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

[Daniel Vetter]

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

>
> >
> >> +
> >>      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
> >>
> >
>


-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch