- (djm) [Makefile.in configure.ac sandbox-seccomp-filter.c] Add sandbox
mode for Linux's new seccomp filter; patch from Will Drewry; feedback
and ok dtucker@
diff --git a/sandbox-seccomp-filter.c b/sandbox-seccomp-filter.c
new file mode 100644
index 0000000..6868129
--- /dev/null
+++ b/sandbox-seccomp-filter.c
@@ -0,0 +1,222 @@
+/*
+ * 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 <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(time),
+ SC_ALLOW(read),
+ SC_ALLOW(write),
+ SC_ALLOW(close),
+ SC_ALLOW(brk),
+ SC_ALLOW(poll),
+#ifdef __NR__newselect
+ SC_ALLOW(_newselect),
+#else
+ SC_ALLOW(select),
+#endif
+ SC_ALLOW(madvise),
+ SC_ALLOW(mmap),
+ 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(void)
+{
+ 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;
+
+ /* 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)
+ fatal("%s: prctl(PR_SET_NO_NEW_PRIVS): %s",
+ __func__, strerror(errno));
+ debug3("%s: attaching seccomp filter program", __func__);
+ if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &preauth_program) == -1)
+ fatal("%s: prctl(PR_SET_SECCOMP): %s",
+ __func__, strerror(errno));
+}
+
+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 */