On 19/06/2023 18:21, Günther Noack wrote:
Hello Mickaël!
On Sat, Jun 17, 2023 at 11:47:55AM +0200, Mickaël Salaün wrote:
This subject is not easy, but I think we're reaching a consensus (see my
3-steps proposal plan below). I answered your questions about the (complex)
interface I proposed, but we should focus on the first step now (your
initial proposal) and get back to the other steps later in another email
thread.
Thanks for the review!
On 10/05/2023 21:21, Günther Noack wrote:
[...]
Some specific things I don't understand well are:
[...]
Thanks, this all make sense now. 👍
Would it not be a more orthogonal API if the "file selection" part
of the Landlock API and the "policy adding" part for these selected
files were independent of each other? Then the .device and
.file_type selection scheme could be used for the existing policies
as well?
Both approaches have pros and cons. I propose a new incremental approach
below that starts with the simple case where there is no direct links
between different rule types (only the third step add that).
* When restricting by dev_t (major and minor number), aren't there use
cases where a system might have 256 CDROM drives, and you'd need to
allow-list all of these minor number combinations?
Indeed, we should be able to just ignore device minors.
They device numbers are listed in
https://www.kernel.org/doc/Documentation/admin-guide/devices.txt
Some major numbers are grab-bags of miscellaneous devices, for example
major numbers 10 and 13; 13/0-31 (maj/min) are joysticks, whereas
13/32-62) are mice.
Maybe this can be specified as ranges on dev_t, so that it would be
possible to specify 13/32-62, without matching the joysticks too?
Indeed, being able to specifying a range would be useful. I'm not sure
about the whole dev_t range or only a minor range though. From a
semantic POV, there is no links between majors.
I think that it might be a feasible approach to start with the
LANDLOCK_ACCESS_FS_IOCTL approach and then look at its usage to
understand whether we see a significant number of programs whose
sandboxes are too coarse because of this.
If more fine-granular control is needed, we can still put the other
approach on top, and the additional complexity from
LANDLOCK_ACCESS_FS_IOCTL that we have to support is not that
dramatically high.
[...]
I agree that IOCTLs are a security risk and that we should propose a simple
solution short-term, and maybe a more complete one long-term.
The main issue with a unique IOCTL access right for a file hierarchy is that
we may not know which device/driver will be the target/server, and hence if
we need to allow some IOCTL for regular files (e.g., fscrypt), we might end
up allowing all IOCTLs.
Here is a plan to incrementally develop a fine-grained IOCTL access control
in 3 steps:
1/ Add a simple IOCTL access right for path_beneath: what you initially
proposed. For systems that already configure nodev mount points, it could be
even more useful (e.g., safely allow IOCTL on /home for fscrypt, and
specific /dev files otherwise).
Ack, I'll continue on that implementation then. 👍
2/ Create a new type of rule to identify file/device type:
struct landlock_inode_type_attr {
__u64 allowed_access; /* same as for path_beneath */
__u64 flags; /* bit 0: ignores device minor */
dev_t device; /* same as stat's st_rdev */
__u16 file_type; /* same as stat's st_mode & S_IFMT */
};
We'll probably need to differentiate the handled accesses for path_beneath
rules and those for inode_type rules to be more useful.
One issue with this type of rule is that it could be used as an oracle to
bypass stat restrictions. We could check if such (virtual) action is allowed
without the current domain though.
3/ Add a new type of rule to match IOCTL commands, with a mechanism to tie
this to inode_type rules (because a command ID is relative to a file
type/device), and potentially the same mechanism to tie inode_type rules to
path_beneath rules.
Each of this step can be implemented one after the other, and each one is
valuable. What do you think?
I think it is a good idea to do it in multiple steps,
as I also believe that step 1) already provides value on its own.
To make sure we are on the same page, let me paraphrase my understanding here:
1) is what I already sent as RFC, with tests and documentation etc.
With 1), callers could allow and deny ioctls on newly opened files,
independent of file path.
Yes
2) makes dev_t and file_type predicates which can be used to limit
file accesses on already opened files.
With 2), callers could allow and deny ioctls and other operations
also on files which are already opened before enablement, such
as the TTY FD inherited from the parent process.
This is not on already opened files, it would be a complementary rule
type to path_beneath ones: if a rule with dev_t/file_type matches and
allowed action FOO, and a path_beneath rules matches and also allowed
action FOO, then FOO is allowed.
However, this could be extended in the future (with a new dedicated
flag) to also support already-opened files.
3) would make it possible to restrict individual ioctl commands,
depending on the dev_t, the file_type, and possibly the path.
Yes. I think we could even only extend the "landlock_inode_attr" struct
to add an ioctl_command field, and potentially others too. I think it
would make sense because IOCTLs are tied to a specific device/file type
(or even the underlying filesystem).
In fact, we can split the third step in two:
3/ Extend landlock_inode_attr with ioctl_command.
4/ Define an inode_category field that contain an arbitrary number which
can be match against a path_beneath rule (with a bitmask to be able to
match others too).
Other steps could allow to match landlock_inode_attr with FDs, add a
landlock_fd_range_attr…
Is that accurate?
In 2) and 3), I'm still contemplating a bit whether we have
alternative approaches, but I believe in any case, it's clear that
they can be built on top of each other without much overhead, and for
many programs, outright denying ioctl on newly opened files with 1)
might be the most straightforward approach which already delivers
value.
Right
Some notes which I collected on the side:
* I'm still worried about the already-opened file descriptors like the
TTY, through which it can be easy to escape a sandbox
(c.f. https://nvd.nist.gov/vuln/detail/CVE-2017-5226) - I would like
to have a solution for that. Step (2) would fix it, but maybe I can
find a workaround to at least detect the problem in step (1)
already.
Yes, good example, Landlock should be able to help with this issue
thanks to the third step.
* I looked at OpenBSD's pledge and unveil. In OpenBSD, the ioctl
policy is based on the properties of an already opened file. They
have hardcoded their ioctl logic in the kernel, which would be a
mistake to do in Linux due to stronger backwards compatibility
guarantees. OpenBSD is making the allow/deny decisions based on
device major/minor numbers and file type (usually block/char
device).
I think it is noteworthy that OpenBSD does *not* use the file path
to make that decision.
Yes, we could have the same behavior by only using the rule type added
with the second step, but we could easily improve that by combining a
path_beneath rule thanks to the third step.
I wonder whether we should replicate that in steps (2) and (3). It
would make for a simpler, more orthogonal API, where the ioctl
policy would become a property of the task, and it would be enforced
independently of the existing path-based logic. When callers
combine that with the overall ioctl check from (1), it might be
flexible enough for practical purposes.
Yes, this will be possible with the four steps.
* We should not rely on the way that ioctl request numbers are
structured, because it is used inconsistently. More background:
https://lwn.net/Articles/428140/ (Thank you for pointing out this
article, Jeff!)
Definitely, but the user space libraries might help with that. ;)
* Should we introduce a "landlock_fd_rights_limit()" syscall?
[...]
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.
Hm, you mean a landlock_fd_rights_limit_range() syscall to limit the
rights for an entire range of FDs?
I have to admit, I'm not familiar with the real-life use cases of
close_range(). In most programs I work with, it's difficult to reason
about their ordering once the program has really started to run. So I
imagine that close_range() is mostly used to "sanitize" the open file
descriptors at the start of main(), and you have a similar use case in
mind for this one as well?
If it's just about closing the range from 0 to 2, I'm not sure it's
worth adding a custom syscall. :)
The advantage of this kind of range is to efficiently manage all potential
FDs, and the main use case is to close (or change, see the flags) everything
*except" 0-2 (i.e. 3-~0), and then avoid a lot of (potentially useless)
syscalls.
The Landlock interface doesn't need to be a syscall. We could just add a new
rule type which could take a FD range and restrict them when calling
landlock_restrict_self(). Something like this:
struct landlock_fd_attr {
__u64 allowed_access;
__u32 first;
__u32 last;
}
Ack, I see it a bit better. Let's discuss this topic separately.
—Günther
