On Tue, Dec 11, 2018 at 2:24 PM Serge E. Hallyn [off-list ref] wrote:

On Sun, Dec 09, 2018 at 11:24:14AM -0700, Tycho Andersen wrote:

quoted

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 <redacted>
CC: Kees Cook <redacted>
CC: Andy Lutomirski <luto@amacapital.net>
CC: Oleg Nesterov <oleg@redhat.com>
CC: Eric W. Biederman <redacted>
CC: "Serge E. Hallyn" <serge@hallyn.com>
CC: Christian Brauner <christian@brauner.io>
CC: Tyler Hicks <redacted>
CC: Akihiro Suda <redacted>
---
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