Re: [Intel-gfx] [PATCH v3 4/9] drm: fix potential null ptr dereferences in drm_{auth, ioctl}

[Desmond Cheong Zhi Xi]

On 19/8/21 12:33 am, Daniel Vetter wrote:
> On Wed, Aug 18, 2021 at 5:37 PM Desmond Cheong Zhi Xi
> <desmondcheongzx@xxxxxxxxx> wrote:
> > On 18/8/21 6:11 pm, Daniel Vetter wrote:
> > > On Wed, Aug 18, 2021 at 03:38:19PM +0800, Desmond Cheong Zhi Xi wrote:
> > > > There are three areas where we dereference struct drm_master without
> > > > checking if the pointer is non-NULL.
> > > >
> > > > 1. drm_getmagic is called from the ioctl_handler. Since
> > > > DRM_IOCTL_GET_MAGIC has no ioctl flags, drm_getmagic is run without
> > > > any check that drm_file.master has been set.
> > > >
> > > > 2. Similarly, drm_getunique is called from the ioctl_handler, but
> > > > DRM_IOCTL_GET_UNIQUE has no ioctl flags. So there is no guarantee that
> > > > drm_file.master has been set.
> > >
> > > I think the above two are impossible, due to the refcounting rules for
> > > struct file.
> >
> > Right, will drop those two parts from the patch.
> >
> > > > 3. drm_master_release can also be called without having a
> > > > drm_file.master set. Here is one error path:
> > > >     drm_open():
> > > >       drm_open_helper():
> > > >         drm_master_open():
> > > >           drm_new_set_master(); <--- returns -ENOMEM,
> > > >                                      drm_file.master not set
> > > >         drm_file_free():
> > > >           drm_master_release(); <--- NULL ptr dereference
> > > >                                      (file_priv->master->magic_map)
> > > >
> > > > Fix these by checking if the master pointers are NULL before use.
> > > >
> > > > Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
> > > > ---
> > > >    drivers/gpu/drm/drm_auth.c  | 16 ++++++++++++++--
> > > >    drivers/gpu/drm/drm_ioctl.c |  5 +++++
> > > >    2 files changed, 19 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
> > > > index f9267b21556e..b7230604496b 100644
> > > > --- a/drivers/gpu/drm/drm_auth.c
> > > > +++ b/drivers/gpu/drm/drm_auth.c
> > > > @@ -95,11 +95,18 @@ EXPORT_SYMBOL(drm_is_current_master);
> > > >    int drm_getmagic(struct drm_device *dev, void *data, struct drm_file *file_priv)
> > > >    {
> > > >       struct drm_auth *auth = data;
> > > > +    struct drm_master *master;
> > > >       int ret = 0;
> > > >
> > > >       mutex_lock(&dev->master_mutex);
> > > > +    master = file_priv->master;
> > > > +    if (!master) {
> > > > +            mutex_unlock(&dev->master_mutex);
> > > > +            return -EINVAL;
> > > > +    }
> > > > +
> > > >       if (!file_priv->magic) {
> > > > -            ret = idr_alloc(&file_priv->master->magic_map, file_priv,
> > > > +            ret = idr_alloc(&master->magic_map, file_priv,
> > > >                               1, 0, GFP_KERNEL);
> > > >               if (ret >= 0)
> > > >                       file_priv->magic = ret;
> > > > @@ -355,8 +362,12 @@ void drm_master_release(struct drm_file *file_priv)
> > > >
> > > >       mutex_lock(&dev->master_mutex);
> > > >       master = file_priv->master;
> > > > +
> > > > +    if (!master)
> > > > +            goto unlock;
> > >
> > > This is a bit convoluted, since we're in the single-threaded release path
> > > we don't need any locking for file_priv related things. Therefore we can
> > > pull the master check out and just directly return.
> > >
> > > But since it's a bit surprising maybe a comment that this can happen when
> > > drm_master_open in drm_open_helper fails?
> >
> > Sounds good. This can actually also happen in the failure path of
> > mock_drm_getfile if anon_inode_getfile fails. I'll leave a short note
> > about both of them.
> >
> > > Another option, and maybe cleaner, would be to move the drm_master_release
> > > from drm_file_free into drm_close_helper. That would be fully symmetrical
> > > and should also fix the bug here?
> > > -Daniel
> > Hmmm maybe the first option to move the check out of the lock might be
> > better. If I'm not wrong, we would otherwise also need to move
> > drm_master_release into drm_client_close.
>
> Do we have to?
>
> If I haven't missed anything, the drm_client stuff only calls
> drm_file_alloc and doesn't set up a master. So this should work?
> -Daniel

Ahhhh ok yes, I understand what's going on better now. I think you're 
right, moving drm_master_release into drm_close_helper should fix all 
the places it can go wrong.

> > > > +
> > > >       if (file_priv->magic)
> > > > -            idr_remove(&file_priv->master->magic_map, file_priv->magic);
> > > > +            idr_remove(&master->magic_map, file_priv->magic);
> > > >
> > > >       if (!drm_is_current_master_locked(file_priv))
> > > >               goto out;
> > > > @@ -379,6 +390,7 @@ void drm_master_release(struct drm_file *file_priv)
> > > >               drm_master_put(&file_priv->master);
> > > >               spin_unlock(&dev->master_lookup_lock);
> > > >       }
> > > > +unlock:
> > > >       mutex_unlock(&dev->master_mutex);
> > > >    }
> > > >
> > > > diff --git a/drivers/gpu/drm/drm_ioctl.c b/drivers/gpu/drm/drm_ioctl.c
> > > > index 26f3a9ede8fe..4d029d3061d9 100644
> > > > --- a/drivers/gpu/drm/drm_ioctl.c
> > > > +++ b/drivers/gpu/drm/drm_ioctl.c
> > > > @@ -121,6 +121,11 @@ int drm_getunique(struct drm_device *dev, void *data,
> > > >
> > > >       mutex_lock(&dev->master_mutex);
> > > >       master = file_priv->master;
> > > > +    if (!master) {
> > > > +            mutex_unlock(&dev->master_mutex);
> > > > +            return -EINVAL;
> > > > +    }
> > > > +
> > > >       if (u->unique_len >= master->unique_len) {
> > > >               if (copy_to_user(u->unique, master->unique, master->unique_len)) {
> > > >                       mutex_unlock(&dev->master_mutex);
> > > > --
> > > > 2.25.1