Re: [PATCH v2 2/2] drm: Protect drm_master pointers in drm_lease.c

[Daniel Vetter]

On Fri, Jun 18, 2021 at 6:54 PM Desmond Cheong Zhi Xi
<desmondcheongzx@xxxxxxxxx> wrote:
>
> On 18/6/21 5:12 pm, Daniel Vetter wrote:
> > On Fri, Jun 18, 2021 at 5:05 AM Desmond Cheong Zhi Xi
> > <desmondcheongzx@xxxxxxxxx> wrote:
> >> On 18/6/21 1:12 am, Daniel Vetter wrote:
> >>> On Tue, Jun 15, 2021 at 10:36:45AM +0800, Desmond Cheong Zhi Xi wrote:
> >>>> This patch ensures that the device's master mutex is acquired before
> >>>> accessing pointers to struct drm_master that are subsequently
> >>>> dereferenced. Without the mutex, the struct drm_master may be freed
> >>>> concurrently by another process calling drm_setmaster_ioctl(). This
> >>>> could then lead to use-after-free errors.
> >>>>
> >>>> Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> >>>> Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> >>>> Reviewed-by: Emil Velikov <emil.l.velikov@xxxxxxxxx>
> >>>> ---
> >>>>    drivers/gpu/drm/drm_lease.c | 58 +++++++++++++++++++++++++++----------
> >>>>    1 file changed, 43 insertions(+), 15 deletions(-)
> >>>>
> >>>> diff --git a/drivers/gpu/drm/drm_lease.c b/drivers/gpu/drm/drm_lease.c
> >>>> index da4f085fc09e..3e6f689236e5 100644
> >>>> --- a/drivers/gpu/drm/drm_lease.c
> >>>> +++ b/drivers/gpu/drm/drm_lease.c
> >>>> @@ -107,10 +107,16 @@ static bool _drm_has_leased(struct drm_master *master, int id)
> >>>>     */
> >>>>    bool _drm_lease_held(struct drm_file *file_priv, int id)
> >>>>    {
> >>>> +    bool ret;
> >>>> +
> >>>>       if (!file_priv || !file_priv->master)
> >>>>               return true;
> >>>>
> >>>> -    return _drm_lease_held_master(file_priv->master, id);
> >>>> +    mutex_lock(&file_priv->master->dev->master_mutex);
> >>>
> >>> So maybe we have a bug somewhere, and the kerneldoc isn't 100% clear, but
> >>> I thought file_priv->master is invariant over the lifetime of file_priv.
> >>> So we don't need a lock to check anything here.
> >>>
> >>> It's the drm_device->master derefence that gets us into trouble. Well also
> >>> file_priv->is_owner is protected by dev->master_mutex.
> >>>
> >>> So I think with your previous patch all the access here in drm_lease.c is
> >>> ok and already protected? Or am I missing something?
> >>>
> >>> Thanks, Daniel
> >>>
> >>
> >> My thinking was that file_priv->master is invariant only if it is the
> >> creator of master. If file_priv->is_master is false, then a call to
> >> drm_setmaster_ioctl will invoke drm_new_set_master, which then allocates
> >> a new master for file_priv, and puts the old master.
> >>
> >> This could be an issue in _drm_lease_held_master, because we dereference
> >> master to get master->dev, master->lessor, and master->leases.
> >>
> >> With the same reasoning, in other parts of drm_lease.c, if there's an
> >> access to drm_file->master that's subsequently dereferenced, I added a
> >> lock around them.
> >>
> >> I could definitely be mistaken on this, so apologies if this scenario
> >> doesn't arise.
> >
> > You're right, I totally missed that setmaster can create a new master
> > instance. And the kerneldoc for drm_file->master doesn't explain this
> > and mention that we must hold drm_device.master_mutex while looking at
> > that pointer. Can you pls do a patch which improves the documentation
> > for that?
> >
>
> Sounds good, I'll add it to the patch series.
>
> > Now for the patch itself I'm not entirely sure what we should do.
> > Leaking the dev->master_mutex into drm_lease.c just because of the
> > setmaster ioctl is kinda unsightly. And we don't really care about the
> > fpriv->master changing under us, we only need to make sure it doesn't
> > get freed. And drm_master is refcounted already.
> >
> > So alternative solution: We add a drm_file_get_master() function which
> > calls drm_master_get under the lock, and we use that instead of
> > directly derefencing drm_file->master? Ofc then needs drm_master_put
> > instead of mutex_unlock. Kerneldoc should then also point at this new
> > function as the correct way to look at drm_file->master state.
> >
> > This way it's 100% clear we're dealing with a lifetime issue and not a
> > consistency issues.
> >
> > What do you think?
> > -Daniel
> >
>
> Makes sense to me, since the drm master itself holds the lease, as long
> as it isn't freed while we're using it, there's no need to prevent the
> value of fpriv->master from changing after we access it in drm_lease.c.
>
> I was going to say that it may be unclear when to use the
>
>         master = drm_file_get_master(file_priv);
>         ...
>         drm_master_put(&master);
>
> pattern, versus when to use
>
>         mutex_lock(&dev->master_mutex);
>         master = file_priv->master;
>         ...
>         mutex_unlock(&dev->master_mutex);
>
> . The second pattern, for example, is used in drm_getunique, and also in
> drm_setversion which calls drm_set_busid.
>
> But on closer inspection, it's clearer to me now that those functions
> need the master_mutex because they access protected fields such as
> unique and unique_len.
>
> Would it then be correct to state in the kerneldoc that
> drm_file_get_master() should be used to look at drm_file->master only if
> we aren't already holding master_mutex + have no other need to grab
> master_mutex?

Yeah that's sounds like a good decider for which variant to pick.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch