[PATCH 0/1] process attribute support for Landlock

[Mickaël Salaün]

On 06/03/2023 20:18, Mickaël Salaün wrote:
> Hi Shervin,
> 
> Thanks for this initial patch.
> 
> On 02/03/2023 19:52, enlightened at chromium.org wrote:
>> From: Shervin Oloumi <enlightened at chromium.org>
>>
>> Hi Mickaël,
>>
>> I'm looking into adding a simple process attribute getter to Landlock so
>> we can determine the sand-boxing state of each process based on
>> /proc/[PID]/attr/current. As ChromeOS is expanding Landlock support,
>> this would help us paint a clear picture of Landlock coverage in the
>> fleet. I prepared a patch as a starting point, and would love to get
>> your feedback.
> 
> It would help to know exactly what are your needs short term, and long
> term. As Günther is wondering, what about nested sandboxing?
> 
> I'm thinking about a new /sys/kernel/security/landlock filesystem to be
> able to audit Landlock domains (i.e. sandboxes). As for your use case,
> it would be useful to be able to tie a process to a Landlock domain
> thanks to IDs.
> 
> Here are the guiding principles I think would make sense:
> 1. A sandboxed thread shall not be able to directly know if it is
> sandbox nor get any specific information from it's restrictions. The
> reason for this principle is to avoid applications to simply jump to
> conclusions (and change behavior) if they see that they are sandboxed
> with Landlock, instead of trying to access resources and falling back
> accordingly. A thread should only be able to inspect its
> own/children/nested domains.
> 2. Access to any Landlock domain information should be checked according
> to PTRACE_MODE_READ_FSCREDS, the Landlock domain hierarchy (cf.
> ptrace.c:domain_scope_le), and the first principle.

We could get some inspiration from pidfd and read the domain ID (or even 
the domain hierarchy) from /proc/self/fdinfo/*. This doesn't require a 
symlink (just a regular file), and it enables to have a way to control 
the domain lifetime by keeping the FD opened (e.g. to look into 
/sys/kernel/security/landlock/*). For now, we can then postpone the 
domain ID design (and the related fdinfo specificity).

To summarize, we would be able to identify if Landlock is enabled 
(according to the "attr/landlock" directory existence) and if a thread 
is sandboxed (according to the "attr/landlock/domain" file existence), 
but nothing more for now. The "domain" file won't even need any file 
operation.

I'd still like to know the exact requirements to identify future 
developments.


> 3. Any (domain) ID should be unique to the whole system (or maybe to the
> reader's PID namespace, and then in theory relative to the /proc
> content) to make it possible to compare Landlock domains (like
> /proc/[pid]/ns/* symlinks enable), and avoid trivial races.
> 4. These IDs should be the same during the whole lifetime of the related
> domain.
> 5. These IDs should not enable to infer information from other Landlock
> domains (e.g. how many are in use, current and parent domains), nor the
> kernel internals (e.g. addresses).
> 6. These IDs should not be sequential nor easily guessed to avoid
> anti-patterns (cf. file descriptors).
> 7. These IDs should be CRIU-friendly, to be able to easily restore such
> state. This doesn't help the previous principles and I don't know how/if
> CRIU supports namespace IDs though.
> 
> The /proc/[pid]/ns/* symlinks should be a good inspiration for a
> /proc/[pid]/attr/landlock/domain symlink with similar properties. Such
> file could then be used to pin or enforce the same Landlock domain on
> other threads in the future (out of scope for this patch series). Being
> able to open such "domain" file would make it possible to avoid races
> while reading the related ID and looking for the related entry in
> /sys/kernel/security/landlock/ by holding this file open.
> 
> It would be nice if the /proc/[pid]/attr/landlock directory would only
> exists if Landlock is enabled.
> 
> Similarly, /proc/[pid]/attr/landlock/domain should only exist (or be
> viewable) for a thread if [pid] is part of one of its child domain.
> 
> For now, I don't see any file in /proc/[pid]/attr/landlock/ other than
> "domain" that would make sense, but a dedicated directory is useful anyway.
> 
> I though about an entire file hierarchy to reflect a Landlock domain
> (e.g., with rule attributes), but that would make the /proc filesystem
> dynamically deep, so this should be dedicated to the
> /sys/kernel/security/landlock filesystem, but tied with /proc in some
> way, in this case with same domain IDs.
> 
> 
>>
>> One area I am not very sure of is the case where more than one LSM is in
>> use. In such cases each LSM could have its own process attribute
>> getters and setters. What I learned is that when this is the case, the
>> kernel only calls the hook function for the LSM that is loaded first in
>> the CONFIG_LSM option. For example if landlock comes first
>> (CONFIG_LSM=landlock,...), then the kernel only calls the hook function
>> for Landlock, when the userspace interacts with process attribute files.
>> This is not a blocker for us, as we only currently care about reading
>> the Landlock related attributes, and my understanding is that this is
>> working as intended, but wanted to get your input.
> 
> Using the /proc/[pid]/attr/landlock/domain path will remove this issue.
> 
>>
>> Shervin Oloumi (1):
>>     lsm: adds process attribute getter for Landlock
>>
>>    fs/proc/base.c         | 11 +++++++++++
>>    security/landlock/fs.c | 33 +++++++++++++++++++++++++++++++++
>>    2 files changed, 44 insertions(+)
>>
>>
>> base-commit: e2ca6ba6ba0152361aa4fcbf6067db71b2c7a770