
#include "includes.h"
#ifdef SANDBOX_SECCOMP_FILTER
#include <sys/types.h>
#include <sys/resource.h>
#include <sys/prctl.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <elf.h>
#include <asm/unistd.h>
#include <errno.h>
#include <signal.h>
#include <stdarg.h>
#include <stddef.h> /* for offsetof */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include "log.h"
#include "ssh-sandbox.h"
#include "xmalloc.h"
/* Linux seccomp_filter sandbox */
#define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
/* Use a signal handler to emit violations when debugging */
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
# undef SECCOMP_FILTER_FAIL
# define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
/* Simple helpers to avoid manual errors (but larger BPF programs). */
#define SC_DENY(_nr, _errno) \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
#define SC_ALLOW(_nr) \
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
/* Syscall filtering set for preauth. */
static const struct sock_filter preauth_insns[] = {
/* Ensure the syscall arch convention is as expected. */
BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
offsetof(struct seccomp_data, arch)),
BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
/* Load the syscall number for checking. */
BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
offsetof(struct seccomp_data, nr)),
SC_DENY(open, EACCES),
SC_ALLOW(getpid),
SC_ALLOW(gettimeofday),
SC_ALLOW(clock_gettime),
#ifdef __NR_time /* not defined on EABI ARM */
SC_ALLOW(time),
#endif
SC_ALLOW(read),
SC_ALLOW(write),
SC_ALLOW(close),
SC_ALLOW(shutdown),
SC_ALLOW(brk),
SC_ALLOW(poll),
#ifdef __NR__newselect
SC_ALLOW(_newselect),
#else
SC_ALLOW(select),
#endif
SC_ALLOW(madvise),
#ifdef __NR_mmap2 /* EABI ARM only has mmap2() */
SC_ALLOW(mmap2),
#endif
#ifdef __NR_mmap
SC_ALLOW(mmap),
#endif
SC_ALLOW(munmap),
SC_ALLOW(exit_group),
#ifdef __NR_rt_sigprocmask
SC_ALLOW(rt_sigprocmask),
#else
SC_ALLOW(sigprocmask),
#endif
BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
};
static const struct sock_fprog preauth_program = {
.len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
.filter = (struct sock_filter *)preauth_insns,
};
struct ssh_sandbox {
pid_t child_pid;
};
struct ssh_sandbox *
ssh_sandbox_init(struct monitor *monitor)
{
struct ssh_sandbox *box;
/*
* Strictly, we don't need to maintain any state here but we need
* to return non-NULL to satisfy the API.
*/
debug3("%s: preparing seccomp filter sandbox", __func__);
box = xcalloc(1, sizeof(*box));
box->child_pid = 0;
return box;
}
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
extern struct monitor *pmonitor;
void mm_log_handler(LogLevel level, const char *msg, void *ctx);
static void
ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
{
char msg[256];
snprintf(msg, sizeof(msg),
"%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
__func__, info->si_arch, info->si_syscall, info->si_call_addr);
mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
_exit(1);
}
static void
ssh_sandbox_child_debugging(void)
{
struct sigaction act;
sigset_t mask;
debug3("%s: installing SIGSYS handler", __func__);
memset(&act, 0, sizeof(act));
sigemptyset(&mask);
sigaddset(&mask, SIGSYS);
act.sa_sigaction = &ssh_sandbox_violation;
act.sa_flags = SA_SIGINFO;
if (sigaction(SIGSYS, &act, NULL) == -1)
fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
fatal("%s: sigprocmask(SIGSYS): %s",
__func__, strerror(errno));
}
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
void
ssh_sandbox_child(struct ssh_sandbox *box)
{
struct rlimit rl_zero;
int nnp_failed = 0;
/* Set rlimits for completeness if possible. */
rl_zero.rlim_cur = rl_zero.rlim_max = 0;
if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
__func__, strerror(errno));
if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
__func__, strerror(errno));
if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
__func__, strerror(errno));
#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
ssh_sandbox_child_debugging();
#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
__func__, strerror(errno));
nnp_failed = 1;
}
debug3("%s: attaching seccomp filter program", __func__);
if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
debug("%s: prctl(PR_SET_SECCOMP): %s",
__func__, strerror(errno));
else if (nnp_failed)
fatal("%s: SECCOMP_MODE_FILTER activated but "
"PR_SET_NO_NEW_PRIVS failed", __func__);
}
void
ssh_sandbox_parent_finish(struct ssh_sandbox *box)
{
free(box);
debug3("%s: finished", __func__);
}
void
ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
{
box->child_pid = child_pid;
}
#endif /* SANDBOX_SECCOMP_FILTER */
|
