Message-ID: <CALmYWFvT8EBh+DwW0gw8yU36FKQxtsmO2s5sU6DD8_pfA9yxYg@mail.gmail.com>
Date:   Thu, 28 Sep 2023 13:04:01 -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 Thu, Sep 28, 2023 at 8:16 AM Mickaël Salaün <mic@...ikod.net> wrote:
>
> On Tue, Sep 26, 2023 at 02:19:51PM -0700, Jeff Xu wrote:
> > 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.
>
> Yes, there is always a way to find a pattern, and the best tool might be
> Coccinelle, but I think it's harder to miss with such tail calls.
>
> >
> > > * 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.
>
> landlock_log_request() should only be called at the end of LSM hooks.
> Tail calls is basically when you call a function at the end of the
> caller. This enables replacing "call" with "jmp" and save stack space.
> landlock_log_request() can fit with this pattern (if not using the
> caller's stack, which is not currently the case). See this tail call
> optimization example: https://godbolt.org/z/r88ofcW6x
>
Thanks for giving the context of the tail call.
Compile options are controlled by makefile, and can be customized. In
the past, I have had different projects setting different O levels for
various reasons, including disable optimization completely. Individual
Compiler implementation  also matters, gcc vs clang, etc. I think the
tail call is probably not essential to the discussion.

> I find it less error prone to not duplicate the error code (once for
> landlock_log_request and another for the caller's returned value). I
> also don't really see the pro of using a variable only to share this
> value. In ptrace.c, an "err" variable is used to check if the error is 0
> or not, but that is handled differently for most hooks.
>
> Makeing landlock_log_request() return a value also encourages us (thanks
> to compiler warnings) to use this value and keep the error handling
> consistent (especially for future code).
>
One general assumption about logging function is that it is not part
of the main business logic, i.e. if the logging statement is
removed/commented out, it doesn't have side effects to the main
business logic. This is probably why most log functions return void.

> Another feature that I'd like to add is to support a "permissive mode",
> that would enable to quickly see the impact of a sandbox without
> applying it for real. This might change some landlock_log_request()
> calls, so we'll see what fits best.
>
It is an excellent feature to have.
To implement that, I guess there will be a common function as a switch
to allow/deny, and logging the decision, depending on the permissive
setting.
>From that point, preparing the code towards that goal makes sense.

> >
> > > 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).
When there is more business logic to landlock_log_request, it is
probably better to rename the function. Most devs might assume the log
function does nothing but logging.  Having some meaningful name, e.g.
check_permissive_and_audit_log,  will help with readability.

Thanks!
-Jeff

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