[PATCH] drm: hold gem reference until object is no longer accessed

[Sam Ravnborg]

Hi Steve.

On Thu, Jul 16, 2020 at 10:29:52PM +0200, Sam Ravnborg wrote:
> Hi Steve and others.
> 
> On Fri, Jul 10, 2020 at 06:40:26PM -0400, Steve Cohen wrote:
> > BUG: KASAN: use-after-free in drm_gem_open_ioctl
> > 
> > There is potential for use-after-free here if the GEM object
> > handle is closed between the idr lookup and retrieving the size
> > from the object since a local reference is not being held at that
> > point. Hold the local reference while the object can still be
> > accessed to resolve this.
> > 
> > Signed-off-by: Steve Cohen <cohens at codeaurora.org>
> > ---
> >  drivers/gpu/drm/drm_gem.c | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> > index 7bf628e..4b2891c 100644
> > --- a/drivers/gpu/drm/drm_gem.c
> > +++ b/drivers/gpu/drm/drm_gem.c
> > @@ -898,14 +898,15 @@ drm_gem_open_ioctl(struct drm_device *dev, void *data,
> >  
> >  	/* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
> >  	ret = drm_gem_handle_create_tail(file_priv, obj, &handle);
> > -	drm_gem_object_put_unlocked(obj);
> >  	if (ret)
> > -		return ret;
> > +		goto out;
> >  
> >  	args->handle = handle;
> >  	args->size = obj->size;
> >  
> > -	return 0;
> > +out:
> > +	drm_gem_object_put_unlocked(obj);
> > +	return ret;
> 
> Lookign at drm_gem_flink_ioctl() that is implmented just above this
> functions there are two things that I noted.
> 
> 1) In drm_gem_flink_ioctl() the label is named "err:" - and my OCD likes
> that similar labels have the same name.
> 
> 2) The function takes the object_name_lock but fails to release it in
> the error situation.
Daniel pointed out on irc that drm_gem_handle_create_tail releases the
lock. If I had read the comment I would have noticed too - sigh.

With the label name fixed to "err:" like used in the function above:
Reviewed-by: Sam Ravnborg <sam at ravnborg.org>

Please re-submit.

	Sam

> 
> Danile Vetter updated the locking in
> 20228c447846da9399ead53fdbbc8ab69b47788a ("drm/gem: completely close gem_open vs. gem_close races")
> 
> but I failed to follow it all.
> 
> 	Sam
> 
> >  }
> >  
> >  /**
> > -- 
> > The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum,
> > a Linux Foundation Collaborative Project
> > 
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