On Sun, Sep 30, 2018 at 10:39 PM Mickaël Salaün <mic@xxxxxxxxxxx> 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 > > >
