Re: [PATCH v24 02/12] landlock: Add ruleset and domain management

[Mickaël Salaün]

On 21/11/2020 08:00, Jann Horn wrote:
> On Thu, Nov 12, 2020 at 9:51 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote:
>> A Landlock ruleset is mainly a red-black tree with Landlock rules as
>> nodes.  This enables quick update and lookup to match a requested
>> access, e.g. to a file.  A ruleset is usable through a dedicated file
>> descriptor (cf. following commit implementing syscalls) which enables a
>> process to create and populate a ruleset with new rules.
>>
>> A domain is a ruleset tied to a set of processes.  This group of rules
>> defines the security policy enforced on these processes and their future
>> children.  A domain can transition to a new domain which is the
>> intersection of all its constraints and those of a ruleset provided by
>> the current process.  This modification only impact the current process.
>> This means that a process can only gain more constraints (i.e. lose
>> accesses) over time.
>>
>> Cc: James Morris <jmorris@xxxxxxxxx>
>> Cc: Jann Horn <jannh@xxxxxxxxxx>
>> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
>> Cc: Serge E. Hallyn <serge@xxxxxxxxxx>
>> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxxxxxxxxxx>
>> ---
>>
>> Changes since v23:
>> * Always intersect access rights.  Following the filesystem change
>>   logic, make ruleset updates more consistent by always intersecting
>>   access rights (boolean AND) instead of combining them (boolean OR) for
>>   the same layer.
> 
> This seems wrong to me. If some software e.g. builds a policy that
> allows it to execute specific libraries and to open input files
> specified on the command line, and the user then specifies a library
> as an input file, this change will make that fail unless the software
> explicitly deduplicates the rules.
> Userspace will be forced to add extra complexity to work around this.

That's a valid use case I didn't think about. Reverting this change is
not an issue.

> 
>>   This defensive approach could also help avoid user
>>   space to inadvertently allow multiple access rights for the same
>>   object (e.g.  write and execute access on a path hierarchy) instead of
>>   dealing with such inconsistency.  This can happen when there is no
>>   deduplication of objects (e.g. paths and underlying inodes) whereas
>>   they get different access rights with landlock_add_rule(2).
> 
> I don't see why that's an issue. If userspace wants to be able to
> access the same object in different ways for different purposes, it
> should be able to do that, no?
> 
> I liked the semantics from the previous version.
> 

I agree, but the real issue is with the ruleset layers applied to the
filesystem, cf. patch 7.