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