On 9/30/18 23:46, Jann Horn wrote:
On Sun, Sep 30, 2018 at 10:39 PM Mickaël Salaün firstname.lastname@example.org 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.
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.
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 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 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.
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