drm/prime: Always add exported buffers to the handle cache
... not only when the dma-buf is freshly created. In contrived
examples someone else could have exported/imported the dma-buf already
and handed us the gem object with a flink name. If such on object gets
reexported as a dma_buf we won't have it in the handle cache already,
which breaks the guarantee that for dma-buf imports we always hand
back an existing handle if there is one.
This is exercised by igt/prime_self_import/with_one_bo_two_files
Now if we extend the locked sections just a notch more we can also
plug th racy buf/handle cache setup in handle_to_fd:
If evil userspace races a concurrent gem close against a prime export
operation we can end up tearing down the gem handle before the dma buf
handle cache is set up. When handle_to_fd gets around to adding the
handle to the cache there will be no one left to clean it up,
effectily leaking the bo (and the dma-buf, since the handle cache
holds a ref on the dma-buf):
Thread A Thread B
handle_to_fd:
lookup gem object from handle
creates new dma_buf
gem_close on the same handle
obj->dma_buf is set, but file priv buf
handle cache has no entry
obj->handle_count drops to 0
drm_prime_add_buf_handle sets up the handle cache
-> We have a dma-buf reference in the handle cache, but since the
handle_count of the gem object already dropped to 0 no on will clean
it up. When closing the drm device fd we'll hit the WARN_ON in
drm_prime_destroy_file_private.
The important change is to extend the critical section of the
filp->prime.lock to cover the gem handle lookup. This serializes with
a concurrent gem handle close.
This leak is exercised by igt/prime_self_import/export-vs-gem_close-race
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 0a5a0ca..1ce88c3 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -195,10 +195,12 @@
* Note: obj->dma_buf can't disappear as long as we still hold a
* handle reference in obj->handle_count.
*/
+ mutex_lock(&filp->prime.lock);
if (obj->dma_buf) {
- drm_prime_remove_buf_handle(&filp->prime,
- obj->dma_buf);
+ drm_prime_remove_buf_handle_locked(&filp->prime,
+ obj->dma_buf);
}
+ mutex_unlock(&filp->prime.lock);
}
static void drm_gem_object_ref_bug(struct kref *list_kref)