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Date: Tue, 11 Dec 2018 16:46:52 -0800
From: Kees Cook <keescook@...omium.org>
To: "Serge E. Hallyn" <serge@...lyn.com>
Cc: Tycho Andersen <tycho@...ho.ws>,
Andy Lutomirski <luto@...capital.net>,
Oleg Nesterov <oleg@...hat.com>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
Christian Brauner <christian@...uner.io>,
Tyler Hicks <tyhicks@...onical.com>,
Akihiro Suda <suda.akihiro@....ntt.co.jp>,
Aleksa Sarai <asarai@...e.de>, Jann Horn <jannh@...gle.com>,
LKML <linux-kernel@...r.kernel.org>,
Linux Containers <containers@...ts.linux-foundation.org>,
Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH v10 4/4] samples: add an example of seccomp user trap
On Tue, Dec 11, 2018 at 2:24 PM Serge E. Hallyn <serge@...lyn.com> wrote:
>
> On Sun, Dec 09, 2018 at 11:24:14AM -0700, Tycho Andersen 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.
> >
> > Signed-off-by: Tycho Andersen <tycho@...ho.ws>
> > CC: Kees Cook <keescook@...omium.org>
> > CC: Andy Lutomirski <luto@...capital.net>
> > CC: Oleg Nesterov <oleg@...hat.com>
> > CC: Eric W. Biederman <ebiederm@...ssion.com>
> > CC: "Serge E. Hallyn" <serge@...lyn.com>
> > CC: Christian Brauner <christian@...uner.io>
> > CC: Tyler Hicks <tyhicks@...onical.com>
> > CC: Akihiro Suda <suda.akihiro@....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
> > v9: * s/mknod/mount in error message
> > * switch to the SECCOMP_GET_NOTIF_SIZES API
> > * add a note about getting ENOENT from SECCOMP_IOCTL_NOTIF_SEND
> > ---
> > samples/seccomp/.gitignore | 1 +
> > samples/seccomp/Makefile | 7 +-
> > samples/seccomp/user-trap.c | 375 ++++++++++++++++++++++++++++++++++++
> > 3 files changed, 382 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..61267cb59c8e
> > --- /dev/null
> > +++ b/samples/seccomp/user-trap.c
> > @@ -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", 4) && !strncmp(target, "/tmp", 4)) {
>
> Of course the host could have a /tmproot or somesuch, so you might want to
> check the first 5 characters :)
I manually bumped this to "/tmp/", 5 in both tests and applied it for -next
-Kees
--
Kees Cook
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