Re: [PATCH v7 6/6] samples: add an example of seccomp user trap

Kees Cook
2018-09-27T22:11:36Z

On Thu, Sep 27, 2018 at 8:11 AM, Tycho Andersen <tycho@tycho.ws> wrote:
> 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.
>
> v5: new in v5
> v7: updates for v7 API changes
>
> 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@ubuntu.com>
> CC: Tyler Hicks <tyhicks@canonical.com>
> CC: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
> ---
> samples/seccomp/.gitignore | 1 +
> samples/seccomp/Makefile | 7 +-
> samples/seccomp/user-trap.c | 312 ++++++++++++++++++++++++++++++++++++
> 3 files changed, 319 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..63c9a5994dc1
> --- /dev/null
> +++ b/samples/seccomp/user-trap.c
> @@ -0,0 +1,312 @@
> +#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>
> +
> +/*
> + * Because of some grossness, we can't include linux/ptrace.h here, so we
> + * re-define PTRACE_SECCOMP_NEW_LISTENER.
> + */
> +#ifndef PTRACE_SECCOMP_NEW_LISTENER
> +#define PTRACE_SECCOMP_NEW_LISTENER 0x420e
> +#endif
> +
> +#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 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->len = sizeof(*resp);
> + 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_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;
> + char c;
> +
> + 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) {
> + if (user_trap_syscall(__NR_mount, 0) < 0) {
> + perror("seccomp");
> + exit(1);
> + }
> +
> + if (setuid(1000) < 0) {
> + perror("setuid");
> + exit(1);
> + }
> +
> + if (write(sk_pair[1], "a", 1) != 1) {
> + perror("write");
> + exit(1);
> + }
> +
> + if (read(sk_pair[1], &c, 1) != 1) {
> + perror("write");
> + exit(1);
> + }
> +
> + if (mkdir("/tmp/foo", 0755) < 0) {
> + perror("mkdir");
> + exit(1);
> + }
> +
> + 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);
> + }
> +
> + if (mount("/tmp/foo", "/tmp/foo", NULL, MS_BIND, NULL) < 0) {
> + perror("mount");
> + exit(1);
> + }
> +
> + exit(0);
> + }
> +
> + if (read(sk_pair[0], &c, 1) != 1) {
> + perror("read ready signal");
> + goto out_kill;
> + }
> +
> + if (ptrace(PTRACE_ATTACH, worker) < 0) {
> + perror("ptrace");
> + goto out_kill;
> + }
> +
> + if (waitpid(worker, NULL, 0) != worker) {
> + perror("waitpid");
> + goto out_kill;
> + }
> +
> + listener = ptrace(PTRACE_SECCOMP_NEW_LISTENER, worker, 0);
> + if (listener < 0) {
> + perror("ptrace get listener");
> + goto out_kill;
> + }
> +
> + if (ptrace(PTRACE_DETACH, worker, NULL, 0) < 0) {
> + perror("ptrace detach");
> + goto out_kill;
> + }
> +
> + if (write(sk_pair[0], "a", 1) != 1) {
> + perror("write");
> + exit(1);
> + }
> +
> + tracer = fork();
> + if (tracer < 0) {
> + perror("fork");
> + goto out_kill;
> + }
> +
> + if (tracer == 0) {
> + while (1) {
> + struct seccomp_notif req = {};
> + struct seccomp_notif_resp resp = {};
> +
> + req.len = sizeof(req);
> + if (ioctl(listener, SECCOMP_NOTIF_RECV, &req) != sizeof(req)) {
> + perror("ioctl recv");
> + goto out_close;
> + }
> +
> + if (handle_req(&req, &resp, listener) < 0)
> + goto out_close;
> +
> + if (ioctl(listener, SECCOMP_NOTIF_SEND, &resp) != sizeof(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
>

handle_req() is well commented, but main() isn't. Since this is
explicitly a "sample", can you add operational comments to main() as
well? I think it might help people follow what is happening (and what
is expected) during main().

Beyond that, yay! Samples! :)

-Kees

--
Kees Cook
Pixel Security