Re: [PATCH 0/3] namei: implement various scoping AT_* flags

On 9/30/18 23:46, Jann Horn wrote:

On Sun, Sep 30, 2018 at 10:39 PM Mickaël Salaün [off-list ref] wrote:

quoted

As a side note, I'm still working on Landlock which can achieve the same
goal but in a more flexible and dynamic way: https://landlock.io

Isn't Landlock mostly intended for userspace that wants to impose a
custom Mandatory Access Control policy on itself, restricting the
whole process?

As far as I can tell, a major usecase for AT_BENEATH are privileged
processes that do not want to restrict all filesystem operations they
perform, but want to sometimes impose limits on filesystem traversal
for the duration of a single system call. For example, a process might
want to first open a file from an untrusted filesystem area with
AT_BENEATH, and afterwards open a configuration file without
AT_BENEATH.

I didn't realized this was the main use case for AT_BENEATH. Landlock is
indeed dedicated to apply a security policy on a set of processes. This
set can be a process and its children (seccomp-like), or another set of
processes that may be identified with a cgroup.

How would you do this in Landlock? Use a BPF map to store per-thread
filesystem restrictions, and then do bpf() calls before and after
every restricted filesystem access to set and unset the policy for the
current syscall?

Another way to apply a security policy could be to tied it to a file
descriptor, similarly to Capsicum, which could enable to create
programmable (real) capabilities. This way, it would be possible to
"wrap" a file descriptor with a Landlock program and use it with
FD-based syscalls or pass it to other processes. This would not require
changes to the FS subsystem, but only the Landlock LSM code. This isn't
done yet but I plan to add this new way to restrict operations on file
descriptors.

Anyway, for the use case you mentioned, the AT_BENEATH flag(s) should be
simple to use and enough for now. We must be careful of the hardcoded
policy though.

quoted

On 9/29/18 12:34, Aleksa Sarai wrote:

quoted

The need for some sort of control over VFS's path resolution (to avoid
malicious paths resulting in inadvertent breakouts) has been a very
long-standing desire of many userspace applications. This patchset is a
revival of Al Viro's old AT_NO_JUMPS[1] patchset with a few additions.

The most obvious change is that AT_NO_JUMPS has been split as dicussed
in the original thread, along with a further split of AT_NO_PROCLINKS
which means that each individual property of AT_NO_JUMPS is now a
separate flag:

  * Path-based escapes from the starting-point using "/" or ".." are
    blocked by AT_BENEATH.
  * Mountpoint crossings are blocked by AT_XDEV.
  * /proc/$pid/fd/$fd resolution is blocked by AT_NO_PROCLINKS (more
      correctly it actually blocks any user of nd_jump_link() because it
      allows out-of-VFS path resolution manipulation).

AT_NO_JUMPS is now effectively (AT_BENEATH|AT_XDEV|AT_NO_PROCLINKS). At
Linus' suggestion in the original thread, I've also implemented
AT_NO_SYMLINKS which just denies _all_ symlink resolution (including
"proclink" resolution).

An additional improvement was made to AT_XDEV. The original AT_NO_JUMPS
path didn't consider "/tmp/.." as a mountpoint crossing -- this patch
blocks this as well (feel free to ask me to remove it if you feel this
is not sane).

Currently I've only enabled these for openat(2) and the stat(2) family.
I would hope we could enable it for basically every *at(2) syscall --
but many of them appear to not have a @flags argument and thus we'll
need to add several new syscalls to do this. I'm more than happy to send
those patches, but I'd prefer to know that this preliminary work is
acceptable before doing a bunch of copy-paste to add new sets of *at(2)
syscalls.

One additional feature I've implemented is AT_THIS_ROOT (I imagine this
is probably going to be more contentious than the refresh of
AT_NO_JUMPS, so I've included it in a separate patch). The patch itself
describes my reasoning, but the shortened version of the premise is that
continer runtimes need to have a way to resolve paths within a
potentially malicious rootfs. Container runtimes currently do this in
userspace[2] which has implicit race conditions that are not resolvable
in userspace (or use fork+exec+chroot and SCM_RIGHTS passing which is
inefficient). AT_THIS_ROOT allows for per-call chroot-like semantics for
path resolution, which would be invaluable for us -- and the
implementation is basically identical to AT_BENEATH (except that we
don't return errors when someone actually hits the root).

I've added some selftests for this, but it's not clear to me whether
they should live here or in xfstests (as far as I can tell there are no
other VFS tests in selftests, while there are some tests that look like
generic VFS tests in xfstests). If you'd prefer them to be included in
xfstests, let me know.

[1]: https://lore.kernel.org/patchwork/patch/784221/
[2]: https://github.com/cyphar/filepath-securejoin

Aleksa Sarai (3):
  namei: implement O_BENEATH-style AT_* flags
  namei: implement AT_THIS_ROOT chroot-like path resolution
  selftests: vfs: add AT_* path resolution tests

 fs/fcntl.c                                    |   2 +-
 fs/namei.c                                    | 158 ++++++++++++------
 fs/open.c                                     |  10 ++
 fs/stat.c                                     |  15 +-
 include/linux/fcntl.h                         |   3 +-
 include/linux/namei.h                         |   8 +
 include/uapi/asm-generic/fcntl.h              |  20 +++
 include/uapi/linux/fcntl.h                    |  10 ++
 tools/testing/selftests/Makefile              |   1 +
 tools/testing/selftests/vfs/.gitignore        |   1 +
 tools/testing/selftests/vfs/Makefile          |  13 ++
 tools/testing/selftests/vfs/at_flags.h        |  40 +++++
 tools/testing/selftests/vfs/common.sh         |  37 ++++
 .../selftests/vfs/tests/0001_at_beneath.sh    |  72 ++++++++
 .../selftests/vfs/tests/0002_at_xdev.sh       |  54 ++++++
 .../vfs/tests/0003_at_no_proclinks.sh         |  50 ++++++
 .../vfs/tests/0004_at_no_symlinks.sh          |  49 ++++++
 .../selftests/vfs/tests/0005_at_this_root.sh  |  66 ++++++++
 tools/testing/selftests/vfs/vfs_helper.c      | 154 +++++++++++++++++
 19 files changed, 707 insertions(+), 56 deletions(-)
 create mode 100644 tools/testing/selftests/vfs/.gitignore
 create mode 100644 tools/testing/selftests/vfs/Makefile
 create mode 100644 tools/testing/selftests/vfs/at_flags.h
 create mode 100644 tools/testing/selftests/vfs/common.sh
 create mode 100755 tools/testing/selftests/vfs/tests/0001_at_beneath.sh
 create mode 100755 tools/testing/selftests/vfs/tests/0002_at_xdev.sh
 create mode 100755 tools/testing/selftests/vfs/tests/0003_at_no_proclinks.sh
 create mode 100755 tools/testing/selftests/vfs/tests/0004_at_no_symlinks.sh
 create mode 100755 tools/testing/selftests/vfs/tests/0005_at_this_root.sh
 create mode 100644 tools/testing/selftests/vfs/vfs_helper.c