samples: add an example of seccomp user trap

The idea here is just to give a demonstration of how one could safely use
the SECCOMP_RET_USER_NOTIF feature to do mount policies. This particular
policy is (as noted in the comment) not very interesting, but it serves to
illustrate how one might apply a policy dodging the various TOCTOU issues.
Signed-off-by: Tycho Andersen <tycho@tycho.ws>
CC: Kees Cook <keescook@chromium.org>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <ebiederm@xmission.com>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian@brauner.io>
CC: Tyler Hicks <tyhicks@canonical.com>
CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
Signed-off-by: Kees Cook <keescook@chromium.org>

samples/seccomp/.gitignore

 bpf-direct
 bpf-fancy
 dropper
+user-trap

samples/seccomp/Makefile

 # SPDX-License-Identifier: GPL-2.0
 ifndef CROSS_COMPILE
-hostprogs-$(CONFIG_SAMPLE_SECCOMP) := bpf-fancy dropper bpf-direct
+hostprogs-$(CONFIG_SAMPLE_SECCOMP) := bpf-fancy dropper bpf-direct user-trap
 
 HOSTCFLAGS_bpf-fancy.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-fancy.o += -idirafter $(objtree)/include
@@ -16,6 +16,10 @@ HOSTCFLAGS_bpf-direct.o += -I$(objtree)/usr/include
 HOSTCFLAGS_bpf-direct.o += -idirafter $(objtree)/include
 bpf-direct-objs := bpf-direct.o
 
+HOSTCFLAGS_user-trap.o += -I$(objtree)/usr/include
+HOSTCFLAGS_user-trap.o += -idirafter $(objtree)/include
+user-trap-objs := user-trap.o
+
 # Try to match the kernel target.
 ifndef CONFIG_64BIT
 
@@ -33,6 +37,7 @@ HOSTCFLAGS_bpf-fancy.o += $(MFLAG)
 HOSTLDLIBS_bpf-direct += $(MFLAG)
 HOSTLDLIBS_bpf-fancy += $(MFLAG)
 HOSTLDLIBS_dropper += $(MFLAG)
+HOSTLDLIBS_user-trap += $(MFLAG)
 endif
 always := $(hostprogs-m)
 endif

samples/seccomp/user-trap.c

+#include <signal.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <string.h>
+#include <stddef.h>
+#include <sys/sysmacros.h>
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+#include <sys/syscall.h>
+#include <sys/user.h>
+#include <sys/ioctl.h>
+#include <sys/ptrace.h>
+#include <sys/mount.h>
+#include <linux/limits.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+
+#define ARRAY_SIZE(x) (sizeof(x) / sizeof(*(x)))
+
+static int seccomp(unsigned int op, unsigned int flags, void *args)
+{
+	errno = 0;
+	return syscall(__NR_seccomp, op, flags, args);
+}
+
+static int send_fd(int sock, int fd)
+{
+	struct msghdr msg = {};
+	struct cmsghdr *cmsg;
+	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
+	struct iovec io = {
+		.iov_base = &c,
+		.iov_len = 1,
+	};
+
+	msg.msg_iov = &io;
+	msg.msg_iovlen = 1;
+	msg.msg_control = buf;
+	msg.msg_controllen = sizeof(buf);
+	cmsg = CMSG_FIRSTHDR(&msg);
+	cmsg->cmsg_level = SOL_SOCKET;
+	cmsg->cmsg_type = SCM_RIGHTS;
+	cmsg->cmsg_len = CMSG_LEN(sizeof(int));
+	*((int *)CMSG_DATA(cmsg)) = fd;
+	msg.msg_controllen = cmsg->cmsg_len;
+
+	if (sendmsg(sock, &msg, 0) < 0) {
+		perror("sendmsg");
+		return -1;
+	}
+
+	return 0;
+}
+
+static int recv_fd(int sock)
+{
+	struct msghdr msg = {};
+	struct cmsghdr *cmsg;
+	char buf[CMSG_SPACE(sizeof(int))] = {0}, c = 'c';
+	struct iovec io = {
+		.iov_base = &c,
+		.iov_len = 1,
+	};
+
+	msg.msg_iov = &io;
+	msg.msg_iovlen = 1;
+	msg.msg_control = buf;
+	msg.msg_controllen = sizeof(buf);
+
+	if (recvmsg(sock, &msg, 0) < 0) {
+		perror("recvmsg");
+		return -1;
+	}
+
+	cmsg = CMSG_FIRSTHDR(&msg);
+
+	return *((int *)CMSG_DATA(cmsg));
+}
+
+static int user_trap_syscall(int nr, unsigned int flags)
+{
+	struct sock_filter filter[] = {
+		BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
+			offsetof(struct seccomp_data, nr)),
+		BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_USER_NOTIF),
+		BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+	};
+
+	struct sock_fprog prog = {
+		.len = (unsigned short)ARRAY_SIZE(filter),
+		.filter = filter,
+	};
+
+	return seccomp(SECCOMP_SET_MODE_FILTER, flags, &prog);
+}
+
+static int handle_req(struct seccomp_notif *req,
+		      struct seccomp_notif_resp *resp, int listener)
+{
+	char path[PATH_MAX], source[PATH_MAX], target[PATH_MAX];
+	int ret = -1, mem;
+
+	resp->id = req->id;
+	resp->error = -EPERM;
+	resp->val = 0;
+
+	if (req->data.nr != __NR_mount) {
+		fprintf(stderr, "huh? trapped something besides mount? %d\n", req->data.nr);
+		return -1;
+	}
+
+	/* Only allow bind mounts. */
+	if (!(req->data.args[3] & MS_BIND))
+		return 0;
+
+	/*
+	 * Ok, let's read the task's memory to see where they wanted their
+	 * mount to go.
+	 */
+	snprintf(path, sizeof(path), "/proc/%d/mem", req->pid);
+	mem = open(path, O_RDONLY);
+	if (mem < 0) {
+		perror("open mem");
+		return -1;
+	}
+
+	/*
+	 * Now we avoid a TOCTOU: we referred to a pid by its pid, but since
+	 * the pid that made the syscall may have died, we need to confirm that
+	 * the pid is still valid after we open its /proc/pid/mem file. We can
+	 * ask the listener fd this as follows.
+	 *
+	 * Note that this check should occur *after* any task-specific
+	 * resources are opened, to make sure that the task has not died and
+	 * we're not wrongly reading someone else's state in order to make
+	 * decisions.
+	 */
+	if (ioctl(listener, SECCOMP_IOCTL_NOTIF_ID_VALID, &req->id) < 0) {
+		fprintf(stderr, "task died before we could map its memory\n");
+		goto out;
+	}
+
+	/*
+	 * Phew, we've got the right /proc/pid/mem. Now we can read it. Note
+	 * that to avoid another TOCTOU, we should read all of the pointer args
+	 * before we decide to allow the syscall.
+	 */
+	if (lseek(mem, req->data.args[0], SEEK_SET) < 0) {
+		perror("seek");
+		goto out;
+	}
+
+	ret = read(mem, source, sizeof(source));
+	if (ret < 0) {
+		perror("read");
+		goto out;
+	}
+
+	if (lseek(mem, req->data.args[1], SEEK_SET) < 0) {
+		perror("seek");
+		goto out;
+	}
+
+	ret = read(mem, target, sizeof(target));
+	if (ret < 0) {
+		perror("read");
+		goto out;
+	}
+
+	/*
+	 * Our policy is to only allow bind mounts inside /tmp. This isn't very
+	 * interesting, because we could do unprivlieged bind mounts with user
+	 * namespaces already, but you get the idea.
+	 */
+	if (!strncmp(source, "/tmp/", 5) && !strncmp(target, "/tmp/", 5)) {
+		if (mount(source, target, NULL, req->data.args[3], NULL) < 0) {
+			ret = -1;
+			perror("actual mount");
+			goto out;
+		}
+		resp->error = 0;
+	}
+
+	/* Even if we didn't allow it because of policy, generating the
+	 * response was be a success, because we want to tell the worker EPERM.
+	 */
+	ret = 0;
+
+out:
+	close(mem);
+	return ret;
+}
+
+int main(void)
+{
+	int sk_pair[2], ret = 1, status, listener;
+	pid_t worker = 0 , tracer = 0;
+
+	if (socketpair(PF_LOCAL, SOCK_SEQPACKET, 0, sk_pair) < 0) {
+		perror("socketpair");
+		return 1;
+	}
+
+	worker = fork();
+	if (worker < 0) {
+		perror("fork");
+		goto close_pair;
+	}
+
+	if (worker == 0) {
+		listener = user_trap_syscall(__NR_mount,
+					     SECCOMP_FILTER_FLAG_NEW_LISTENER);
+		if (listener < 0) {
+			perror("seccomp");
+			exit(1);
+		}
+
+		/*
+		 * Drop privileges. We definitely can't mount as uid 1000.
+		 */
+		if (setuid(1000) < 0) {
+			perror("setuid");
+			exit(1);
+		}
+
+		/*
+		 * Send the listener to the parent; also serves as
+		 * synchronization.
+		 */
+		if (send_fd(sk_pair[1], listener) < 0)
+			exit(1);
+		close(listener);
+
+		if (mkdir("/tmp/foo", 0755) < 0) {
+			perror("mkdir");
+			exit(1);
+		}
+
+		/*
+		 * Try a bad mount just for grins.
+		 */
+		if (mount("/dev/sda", "/tmp/foo", NULL, 0, NULL) != -1) {
+			fprintf(stderr, "huh? mounted /dev/sda?\n");
+			exit(1);
+		}
+
+		if (errno != EPERM) {
+			perror("bad error from mount");
+			exit(1);
+		}
+
+		/*
+		 * Ok, we expect this one to succeed.
+		 */
+		if (mount("/tmp/foo", "/tmp/foo", NULL, MS_BIND, NULL) < 0) {
+			perror("mount");
+			exit(1);
+		}
+
+		exit(0);
+	}
+
+	/*
+	 * Get the listener from the child.
+	 */
+	listener = recv_fd(sk_pair[0]);
+	if (listener < 0)
+		goto out_kill;
+
+	/*
+	 * Fork a task to handle the requests. This isn't strictly necessary,
+	 * but it makes the particular writing of this sample easier, since we
+	 * can just wait ofr the tracee to exit and kill the tracer.
+	 */
+	tracer = fork();
+	if (tracer < 0) {
+		perror("fork");
+		goto out_kill;
+	}
+
+	if (tracer == 0) {
+		struct seccomp_notif *req;
+		struct seccomp_notif_resp *resp;
+		struct seccomp_notif_sizes sizes;
+
+		if (seccomp(SECCOMP_GET_NOTIF_SIZES, 0, &sizes) < 0) {
+			perror("seccomp(GET_NOTIF_SIZES)");
+			goto out_close;
+		}
+
+		req = malloc(sizes.seccomp_notif);
+		if (!req)
+			goto out_close;
+		memset(req, 0, sizeof(*req));
+
+		resp = malloc(sizes.seccomp_notif_resp);
+		if (!resp)
+			goto out_req;
+		memset(resp, 0, sizeof(*resp));
+
+		while (1) {
+			if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
+				perror("ioctl recv");
+				goto out_resp;
+			}
+
+			if (handle_req(req, resp, listener) < 0)
+				goto out_resp;
+
+			/*
+			 * ENOENT here means that the task may have gotten a
+			 * signal and restarted the syscall. It's up to the
+			 * handler to decide what to do in this case, but for
+			 * the sample code, we just ignore it. Probably
+			 * something better should happen, like undoing the
+			 * mount, or keeping track of the args to make sure we
+			 * don't do it again.
+			 */
+			if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, resp) < 0 &&
+			    errno != ENOENT) {
+				perror("ioctl send");
+				goto out_resp;
+			}
+		}
+out_resp:
+		free(resp);
+out_req:
+		free(req);
+out_close:
+		close(listener);
+		exit(1);
+	}
+
+	close(listener);
+
+	if (waitpid(worker, &status, 0) != worker) {
+		perror("waitpid");
+		goto out_kill;
+	}
+
+	if (umount2("/tmp/foo", MNT_DETACH) < 0 && errno != EINVAL) {
+		perror("umount2");
+		goto out_kill;
+	}
+
+	if (remove("/tmp/foo") < 0 && errno != ENOENT) {
+		perror("remove");
+		exit(1);
+	}
+
+	if (!WIFEXITED(status) || WEXITSTATUS(status)) {
+		fprintf(stderr, "worker exited nonzero\n");
+		goto out_kill;
+	}
+
+	ret = 0;
+
+out_kill:
+	if (tracer > 0)
+		kill(tracer, SIGKILL);
+	if (worker > 0)
+		kill(worker, SIGKILL);
+
+close_pair:
+	close(sk_pair[0]);
+	close(sk_pair[1]);
+	return ret;
+}