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Subject[PATCH v8 2/2] samples: add an example of seccomp user trap
Date
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>
---
v5: new in v5
v7: updates for v7 API changes
v8: * add some more comments about what's happening in main() (Kees)
* move from ptrace API to SECCOMP_FILTER_FLAG_NEW_LISTENER
---
samples/seccomp/.gitignore | 1 +
samples/seccomp/Makefile | 7 +-
samples/seccomp/user-trap.c | 345 ++++++++++++++++++++++++++++++++++++
3 files changed, 352 insertions(+), 1 deletion(-)

diff --git a/samples/seccomp/.gitignore b/samples/seccomp/.gitignore
index 78fb78184291..d1e2e817d556 100644
--- a/samples/seccomp/.gitignore
+++ b/samples/seccomp/.gitignore
@@ -1,3 +1,4 @@
bpf-direct
bpf-fancy
dropper
+user-trap
diff --git a/samples/seccomp/Makefile b/samples/seccomp/Makefile
index cf34ff6b4065..4920903c8009 100644
--- a/samples/seccomp/Makefile
+++ b/samples/seccomp/Makefile
@@ -1,6 +1,6 @@
# 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
diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
new file mode 100644
index 000000000000..bba7ac803c6c
--- /dev/null
+++ b/samples/seccomp/user-trap.c
@@ -0,0 +1,345 @@
+#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 mknod? %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", 4) && !strncmp(target, "/tmp", 4)) {
+ 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) {
+ while (1) {
+ struct seccomp_notif req = {};
+ struct seccomp_notif_resp resp = {};
+
+ if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, &req)) {
+ perror("ioctl recv");
+ goto out_close;
+ }
+
+ if (handle_req(&req, &resp, listener) < 0)
+ goto out_close;
+
+ if (ioctl(listener, SECCOMP_IOCTL_NOTIF_SEND, &resp)) {
+ perror("ioctl send");
+ goto out_close;
+ }
+ }
+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;
+}
--
2.17.1
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