Date:   Tue, 26 Sep 2023 14:19:51 -0700
From:   Jeff Xu <jeffxu@...gle.com>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Eric Paris <eparis@...hat.com>, James Morris <jmorris@...ei.org>,
        Paul Moore <paul@...l-moore.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Ben Scarlato <akhna@...gle.com>,
        Günther Noack <gnoack@...gle.com>,
        Jorge Lucangeli Obes <jorgelo@...gle.com>,
        Konstantin Meskhidze <konstantin.meskhidze@...wei.com>,
        Shervin Oloumi <enlightened@...gle.com>, audit@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [RFC PATCH v1 5/7] landlock: Log file-related requests

On Tue, Sep 26, 2023 at 6:35 AM Mickaël Salaün <mic@...ikod.net> wrote:
>
> On Mon, Sep 25, 2023 at 06:26:28PM -0700, Jeff Xu wrote:
> > On Wed, Sep 20, 2023 at 11:17 PM Mickaël Salaün <mic@...ikod.net> wrote:
> > >
> > > Add audit support for mkdir, mknod, symlink, unlink, rmdir, truncate,
> > > and open requests.
> > >
> > > Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> > > ---
> > >  security/landlock/audit.c | 114 ++++++++++++++++++++++++++++++++++++++
> > >  security/landlock/audit.h |  32 +++++++++++
> > >  security/landlock/fs.c    |  62 ++++++++++++++++++---
> > >  3 files changed, 199 insertions(+), 9 deletions(-)
> > >
>
> > > +static void
> > > +log_request(const int error, struct landlock_request *const request,
> > > +           const struct landlock_ruleset *const domain,
> > > +           const access_mask_t access_request,
> > > +           const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS])
> > > +{
> > > +       struct audit_buffer *ab;
> > > +
> > > +       if (WARN_ON_ONCE(!error))
> > > +               return;
> > > +       if (WARN_ON_ONCE(!request))
> > > +               return;
> > > +       if (WARN_ON_ONCE(!domain || !domain->hierarchy))
> > > +               return;
> > > +
> > > +       /* Uses GFP_ATOMIC to not sleep. */
> > > +       ab = audit_log_start(audit_context(), GFP_ATOMIC | __GFP_NOWARN,
> > > +                            AUDIT_LANDLOCK);
> > > +       if (!ab)
> > > +               return;
> > > +
> > > +       update_request(request, domain, access_request, layer_masks);
> > > +
> > > +       log_task(ab);
> > > +       audit_log_format(ab, " domain=%llu op=%s errno=%d missing-fs-accesses=",
> > > +                        request->youngest_domain,
> > > +                        op_to_string(request->operation), -error);
> > > +       log_accesses(ab, request->missing_access);
> > > +       audit_log_lsm_data(ab, &request->audit);
> > > +       audit_log_end(ab);
> > > +}
> > > +
> > > +// TODO: Make it generic, not FS-centric.
> > > +int landlock_log_request(
> > > +       const int error, struct landlock_request *const request,
> > > +       const struct landlock_ruleset *const domain,
> > > +       const access_mask_t access_request,
> > > +       const layer_mask_t (*const layer_masks)[LANDLOCK_NUM_ACCESS_FS])
> > > +{
> > > +       /* No need to log the access request, only the missing accesses. */
> > > +       log_request(error, request, domain, access_request, layer_masks);
> > > +       return error;
> > > +}
>
> > > @@ -636,7 +638,8 @@ static bool is_access_to_paths_allowed(
> > >  }
> > >
> > >  static int current_check_access_path(const struct path *const path,
> > > -                                    access_mask_t access_request)
> > > +                                    access_mask_t access_request,
> > > +                                    struct landlock_request *const request)
> > >  {
> > >         const struct landlock_ruleset *const dom =
> > >                 landlock_get_current_domain();
> > > @@ -650,7 +653,10 @@ static int current_check_access_path(const struct path *const path,
> > >                                        NULL, 0, NULL, NULL))
> > >                 return 0;
> > >
> > > -       return -EACCES;
> > > +       request->audit.type = LSM_AUDIT_DATA_PATH;
> > > +       request->audit.u.path = *path;
> > > +       return landlock_log_request(-EACCES, request, dom, access_request,
> > > +                                   &layer_masks);
> >
> > It might be more readable to let landlock_log_request return void.
> > Then the code will look like below.
> >
> > landlock_log_request(-EACCES, request, dom, access_request,  &layer_masks);
> > return -EACCES;
> >
> > The allow/deny logic will be in this function, i.e. reader
> > doesn't need to check landlock_log_request's implementation to find
> > out it never returns 0.
>
> I did that in an early version of this patch, but I finally choose to write
> 'return lanlock_log_request();` for mainly two reasons:
> * to help not forget to call this function at any non-zero return values
>   (which can easily be checked with grep),

"grep -A 2 landlock_log_request" would serve the same purpose though.

> * to do tail calls.
>
> I guess compiler should be smart enough to do tail calls with a variable
> set indirection, but I'd like to check that.
>

What are tail calls and what is the benefit of this code pattern ?
i.e. pass the return value into landlock_log_request() and make it a
single point of setting return value for all landlock hooks.

> To make it easier to read (and to not forget returning the error), the
> landlock_log_request() calls a void log_request() helper, and returns
> the error itself. It is then easy to review and know what's happening
> without reading log_request().
>
> I'd like the compiler to check itself that every LSM hook returned
> values are either 0 or comming from landlock_log_request() but I think
> it's not possible right now. Coccinelle might help here though.
>
> BTW, in a next version, we might have landlock_log_request() called even
> for allowed requests (i.e. returned value of 0).

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