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@xxxxxxxxxxxxxx>
> > ---
> > 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@xxxxxxxxxxxx>
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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