netdev - Re: [PATCH v3 2/6] Landlock: Adding file_send

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Message-ID: <CAG48ez1PcHRDhRjtsq_JAr5e6z=XNjB1Mi_jjtr8EsRphnnb2g@mail.gmail.com>
Date: Fri, 16 Aug 2024 00:10:44 +0200
From: Jann Horn <jannh@...gle.com>
To: Tahera Fahimi <fahimitahera@...il.com>
Cc: outreachy@...ts.linux.dev, mic@...ikod.net, gnoack@...gle.com, 
	paul@...l-moore.com, jmorris@...ei.org, serge@...lyn.com, 
	linux-security-module@...r.kernel.org, linux-kernel@...r.kernel.org, 
	bjorn3_gh@...tonmail.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3 2/6] Landlock: Adding file_send_sigiotask signal
 scoping support

On Thu, Aug 15, 2024 at 11:28 PM Tahera Fahimi <fahimitahera@...il.com> wrote:
>
> On Thu, Aug 15, 2024 at 10:25:15PM +0200, Jann Horn wrote:
> > On Thu, Aug 15, 2024 at 8:29 PM Tahera Fahimi <fahimitahera@...il.com> wrote:
> > > This patch adds two new hooks "hook_file_set_fowner" and
> > > "hook_file_free_security" to set and release a pointer to the
> > > domain of the file owner. This pointer "fown_domain" in
> > > "landlock_file_security" will be used in "file_send_sigiotask"
> > > to check if the process can send a signal.
> > >
> > > Signed-off-by: Tahera Fahimi <fahimitahera@...il.com>
> > > ---
> > >  security/landlock/fs.c   | 18 ++++++++++++++++++
> > >  security/landlock/fs.h   |  6 ++++++
> > >  security/landlock/task.c | 27 +++++++++++++++++++++++++++
> > >  3 files changed, 51 insertions(+)
> > >
> > > diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> > > index 7877a64cc6b8..d05f0e9c5e54 100644
> > > --- a/security/landlock/fs.c
> > > +++ b/security/landlock/fs.c
> > > @@ -1636,6 +1636,21 @@ static int hook_file_ioctl_compat(struct file *file, unsigned int cmd,
> > >         return -EACCES;
> > >  }
> > >
> > > +static void hook_file_set_fowner(struct file *file)
> > > +{
> > > +       write_lock_irq(&file->f_owner.lock);
> >
> > Before updating landlock_file(file)->fown_domain, this hook must also
> > drop a reference on the old domain - maybe by just calling
> > landlock_put_ruleset_deferred(landlock_file(file)->fown_domain) here.
> Hi Jann,
>
> Thanks for the feedback :)
> It totally make sense.
> > > +       landlock_file(file)->fown_domain = landlock_get_current_domain();
> > > +       landlock_get_ruleset(landlock_file(file)->fown_domain);
> > > +       write_unlock_irq(&file->f_owner.lock);
> > > +}
> > > +
> > > +static void hook_file_free_security(struct file *file)
> > > +{
> > > +       write_lock_irq(&file->f_owner.lock);
> > > +       landlock_put_ruleset(landlock_file(file)->fown_domain);
> I was thinking of if we can replace this landlock_put_ruleset with
> landlock_put_ruleset_deferred. In this case, it would be better use of
> handling the lock?

I don't think you have to take the "file->f_owner.lock" in this hook -
the file has already been torn down pretty far, nothing is going to be
able to trigger the file_set_fowner hook anymore.

But either way, you're right that we can't just use
landlock_put_ruleset() here because landlock_put_ruleset() can sleep
and the file_free_security hook can be invoked from non-sleepable
context. (This only happens when fput() directly calls file_free(),
and I think that only happens with ->fown_domain==NULL, so technically
it would also be fine to do something like "if (domain)
landlock_put_ruleset(domain);".)
If you test your current code in a kernel that was built with
CONFIG_DEBUG_ATOMIC_SLEEP=y, this will probably print an warning
message in the kernel log (dmesg). You're right that using
landlock_put_ruleset_deferred() instead would fix that.

I think the right solution here is probably just to do:

static void hook_file_free_security(struct file *file)
{
  landlock_put_ruleset_deferred(landlock_file(file)->fown_domain);
}

Alternatively it would also work to do this - this code is probably a
bit more efficient but also a little less clear:

static void hook_file_free_security(struct file *file)
{
  /* don't trigger might_sleep() for tearing down unopened file */
  if (landlock_file(file)->fown_domain)
    landlock_put_ruleset(landlock_file(file)->fown_domain);
}

>
> > > +       write_unlock_irq(&file->f_owner.lock);
> > > +}