[PATCH review 0/7] Bind mount escape fixes
Eric W. Biederman
ebiederm at xmission.com
Sat Aug 15 18:35:19 UTC 2015
It is possible in some situations to rename a file or directory through
one mount point such that it can start out inside of a bind mount and
after the rename wind up outside of the bind mount. Unfortunately with
user namespaces these conditions can be trivially created by creating a
bind mount under an existing bind mount.
I have identified four situations in which this may be a problem.
- __d_path and d_absolute_path need to error on disconnected paths
that can not reach some root directory or lsm path based security
checks can incorrectly succeed.
- Normal path name resolution following .. can lead to a directory
that is outside of the original loopback mount.
- file handle reconsititution aka exportfs_decode_fh can yield a dentry
from which d_parent can be followed up to mnt->sb->s_root, but
d_parent can not be followed up to mnt->mnt_root.
- Mounts on a path that has been renamed outside of a loopback mount
become unreachable, as there is no possible path that can be passed
to umount to unmount them.
o File handle reconsitituion problems can be prevented by enabling
the nfsd subtree checks for nfs exports, and open_by_handle_at
requires capable(CAP_DAC_READ_SEARCH) so is only usable by the global
root. This makes any problems difficult if not impossible to exploit
in practice so I have not yet written code to address that issue.
o The functions __d_path and d_absolute_path are agumented so that the
security modules will not be fed a problematic path to work with.
o Following of .. has been agumented to test that after d_parent has
been resolved the original directory is connected, and if not
an error of -ENOENT is returned.
o I do not worry about mounts that are disconnected from their bind
mount as these mounts can always be freed by either umount -l on
the bind mount they have escaped from, or by freeing the mount
namespace. So I do not believe there is an actual problem.
Pathname resolution is a common fast path and most of the code in this
patchset to support keeping .. from becoming expensive in the common
After hearing the Al's feedback and running some numbers I have given
up attempting to keeping the number of d_ancestor calls during pathname
resolution to an absolute minimum. It appears that simply preventing
calls d_ancestor unless a directory has escaped is good enough. This
change in approach has significantly simplified the code.
The implementation change this round is I have dropped my patch cleaning
up d_splice_alias. Al Viro found a race that makes the technique I was
using fundamentally racy. I now have d_splice_alias taking mount_lock
around rename_lock. Since I don't have to sleep in d_splice_alias
change is minimal and sufficient for this purpose.
Barring some other idiocy I think this will be the final version of this
These changes are all against v4.2-rc1.
For those who like to see everything in a single tree the code is at:
Eric W. Biederman (7):
dcache: Handle escaped paths in prepend_path
dcache: Reduce the scope of i_lock in d_splice_alias
mnt: Track which mounts use a dentry as root.
dcache: Implement d_common_ancestor
dcache: Only read d_flags once in d_is_dir
mnt: Track when a directory escapes a bind mount
vfs: Test for and handle paths that are unreachable from their mnt_root
fs/dcache.c | 91 +++++++++++++++++++++++++++--
fs/mount.h | 9 +++
fs/namei.c | 26 ++++++++-
fs/namespace.c | 152 +++++++++++++++++++++++++++++++++++++++++++++++--
include/linux/dcache.h | 11 +++-
include/linux/mount.h | 1 +
6 files changed, 279 insertions(+), 11 deletions(-)
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