On 27/07/2023 11:32, Günther Noack wrote:
Hello Mickaël!
Overall, I believe that I thought too far ahead here and now we've
been mixing the discussions for different steps from the three-step
approach that we discussed in [1].
In order to make progress here, let me try to disentangle in this mail
which parts we need for the current step (1) and which parts are only
needed for later steps.
[1] https://lore.kernel.org/linux-security-module/d4f1395c-d2d4-1860-3a02-2a0c023dd761@xxxxxxxxxxx/
On Mon, Jul 24, 2023 at 09:03:42PM +0200, Mickaël Salaün wrote:
On 14/07/2023 00:38, Günther Noack wrote:
On Wed, Jul 12, 2023 at 07:48:29PM +0200, Mickaël Salaün wrote:
A useful split would be at least between devices and regular
files/directories, something like this:
- LANDLOCK_ACCESS_FS_IOCTL_DEV: allows IOCTLs on character or block devices,
which should be targeted on specific paths.
- LANDLOCK_ACCESS_FS_IOCTL_NODEV: allows IOCTLs on regular files,
directories, unix sockets, pipes, and symlinks. These are targeting
filesystems (e.g. ext4's fsverity) or common Linux file types.
To make sure we are on the same page, let me paraphrase:
You are suggesting that we should split the LANDLOCK_ACCESS_FS_IOCTL
right into a LANDLOCK_ACCESS_FS_IOCTL_DEV part (for block and
character devices) and a LANDLOCK_ACCESS_FS_IOCTL_NODEV part (for
regular files, directories, named(!) unix sockets, named(!) pipes and
symlinks)? The check would presumably be done during the open(2) call
and then store the access right on the freshly opened struct file?
Correct
OK, I'll add this to step (1) then.
(It is more clearly a philosophy of "protecting resources", rather
than a philosophy of limiting access to the thousands of potentially
buggy ioctl implementations. - But I think it might be reasonable to
permit unnamed pipes and socketpairs - they are useful mechanisms and
seem harmless as long as their implementations don't have bugs.)
The goal of Landlock is to limit access to new resources (e.g. new FD
obtained from an existing FD or a path).
Unnamed pipes and socketpairs are not a way to (directly) access new kernel
resources/data, hence out of scope for Landlock. Abstract unix sockets will
need to be restricted though, but not with a path_beneath rule (and not
right now with this patch series).
OK, fair enough. I can see that this is conceptually cleaner within
Landlock.
Let's go for that approach then, where Landlock only restricts newly
opened path-based files, and where we leave inherited file descriptors
and the ones created through pipe(2), socketpair(2) and timerfds as
they are for now.
It feels like TIOCSTI (and TIOCLINUX) are probably the biggest
problems when it comes to these inherited files. With some luck,
TIOCSTI will be turned off by distributions soon (and TIOCLINUX only
works on the text console, which is luckily not in use that much).
Fingers crossed.
I think it makes sense because the underlying filesystems should already
check for read/write access, which is not the case for block/char devices.
Pipe and unix socket IOCTLs are quite specific but don't touch the
underlying filesystem, and it should be allowed to properly use them. It
should be noted that the pipe and socket IOCTL implementations don't care
about their file mode though; I guess the rationale might be that IOCTLs may
be required to (efficiently) either read or write.
I don't understand your remark about the read/write access.
I meant that regular file/directory IOCTLs (e.g. fscrypt) should already
check for read or write access according to the IOCTL command. This doesn't
seem to be the case for devices because they don't modify and are unrelated
to the underlying filesystem.
Pipes have a read end and a write end, where only one of the two
operations should work. Unix sockets are always bidirectional, if I
remember this correctly.
Yes, but the pipe and socket IOCTL commands don't check if the related FD is
opened with read nor write.
I still don't understand your remarks about ioctl commands not
checking read and write flags. To clarify: What you are talking about
is that the implementations of individual ioctl commands should all
check the read and write mode flags on the struct file, is that right?
I'm puzzled how you come to the conclusion that devices don't do such
checks - did you read some ioctl command implementations, or is it a
more underlying principle that I was not aware of so far?
I took a look at fscrypt IOCTLs for instance, and other FS IOCTLs seems
to correctly check for FD's read/write mode according to the IOCTL
behavior. From what I've seen, IOCTLs implemented by device drivers
don't care about file mode.
So far, I've always been under the impression that the modes on device
files are also reflected on the associated struct file after they are
opened. As in, you open a block device for reading to read its
contents, but you need to open it for writing to modify it. Are these
rights not respected by the ioctl commands?
File mode is correctly tracked but ignore by most IOCTL handlers,
probably because file mode is dedicated to raw files and directories.
In any case - I believe the only reason why we are discussing this is
to justify the DEV/NODEV split, and that one in itself is not
controversial to me, even when I admittedly don't fully follow your
reasoning.
The main reason is than I don't want applications/users to not be
allowed to use "legitimate" IOCTL, for instance to correctly encrypt
their own files. If we only have one IOCTL right, we cannot easily
differentiate between the targeted file types. However, this split might
be too costly, cf. my comment in the below summary.
The thing that struck me about the above list of criteria is that each
of them seems to have gaps. As an example, take timerfds
(timerfd_create(2)):
* these do not get opened through a file system path, so the *file
path* can not restrict them.
* they are not character or block devices and do not have a device ID.
* they don't match any of the file types in filp->f_mode.
Indeed, we may need a way to identify this kind of FD in the future but it
should not be an issue for now with the path_beneath rules. I guess we could
match the anon_inode:[*] name, but I would prefer to avoid using strings. A
better way to identify this kind of FD would be to pass a similar one in a
rule, in the same way as for path_beneath (as I suggested in a previous
email).
(In retrospect, the timerfd was a bad example, because it is acquired
through something else than open(2). I don't currently have an
immediate example at hand for an anon_inode which is reachable through
the file system (except /proc/.../fd).)
Agreed that matching the "anon_inode:*" name would be a hack. That
does not look like it was meant as a reliable interface.
This discussion seems like it belongs to step (2) and later though, so
wouldn't block the first patch set, I think.
Yes :)
So in order to permit the TFD_IOC_SET_TICKS ioctl on them, these three
criteria can't be used to describe a timerfd.
Correct, and timerfd don't give access to (FS-related) data.
If we want to restrict this kind of FD (and if it's worth it), we can follow
the same approach as for restricting new socket creation. Restricting
specific IOCTLs on these FDs would require a capability-based approach (cf.
Capsicum): explicitly attach restrictions to a FD, not a process, and the
mechanism is almost there thanks to the truncate access right patches. It
would make sense for Landlock to support this kind of FD capabilities, but
maybe not right now.
+1 let's discuss in a follow-up patch set.
For completeness: I forgot to list here: The other reason where a
check during ioctl() is needed is the case as for the timerfd, the
pipe(2) and socketpair(2), where a file is created through a simple
syscall, but without spelling out a path. If these kinds of files are
in scope for ioctl protection, it can't be done during the open()
check alone, I suspect?
Correct, but we don't need this kind of restriction for now.
OK
As I explained before, I don't think we should care about inherited or
passed FDs. Other ways to get FDs (e.g. landlock_create_ruleset) should
probably not be a priority for now.
I don't know what we should do about the "basic Unix tool" and
TIOCSTI/TIOCLINUX case, where it is possible to gain control over the
shell running in the tty that we get as stdout fd.
I'm in that situation with the little web application I run at home,
but the patch that you have sent for GNU tar at some point (and which
we should really revive :)) has the same problem: If an attacker
manages to do a Remote Code Execution in that tar process, they can
ioctl(1, TIOCSTI, ...) their way out into the shell which invoked tar,
and which is not restricted with tar's Landlock policy.
(I don't really see tar create a pty/tty pair either and shovel data
between them in a sidecar process or thread, just to protect against
that.)
Indeed, and that's why sandboxing an application might raise some
challenges. We should note that a sandboxed application might only be safely
used in some cases (e.g. pipe stdio and close other FDs), but I agree that
this is not satisfactory for now, and there are still gaps.
OK, I'll make sure it shows up in the documentation.
Remark: For the specific TIOCSTI problem, I'm seeing a glimmer of
light with this patch set which has appeared in the meantime:
https://lore.kernel.org/all/20230710002645.v565c7xq5iddruse@begin/
(This will still require that distributions flip that Kconfig option
off, but the only(?) known user of TIOCSTI, BRLTTY, would continue
working.)
I hope most distros are disabling CONFIG_LEGACY_TIOCSTI, otherwise users
should still be able to tweak the related sysctl.
+1
I would be more comfortable with doing the checks only at open(2) time
if the above patch landed in distributions so that you would need to
have CAP_SYS_ADMIN in order to use TIOCSTI.
Do you think this is realistic? If this does not get flipped by
distributions, Landlock would continue to have these TIOCSTI problems
on these platforms (unless its users do the pty/tty pair thing, but
that seems like an unrealistic demand).
What if we use an FD to identify an inode with landlock_inode_attr rule? We
could have some flag to "pin" the restriction on this specific FD/inode, or
only match the device type, or the file type… We need to think a bit more
about the implications though.
This would also be a later step and not be part of step (1).
The idea of using an FD as an "example" is interesting, but I'm not
fully sold on it yet. I need to ponder it more. Some specific
points:
* Creating such FDs might have unwanted side effects, or be
disproportionally difficult, when they are just created for the
purpose of defining a Landlock ruleset.
* The matching is unclear to me. In particular, we've discussed
before to restrict dev_t ranges (fixed major, range on minor) - I
don't know how that would be done with this approach.
Those are valid concerns, but yes, let's get back to this discussion
after the first step. :)
I don't see how
specifying the file type and device ID range as plain numbers could
lead to a race condition.
No race condition but side channel issues. For instance, a landlocked
process could infer some file properties by adding and testing a Landlock
rule even if it is not allowed to read such file properties (because of
potential Landlock or other restrictions). Using a FD enables Landlock to
check that the process is indeed allowed to read such properties, or we may
decide that it is not necessary to do so.
Understood - so IIUC the scenario is that a process is not permitted
to read file attributes, but it'll be able to infer the device ID by
defining a dev_t-based Landlock rule and then observing whether ioctl
still works.
Right. I think it should be possible to still check if this kind of file
attribute would be allowed to be read by the process, when performing
the IOCTL on it. We need to think about the implications, and if it's
worth it.
It would be interesting to see if there are similar cover channels with
other Linux interfaces.
(I'll also postpone it to step (2) or later then)
If yes, I do agree that a list of permitted ioctls is similar to the
access rights flags that we already have, and it would have to get
passed around in a similar fashion (as "synthetic access rights"),
albeit using a different data structure.
I'm still skeptical of the API approach where we tie previously
unrelated rules together, if that is what you mean here. I find this
difficult to explain and reason about. But in doubt we'll see in the
implementation how unwieldy it actually gets.
Right, we don't need to implement the synthetic access rights with the
current step though.
+1 OK
The upside of that approach would be that it could also be used to selectively
restrict specific known-evil ioctls, and letting all others continue to work.
For example, sandboxing or sudo-like programs could filter out TIOCSTI and
TIOCLINUX.
By the way, selectively restricting known-bad ioctls is still not
possible with the approach we discussed now, I think. Maybe TIOCSTI
is the only bad one... I hope.
It would not be possible with the landlock_path_beneath_attr, but the
landlock_inode_attr could be enough.
Will ponder it -- it has the limitation as I said above that it can't
restrict device ranges, but it could be used to apply restrictions to
specific opened inodes.
One difference I see with this approach is that the rights would not
transfer along with the opened file when the files get passed between
processes. So the policy for that inode would apply to the enforcing
process and its new children, but it would not apply to other
processes which the file is given to.
Good point. We need to come up with something consistent.
---
Summarizing this, I believe that the parts that we need for step (1)
are the following ones:
(1) Identify and blanket-permit a small list of ioctl cmds which work
on all file descriptors (FIOCLEX, FIONCLEX, FIONBIO, FIOASYNC, and
others?)
Compare
https://lore.kernel.org/linux-security-module/6dfc0198-9010-7c54-2699-d3b867249850@xxxxxxxxxxx/
(2) Split into LANDLOCK_ACCESS_FS_IOCTL into a ..._DEV and a ..._NODEV part.
I'm a bit annoyed that this access rights mix action and object property
and I'm worried that this might be a pain for future FS-related rule
types (e.g. reusing all/most ACCESS_FS rights).
At first glance, a cleaner way would be to add a file_type field to
landlock_path_beneath_attr (i.e. a subset of the landlock_inode_attr)
but that would make struct landlock_rule and landlock_layer too complex,
so not a good approach.
Unless someone has a better idea, let's stick to your first proposal and
only implement LANDLOCK_ACCESS_FS_IOCTL (with the FIOCLEX-like
exceptions). We should clearly explain that IOCTLs should be allowed for
non-device file hierarchies: containing regular/data file (e.g. /home
with the fscrypt use case), pipe, socket…
I'm still trying to convince myself which approach is the best, but for
now the simplest one wins.
(3) Point out in documentation that this IOCTL restriction scheme only
applies to newly opened FDs and in particular point out the common
use case where that is a TTY, and what to do in this case.
If you agree, I'd go ahead and implement that as step (1) and we can
discuss the more advanced ideas in the context of a follow-up.
Sounds good!
Thanks,
—Günther
