linux-kernel - Re: [RFC PATCH 3/3] add listmnt(2) syscall

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Date:   Wed, 4 Oct 2023 15:22:54 -0400
From:   Paul Moore <paul@...l-moore.com>
To:     Miklos Szeredi <miklos@...redi.hu>
Cc:     Christian Brauner <brauner@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew House <mattlloydhouse@...il.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [RFC PATCH 3/3] add listmnt(2) syscall

On Thu, Sep 28, 2023 at 6:07 AM Miklos Szeredi <miklos@...redi.hu> wrote:
> On Tue, 19 Sept 2023 at 18:48, Paul Moore <paul@...l-moore.com> wrote:
>
> > > Ideally we avoid multiple capable(CAP_SYS_ADMIN) calls by only doing it
> > > once and saving the return value. capable() call's aren't that cheap.
> >
> > Agreed.  The capability check doesn't do any subject/object
> > comparisons so calling it for each mount is overkill.  However, I
> > would think we would want the LSM hook called from inside the loop as
> > that could involve a subject (@current) and object (individual mount
> > point) comparison.

My apologies, I was traveling and while I was quickly checking my
email each day this message was lost.  I'm very sorry for the delay in
responding.

> The security_sb_statfs() one?

Yes.

> Should a single failure result in a complete failure?

My opinion is that it should only result in the failure of that
listing/stat'ing that particular mount; if other mounts are allowed to
be queried than the operation should be allowed to continue.

> Why is it not enough to check permission on the parent?

Each mount has the potential to have a unique security identify in the
context of the LSM, and since the LSM access controls are generally
intended to support a subject-verb-object access control policy we
need to examine the subject and object together (the subject here is
@current, the object is the individual mount, and the verb is the
stat/list operation).

Does that make sense?

I'm looking at the v3 patchset right now, I've got some small nits,
but I'll add those to that thread.

-- 
paul-moore.com