Object creation is a careful dance where we must guarantee that the object is fully constructed before it is visible to other threads, and GEM buffer objects are no difference.
Final publishing happens by calling drm_gem_handle_create(). After that the only allowed thing to do is call drm_gem_object_put() because a concurrent call to the GEM_CLOSE ioctl with a correctly guessed id (which is trivial since we have a linear allocator) can already tear down the object again.
Luckily most drivers get this right, the very few exceptions I've pinged the relevant maintainers for. Unfortunately we also need drm_gem_handle_create() when creating additional handles for an already existing object (e.g. GETFB ioctl or the various bo import ioctl), and hence we cannot have a drm_gem_handle_create_and_put() as the only exported function to stop these issues from happening.
Now unfortunately the implementation of drm_gem_handle_create() isn't living up to standards: It does correctly finishe object initialization at the global level, and hence is safe against a concurrent tear down. But it also sets up the file-private aspects of the handle, and that part goes wrong: We fully register the object in the drm_file.object_idr before calling drm_vma_node_allow() or obj->funcs->open, which opens up races against concurrent removal of that handle in drm_gem_handle_delete().
Fix this with the usual two-stage approach of first reserving the handle id, and then only registering the object after we've completed the file-private setup.
Jacek reported this with a testcase of concurrently calling GEM_CLOSE on a freshly-created object (which also destroys the object), but it should be possible to hit this with just additional handles created through import or GETFB without completed destroying the underlying object with the concurrent GEM_CLOSE ioctl calls.
Note that the close-side of this race was fixed in f6cd7daecff5 ("drm: Release driver references to handle before making it available again"), which means a cool 9 years have passed until someone noticed that we need to make this symmetry or there's still gaps left :-/ Without the 2-stage close approach we'd still have a race, therefore that's an integral part of this bugfix.
More importantly, this means we can have NULL pointers behind allocated id in our drm_file.object_idr. We need to check for that now:
- drm_gem_handle_delete() checks for ERR_OR_NULL already
- drm_gem.c:object_lookup() also chekcs for NULL
- drm_gem_release() should never be called if there's another thread still existing that could call into an IOCTL that creates a new handle, so cannot race. For paranoia I added a NULL check to drm_gem_object_release_handle() though.
- most drivers (etnaviv, i915, msm) are find because they use idr_find(), which maps both ENOENT and NULL to NULL.
- drivers using idr_for_each_entry() should also be fine, because idr_get_next does filter out NULL entries and continues the iteration.
- The same holds for drm_show_memory_stats().
v2: Use drm_WARN_ON (Thomas)
Reported-by: Jacek Lawrynowicz jacek.lawrynowicz@linux.intel.com Tested-by: Jacek Lawrynowicz jacek.lawrynowicz@linux.intel.com Reviewed-by: Thomas Zimmermann tzimmermann@suse.de Cc: stable@vger.kernel.org Cc: Jacek Lawrynowicz jacek.lawrynowicz@linux.intel.com Cc: Maarten Lankhorst maarten.lankhorst@linux.intel.com Cc: Maxime Ripard mripard@kernel.org Cc: Thomas Zimmermann tzimmermann@suse.de Cc: David Airlie airlied@gmail.com Cc: Simona Vetter simona@ffwll.ch Signed-off-by: Simona Vetter simona.vetter@intel.com Signed-off-by: Simona Vetter simona.vetter@ffwll.ch --- drivers/gpu/drm/drm_gem.c | 10 +++++++++- include/drm/drm_file.h | 3 +++ 2 files changed, 12 insertions(+), 1 deletion(-)
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c index bc505d938b3e..1aa9192c4cc6 100644 --- a/drivers/gpu/drm/drm_gem.c +++ b/drivers/gpu/drm/drm_gem.c @@ -316,6 +316,9 @@ drm_gem_object_release_handle(int id, void *ptr, void *data) struct drm_file *file_priv = data; struct drm_gem_object *obj = ptr;
+ if (drm_WARN_ON(obj->dev, !data)) + return 0; + if (obj->funcs->close) obj->funcs->close(obj, file_priv);
@@ -436,7 +439,7 @@ drm_gem_handle_create_tail(struct drm_file *file_priv, idr_preload(GFP_KERNEL); spin_lock(&file_priv->table_lock);
- ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT); + ret = idr_alloc(&file_priv->object_idr, NULL, 1, 0, GFP_NOWAIT);
spin_unlock(&file_priv->table_lock); idr_preload_end(); @@ -457,6 +460,11 @@ drm_gem_handle_create_tail(struct drm_file *file_priv, goto err_revoke; }
+ /* mirrors drm_gem_handle_delete to avoid races */ + spin_lock(&file_priv->table_lock); + obj = idr_replace(&file_priv->object_idr, obj, handle); + WARN_ON(obj != NULL); + spin_unlock(&file_priv->table_lock); *handlep = handle; return 0;
diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h index eab7546aad79..115763799625 100644 --- a/include/drm/drm_file.h +++ b/include/drm/drm_file.h @@ -300,6 +300,9 @@ struct drm_file { * * Mapping of mm object handles to object pointers. Used by the GEM * subsystem. Protected by @table_lock. + * + * Note that allocated entries might be NULL as a transient state when + * creating or deleting a handle. */ struct idr object_idr;