On Sat, Sep 29, 2018 at 08:34:50PM +1000, 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). Imho, these flags are very much needed and they all are pretty useful not just for container runtimes but in general. > > 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. We should really make sure that we can't make due with openat() alone before adding a bunch of new syscalls. So there's no need to rush into this. :) > > 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 > > -- > 2.19.0 >
