[PATCH] drm/syncobj: Fix use-after-free

[Daniel Vetter]

On Tue, Jan 19, 2021 at 02:08:12PM +0100, Christian König wrote:
> Am 19.01.21 um 14:03 schrieb Daniel Vetter:
> > While reviewing Christian's annotation patch I noticed that we have a
> > user-after-free for the WAIT_FOR_SUBMIT case: We drop the syncobj
> > reference before we've completed the waiting.
> > 
> > Of course usually there's nothing bad happening here since userspace
> > keeps the reference, but we can't rely on userspace to play nice here!
> > 
> > Signed-off-by: Daniel Vetter <daniel.vetter at intel.com>
> > Fixes: bc9c80fe01a2 ("drm/syncobj: use the timeline point in drm_syncobj_find_fence v4")
> > Cc: Christian König <christian.koenig at amd.com>
> > Cc: Lionel Landwerlin <lionel.g.landwerlin at intel.com>
> > Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > Cc: Maxime Ripard <mripard at kernel.org>
> > Cc: Thomas Zimmermann <tzimmermann at suse.de>
> > Cc: David Airlie <airlied at linux.ie>
> > Cc: Daniel Vetter <daniel at ffwll.ch>
> > Cc: dri-devel at lists.freedesktop.org
> > Cc: <stable at vger.kernel.org> # v5.2+
> 
> Reviewed-by: Christian König <christian.koenig at amd.com>

Pushed to drm-misc-fixes, thanks for reviewing.
-Daniel

> 
> > ---
> >   drivers/gpu/drm/drm_syncobj.c | 8 +++++---
> >   1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
> > index 6e74e6745eca..349146049849 100644
> > --- a/drivers/gpu/drm/drm_syncobj.c
> > +++ b/drivers/gpu/drm/drm_syncobj.c
> > @@ -388,19 +388,18 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
> >   		return -ENOENT;
> >   	*fence = drm_syncobj_fence_get(syncobj);
> > -	drm_syncobj_put(syncobj);
> >   	if (*fence) {
> >   		ret = dma_fence_chain_find_seqno(fence, point);
> >   		if (!ret)
> > -			return 0;
> > +			goto out;
> >   		dma_fence_put(*fence);
> >   	} else {
> >   		ret = -EINVAL;
> >   	}
> >   	if (!(flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_FOR_SUBMIT))
> > -		return ret;
> > +		goto out;
> >   	memset(&wait, 0, sizeof(wait));
> >   	wait.task = current;
> > @@ -432,6 +431,9 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
> >   	if (wait.node.next)
> >   		drm_syncobj_remove_wait(syncobj, &wait);
> > +out:
> > +	drm_syncobj_put(syncobj);
> > +
> >   	return ret;
> >   }
> >   EXPORT_SYMBOL(drm_syncobj_find_fence);
> 

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