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

[Jann Horn]

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.

>   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.