Thanks for this RFC, this is interesting!
I previously thought a lot about IOCTLs restrictions and here are some
notes:
IOCTLs are everywhere, from devices to filesystems (see fscrypt). Each
different file type may behave differently for the same IOCTL
command/ID. It is also worth noting that there are a lot of different
IOCTLs, they are growing over time, some might be dedicated to get some
data (i.e. read) and others to change the state of a device (i.e.
write), some might be innocuous (e.g. FIOCLEX, FIONCLEX) and others
potentially harmful. _IOC_READ and _IOC_WRITE can be useful but they are
not mandatory, there are exceptions, and it may be difficult to identify
if a command pertains to one, the other, or both kind of actions.
I then think it would be very useful to be able to tie file/device types
to a set of IOCTLs, letting user space libraries define and classify the
IOCTL semantic.
Instead of relying on a LANDLOCK_ACCESS_FS_IOCTL which would allow or
deny all IOCTLs, we can extend the path_beneath struct to manage IOCTLs
in addition to regular file accesses. Because dealing with a set of
IOCTLs would imply to deal with a lot of data and combinations, I
thought about creating groups of IOCTLs (defining access semantic) that
could be matched against file hierarchies. The composability nature of
Landlock domains is also another constraint to keep in mind.
// New rule type dedicated to define groups of IOCTLs.
struct landlock_ioctl_attr {
__u32 command; // IOCTL number/ID
dev_t device; // must be 0 for regular file and directory
__u8 file_type;
__u8 id_mask; // if 0, then applied globally
};
We could use landlock_add_rule(2) to fill a set of landlock_ioctl_attr
into a ruleset and use them with landlock_path_beneath_attr entries:
// LANDLOCK_RULE_PATH_BENEATH, leveraging the extensible design of
// landlock_path_beneath_attr, hence the same first fields.
struct landlock_path_beneath_attr {
__u64 allowed_access;
__s32 parent_fd;
__u16 allowed_ioctl_id_mask;
};
landlock_ioctl_attr includes a 8-bit mask for which each bit identifies
a set of allowed IOCTLs per device/file type. This mask is then tied to
a path_beneath_attr. We cannot use number IDs because of dev_t+IOCTL->ID
intersection conflicts. Using an id_mask enables to group (specific)
IOCTLs together, then creating synthetic access rights.
When merging a ruleset with a domain, each IOCTL ID mask is shifted and
ORed with the other layer ones to get a (8*16) 128-bit mask, stored in
an IOCTL/dev_t table and in the related landlock_object. When looking
for an IOCTL request, Landlock first looks into the IOCTL set ID table
and get the global set ID mask, which kind of translates to a
composition of synthetic access rights (stored with the
landlock_layer.ioctl_access bitmask). We then walk through all the
inodes to match the whole mask.
I realize that this is complex and this explanation might be confusing
though. What do you think?
On 02/05/2023 19:17, Günther Noack wrote:
Hello!
These patches add ioctl support to Landlock.
It's an early version - it potentially needs more tests and
documentation. I'd like to circulate the patches early to discuss
whether this approach is feasible.
Objective
~~~~~~~~~
Make ioctl(2) requests restrictable with Landlock,
in a way that is useful for real-world applications.
Proposed approach
~~~~~~~~~~~~~~~~~
Introduce the LANDLOCK_ACCESS_FS_IOCTL right, which restricts the use
of ioctl(2) on file descriptors.
We attach the LANDLOCK_ACCESS_FS_IOCTL right to opened file
descriptors, as we already do for LANDLOCK_ACCESS_FS_TRUNCATE.
I believe that this approach works for the majority of use cases, and
offers a good trade-off between Landlock API and implementation
complexity and flexibility when the feature is used.
Notable implications of this approach
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
* Existing inherited file descriptors stay unaffected
when a program enables Landlock.
This means in particular that in common scenarios,
the terminal's ioctls (ioctl_tty(2)) continue to work.
* ioctl(2) continues to be available for file descriptors acquired
through means other than open(2). Example: Network sockets.
Examples
~~~~~~~~
Starting a sandboxed shell from $HOME with samples/landlock/sandboxer:
LL_FS_RO=/ LL_FS_RW=. ./sandboxer /bin/bash
The LANDLOCK_ACCESS_FS_IOCTL right is part of the "read-write" rights
here, so we expect that newly opened files outside of $HOME don't work
with ioctl(2).
* "stty" works: It probes terminal properties
* "stty </dev/tty" fails: /dev/tty can be reopened, but the ioctl is
denied.
* "eject" fails: ioctls to use CD-ROM drive are denied.
* "ls /dev" works: It uses ioctl to get the terminal size for
columnar layout
* The text editors "vim" and "mg" work. (GNU Emacs fails because it
attempts to reopen /dev/tty.)
Alternatives considered: Allow-listing specific ioctl requests
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
It would be technically possible to keep an allow-list of ioctl
requests and thereby control in more detail which ioctls should work.
I believe that this is not needed for the majority of use cases and
that it is a reasonable trade-off to make here (but I'm happy to hear
about counterexamples). The reasoning is:
* Many programs do not need ioctl at all,
and denying all ioctl(2) requests OK for these.
* Other programs need ioctl, but only for the terminal FDs.
This is supported because these file descriptors are usually
inherited from the parent process - so the parent process gets to
control the ioctl(2) policy for them.
* Some programs need ioctl on specific files that they are opening
themselves. They can allow-list these file paths for ioctl(2).
This makes the programs work, but it restricts a variety of other
ioctl requests which are otherwise possible through opening other
files.
Because the LANDLOCK_ACCESS_FS_IOCTL right is attached to the file
descriptor, programs have flexible options to control which ioctl
operations should work, without the implementation complexity of
additional ioctl allow-lists in the kernel.
Finally, the proposed approach is simpler in implementation and has
lower API complexity, but it does *not* preclude us from implementing
per-ioctl-request allow lists later, if that turns out to be necessary
at a later point.
I value this simplicity, but I'm also wondering about how much this
allow/deny all IOCTLs approach would be useful in real case scenarios. ;)
Related Work
~~~~~~~~~~~~
OpenBSD's pledge(2) [1] restricts ioctl(2) independent of the file
descriptor which is used. The implementers maintain multiple
allow-lists of predefined ioctl(2) operations required for different
application domains such as "audio", "bpf", "tty" and "inet".
OpenBSD does not guarantee ABI backwards compatibility to the same
extent as Linux does, so it's easier for them to update these lists in
later versions. It might not be a feasible approach for Linux though.
[1] https://man.openbsd.org/OpenBSD-7.3/pledge.2
Feedback I'm looking for
~~~~~~~~~~~~~~~~~~~~~~~~
Some specific points I would like to get your opinion on:
* Is this the right general approach to restricting ioctl(2)?
It will probably be possible to find counter-examples where the
alternative (see below) is better. I'd be interested in these, and
in how common they are, to understand whether we have picked the
right trade-off here.
In the long term, I'd like Landlock to be able to restrict a set of
IOCTLs, taking into account the type of file/device. Being able to deny
all IOCTLs might be useful and is much easier to implement though.
* Should we introduce a "landlock_fd_rights_limit()" syscall?
We could potentially implement a system call for dropping the
LANDLOCK_ACCESS_FS_IOCTL and LANDLOCK_ACCESS_FS_TRUNCATE rights from
existing file descriptors (independent of the type of file
descriptor, even).
Possible use cases would be to (a) restrict the rights on inherited
file descriptors like std{in,out,err} and to (b) restrict ioctl and
truncate operations on file descriptors that are not acquired
through open(2), such as network sockets.
This would be similar to the cap_rights_limit(2) system call in
FreeBSD's Capsicum.
This idea feels somewhat orthogonal to the ioctl patch, but it would
start to be more useful if the ioctl right exists.
This is indeed interesting, and that should be useful for the cases you
explained. I think supporting IOCTLs is more important for now, but a
new syscall to restrict FDs could be useful in the future. We should
also think about batch operations on FD (see the close_range syscall),
for instance to deny all IOCTLs on inherited or received FDs.
Günther Noack (4):
landlock: Increment Landlock ABI version to 4
landlock: Add LANDLOCK_ACCESS_FS_IOCTL access right
selftests/landlock: Test ioctl support
samples/landlock: Add support for LANDLOCK_ACCESS_FS_IOCTL
include/uapi/linux/landlock.h | 19 ++++--
samples/landlock/sandboxer.c | 12 +++-
security/landlock/fs.c | 20 +++++-
security/landlock/limits.h | 2 +-
security/landlock/syscalls.c | 2 +-
tools/testing/selftests/landlock/base_test.c | 2 +-
tools/testing/selftests/landlock/fs_test.c | 67 +++++++++++++++++++-
7 files changed, 107 insertions(+), 17 deletions(-)
base-commit: 457391b0380335d5e9a5babdec90ac53928b23b4
