[PATCH] drm/vgem: Close use-after-free race in vgem_gem_create

* [PATCH] drm/vgem: Close use-after-free race in vgem_gem_create
@ 2020-02-02 13:21 ` Daniel Vetter
  0 siblings, 0 replies; 15+ messages in thread
From: Daniel Vetter @ 2020-02-02 13:21 UTC (permalink / raw)
  To: DRI Development
  Cc: Intel Graphics Development, Daniel Vetter, Dan Carpenter,
	Hillf Danton, stable, Emil Velikov, Sean Paul, Chris Wilson,
	Eric Anholt, Sam Ravnborg, Rob Clark, Daniel Vetter

There's two references floating around here (for the object reference,
not the handle_count reference, that's a different thing):

- The temporary reference held by vgem_gem_create, acquired by
  creating the object and released by calling
  drm_gem_object_put_unlocked.

- The reference held by the object handle, created by
  drm_gem_handle_create. This one generally outlives the function,
  except if a 2nd thread races with a GEM_CLOSE ioctl call.

So usually everything is correct, except in that race case, where the
access to gem_object->size could be looking at freed data already.
Which again isn't a real problem (userspace shot its feet off already
with the race, we could return garbage), but maybe someone can exploit
this as an information leak.

Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Hillf Danton <hdanton@sina.com>
Cc: Reported-by: syzbot+0dc4444774d419e916c8@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Anholt <eric@anholt.net>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Rob Clark <robdclark@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
---
 drivers/gpu/drm/vgem/vgem_drv.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/vgem/vgem_drv.c b/drivers/gpu/drm/vgem/vgem_drv.c
index 5bd60ded3d81..909eba43664a 100644
--- a/drivers/gpu/drm/vgem/vgem_drv.c
+++ b/drivers/gpu/drm/vgem/vgem_drv.c
@@ -196,9 +196,10 @@ static struct drm_gem_object *vgem_gem_create(struct drm_device *dev,
 		return ERR_CAST(obj);
 
 	ret = drm_gem_handle_create(file, &obj->base, handle);
-	drm_gem_object_put_unlocked(&obj->base);
-	if (ret)
+	if (ret) {
+		drm_gem_object_put_unlocked(&obj->base);
 		return ERR_PTR(ret);
+	}
 
 	return &obj->base;
 }
@@ -221,7 +222,9 @@ static int vgem_gem_dumb_create(struct drm_file *file, struct drm_device *dev,
 	args->size = gem_object->size;
 	args->pitch = pitch;
 
-	DRM_DEBUG("Created object of size %lld\n", size);
+	drm_gem_object_put_unlocked(gem_object);
+
+	DRM_DEBUG("Created object of size %llu\n", args->size);
 
 	return 0;
 }
-- 
2.24.1


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