328 lines
8.3 KiB
C
328 lines
8.3 KiB
C
/*
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* Copyright (c) 2012 Will Drewry <wad@dataspill.org>
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*
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* Permission to use, copy, modify, and distribute this software for any
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* purpose with or without fee is hereby granted, provided that the above
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* copyright notice and this permission notice appear in all copies.
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*
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* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
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* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
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* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
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* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
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* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
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* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
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* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
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*/
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/*
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* Uncomment the SANDBOX_SECCOMP_FILTER_DEBUG macro below to help diagnose
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* filter breakage during development. *Do not* use this in production,
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* as it relies on making library calls that are unsafe in signal context.
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*
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* Instead, live systems the auditctl(8) may be used to monitor failures.
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* E.g.
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* auditctl -a task,always -F uid=<privsep uid>
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*/
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/* #define SANDBOX_SECCOMP_FILTER_DEBUG 1 */
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/* XXX it should be possible to do logging via the log socket safely */
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#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
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/* Use the kernel headers in case of an older toolchain. */
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# include <asm/siginfo.h>
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# define __have_siginfo_t 1
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# define __have_sigval_t 1
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# define __have_sigevent_t 1
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#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
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#include "includes.h"
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#ifdef SANDBOX_SECCOMP_FILTER
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#include <sys/types.h>
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#include <sys/resource.h>
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#include <sys/prctl.h>
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#include <linux/net.h>
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#include <linux/audit.h>
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#include <linux/filter.h>
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#include <linux/seccomp.h>
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#include <elf.h>
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#include <asm/unistd.h>
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#include <errno.h>
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#include <signal.h>
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#include <stdarg.h>
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#include <stddef.h> /* for offsetof */
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#include <stdio.h>
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#include <stdlib.h>
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#include <string.h>
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#include <unistd.h>
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#include "log.h"
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#include "ssh-sandbox.h"
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#include "xmalloc.h"
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/* Linux seccomp_filter sandbox */
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#define SECCOMP_FILTER_FAIL SECCOMP_RET_KILL
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/* Use a signal handler to emit violations when debugging */
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#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
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# undef SECCOMP_FILTER_FAIL
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# define SECCOMP_FILTER_FAIL SECCOMP_RET_TRAP
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#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
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/* Simple helpers to avoid manual errors (but larger BPF programs). */
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#define SC_DENY(_nr, _errno) \
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ERRNO|(_errno))
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#define SC_ALLOW(_nr) \
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 1), \
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW)
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#define SC_ALLOW_ARG(_nr, _arg_nr, _arg_val) \
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_ ## _nr, 0, 4), \
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/* load first syscall argument */ \
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
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offsetof(struct seccomp_data, args[(_arg_nr)])), \
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, (_arg_val), 0, 1), \
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW), \
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/* reload syscall number; all rules expect it in accumulator */ \
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS, \
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offsetof(struct seccomp_data, nr))
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/* Syscall filtering set for preauth. */
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static const struct sock_filter preauth_insns[] = {
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/* Ensure the syscall arch convention is as expected. */
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
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offsetof(struct seccomp_data, arch)),
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BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SECCOMP_AUDIT_ARCH, 1, 0),
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
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/* Load the syscall number for checking. */
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BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
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offsetof(struct seccomp_data, nr)),
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/* Syscalls to non-fatally deny */
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#ifdef __NR_lstat
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SC_DENY(lstat, EACCES),
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#endif
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#ifdef __NR_lstat64
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SC_DENY(lstat64, EACCES),
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#endif
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#ifdef __NR_fstat
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SC_DENY(fstat, EACCES),
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#endif
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#ifdef __NR_fstat64
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SC_DENY(fstat64, EACCES),
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#endif
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#ifdef __NR_open
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SC_DENY(open, EACCES),
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#endif
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#ifdef __NR_openat
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SC_DENY(openat, EACCES),
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#endif
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#ifdef __NR_newfstatat
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SC_DENY(newfstatat, EACCES),
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#endif
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#ifdef __NR_stat
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SC_DENY(stat, EACCES),
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#endif
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#ifdef __NR_stat64
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SC_DENY(stat64, EACCES),
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#endif
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/* Syscalls to permit */
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#ifdef __NR_brk
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SC_ALLOW(brk),
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#endif
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#ifdef __NR_clock_gettime
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SC_ALLOW(clock_gettime),
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#endif
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#ifdef __NR_close
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SC_ALLOW(close),
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#endif
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#ifdef __NR_exit
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SC_ALLOW(exit),
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#endif
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#ifdef __NR_exit_group
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SC_ALLOW(exit_group),
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#endif
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#ifdef __NR_getpgid
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SC_ALLOW(getpgid),
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#endif
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#ifdef __NR_getpid
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SC_ALLOW(getpid),
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#endif
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#ifdef __NR_getrandom
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SC_ALLOW(getrandom),
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#endif
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#ifdef __NR_gettimeofday
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SC_ALLOW(gettimeofday),
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#endif
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#ifdef __NR_madvise
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SC_ALLOW(madvise),
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#endif
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#ifdef __NR_mmap
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SC_ALLOW(mmap),
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#endif
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#ifdef __NR_mmap2
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SC_ALLOW(mmap2),
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#endif
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#ifdef __NR_mremap
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SC_ALLOW(mremap),
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#endif
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#ifdef __NR_munmap
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SC_ALLOW(munmap),
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#endif
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#ifdef __NR__newselect
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SC_ALLOW(_newselect),
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#endif
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#ifdef __NR_poll
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SC_ALLOW(poll),
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#endif
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#ifdef __NR_pselect6
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SC_ALLOW(pselect6),
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#endif
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#ifdef __NR_read
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SC_ALLOW(read),
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#endif
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#ifdef __NR_rt_sigprocmask
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SC_ALLOW(rt_sigprocmask),
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#endif
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#ifdef __NR_select
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SC_ALLOW(select),
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#endif
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#ifdef __NR_shutdown
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SC_ALLOW(shutdown),
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#endif
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#ifdef __NR_sigprocmask
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SC_ALLOW(sigprocmask),
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#endif
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#ifdef __NR_time
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SC_ALLOW(time),
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#endif
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#ifdef __NR_write
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SC_ALLOW(write),
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#endif
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#ifdef __NR_socketcall
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SC_ALLOW_ARG(socketcall, 0, SYS_SHUTDOWN),
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#endif
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/* Default deny */
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BPF_STMT(BPF_RET+BPF_K, SECCOMP_FILTER_FAIL),
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};
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static const struct sock_fprog preauth_program = {
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.len = (unsigned short)(sizeof(preauth_insns)/sizeof(preauth_insns[0])),
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.filter = (struct sock_filter *)preauth_insns,
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};
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struct ssh_sandbox {
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pid_t child_pid;
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};
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struct ssh_sandbox *
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ssh_sandbox_init(struct monitor *monitor)
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{
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struct ssh_sandbox *box;
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/*
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* Strictly, we don't need to maintain any state here but we need
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* to return non-NULL to satisfy the API.
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*/
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debug3("%s: preparing seccomp filter sandbox", __func__);
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box = xcalloc(1, sizeof(*box));
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box->child_pid = 0;
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return box;
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}
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#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
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extern struct monitor *pmonitor;
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void mm_log_handler(LogLevel level, const char *msg, void *ctx);
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static void
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ssh_sandbox_violation(int signum, siginfo_t *info, void *void_context)
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{
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char msg[256];
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snprintf(msg, sizeof(msg),
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"%s: unexpected system call (arch:0x%x,syscall:%d @ %p)",
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__func__, info->si_arch, info->si_syscall, info->si_call_addr);
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mm_log_handler(SYSLOG_LEVEL_FATAL, msg, pmonitor);
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_exit(1);
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}
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static void
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ssh_sandbox_child_debugging(void)
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{
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struct sigaction act;
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sigset_t mask;
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debug3("%s: installing SIGSYS handler", __func__);
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memset(&act, 0, sizeof(act));
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sigemptyset(&mask);
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sigaddset(&mask, SIGSYS);
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act.sa_sigaction = &ssh_sandbox_violation;
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act.sa_flags = SA_SIGINFO;
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if (sigaction(SIGSYS, &act, NULL) == -1)
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fatal("%s: sigaction(SIGSYS): %s", __func__, strerror(errno));
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if (sigprocmask(SIG_UNBLOCK, &mask, NULL) == -1)
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fatal("%s: sigprocmask(SIGSYS): %s",
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__func__, strerror(errno));
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}
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#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
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void
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ssh_sandbox_child(struct ssh_sandbox *box)
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{
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struct rlimit rl_zero;
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int nnp_failed = 0;
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/* Set rlimits for completeness if possible. */
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rl_zero.rlim_cur = rl_zero.rlim_max = 0;
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if (setrlimit(RLIMIT_FSIZE, &rl_zero) == -1)
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fatal("%s: setrlimit(RLIMIT_FSIZE, { 0, 0 }): %s",
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__func__, strerror(errno));
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if (setrlimit(RLIMIT_NOFILE, &rl_zero) == -1)
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fatal("%s: setrlimit(RLIMIT_NOFILE, { 0, 0 }): %s",
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__func__, strerror(errno));
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if (setrlimit(RLIMIT_NPROC, &rl_zero) == -1)
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fatal("%s: setrlimit(RLIMIT_NPROC, { 0, 0 }): %s",
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__func__, strerror(errno));
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#ifdef SANDBOX_SECCOMP_FILTER_DEBUG
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ssh_sandbox_child_debugging();
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#endif /* SANDBOX_SECCOMP_FILTER_DEBUG */
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debug3("%s: setting PR_SET_NO_NEW_PRIVS", __func__);
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if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
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debug("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
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__func__, strerror(errno));
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nnp_failed = 1;
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}
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debug3("%s: attaching seccomp filter program", __func__);
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if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
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debug("%s: prctl(PR_SET_SECCOMP): %s",
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__func__, strerror(errno));
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else if (nnp_failed)
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fatal("%s: SECCOMP_MODE_FILTER activated but "
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"PR_SET_NO_NEW_PRIVS failed", __func__);
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}
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void
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ssh_sandbox_parent_finish(struct ssh_sandbox *box)
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{
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free(box);
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debug3("%s: finished", __func__);
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}
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void
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ssh_sandbox_parent_preauth(struct ssh_sandbox *box, pid_t child_pid)
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{
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box->child_pid = child_pid;
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}
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#endif /* SANDBOX_SECCOMP_FILTER */
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