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

[Jann Horn]

On Sun, Sep 30, 2018 at 10:39 PM Mickaël Salaün <mic at digikod.net> wrote:
> 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.

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?

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