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samples: add an example of seccomp user trap

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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>
This commit is contained in:
Tycho Andersen 2018-12-09 11:24:14 -07:00 committed by Kees Cook
parent 6a21cc50f0
commit fec7b66905
3 changed files with 382 additions and 1 deletions
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1
samples/seccomp/.gitignore vendored
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@ -1,3 +1,4 @@
bpf-direct
bpf-fancy
dropper
user-trap

7
samples/seccomp/Makefile
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@ -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

375
samples/seccomp/user-trap.c Normal file
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@ -0,0 +1,375 @@
#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;
}