On 14/02/2023 12:50, Tvrtko Ursulin wrote:
> From: Tvrtko Ursulin <[email protected]>
>
> Currently drm_gem_handle_create_tail exposes the handle to userspace
> before the buffer object constructions is complete. This allowing
> of working against a partially constructed object, which may also be in
> the process of having its creation fail, can have a range of negative
> outcomes.
>
> A lot of those will depend on what the individual drivers are doing in
> their obj->funcs->open() callbacks, and also with a common failure mode
> being -ENOMEM from drm_vma_node_allow.
>
> We can make sure none of this can happen by allocating a handle last,
> although with a downside that more of the function now runs under the
> dev->object_name_lock.
>
> Looking into the individual drivers open() hooks, we have
> amdgpu_gem_object_open which seems like it could have a potential security
> issue without this change.
>
> A couple drivers like qxl_gem_object_open and vmw_gem_object_open
> implement no-op hooks so no impact for them.
>
> A bunch of other require a deeper look by individual owners to asses for
> impact. Those are lima_gem_object_open, nouveau_gem_object_open,
> panfrost_gem_open, radeon_gem_object_open and virtio_gpu_gem_object_open.
I've looked over the panfrost code, and I can't see how this could
create a security hole there. It looks like there's a path which can
confuse the shrinker (so objects might not be purged when they could
be[1]) but they would be freed properly in the normal path - so no worse
than user space could already do.
[1] gpu_usecount is incremented in panfrost_lookup_bos() per bo, but not
decremented on failure.
> Putting aside the risk assesment of the above, some common scenarios to
> think about are along these lines:
>
> 1)
> Userspace closes a handle by speculatively "guessing" it from a second
> thread.
>
> This results in an unreachable buffer object so, a memory leak.
>
> 2)
> Same as 1), but object is in the process of getting closed (failed
> creation).
>
> The second thread is then able to re-cycle the handle and idr_remove would
> in the first thread would then remove the handle it does not own from the
> idr.
This, however, looks plausible - and I can see how this could
potentially trigger a security hole in user space.
> 3)
> Going back to the earlier per driver problem space - individual impact
> assesment of allowing a second thread to access and operate on a partially
> constructed handle / object. (Can something crash? Leak information?)
>
> In terms of identifying when the problem started I will tag some patches
> as references, but not all, if even any, of them actually point to a
> broken state. I am just identifying points at which more opportunity for
> issues to arise was added.
>
> References: 304eda32920b ("drm/gem: add hooks to notify driver when object
> handle is created/destroyed")
> References: ca481c9b2a3a ("drm/gem: implement vma access management")
> References: b39b5394fabc ("drm/gem: Add drm_gem_object_funcs")
> Cc: [email protected]
> Cc: Rob Clark <[email protected]>
> Cc: Ben Skeggs <[email protected]>
> Cc: David Herrmann <[email protected]>
> Cc: Noralf Trønnes <[email protected]>
> Cc: David Airlie <[email protected]>
> Cc: Daniel Vetter <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: [email protected]
> Cc: Steven Price <[email protected]>
> Cc: [email protected]
> Cc: [email protected]
> Cc: Zack Rusin <[email protected]>
FWIW I think this makes the code easier to reason about, so
Reviewed-by: Steven Price <[email protected]>
> ---
> drivers/gpu/drm/drm_gem.c | 48 +++++++++++++++++++--------------------
> 1 file changed, 24 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
> index aa15c52ae182..e3d897bca0f2 100644
> --- a/drivers/gpu/drm/drm_gem.c
> +++ b/drivers/gpu/drm/drm_gem.c
> @@ -356,52 +356,52 @@ drm_gem_handle_create_tail(struct drm_file *file_priv,
> u32 *handlep)
> {
> struct drm_device *dev = obj->dev;
> - u32 handle;
> int ret;
>
> WARN_ON(!mutex_is_locked(&dev->object_name_lock));
> if (obj->handle_count++ == 0)
> drm_gem_object_get(obj);
>
> + ret = drm_vma_node_allow(&obj->vma_node, file_priv);
> + if (ret)
> + goto err_put;
> +
> + if (obj->funcs->open) {
> + ret = obj->funcs->open(obj, file_priv);
> + if (ret)
> + goto err_revoke;
> + }
> +
> /*
> - * Get the user-visible handle using idr. Preload and perform
> - * allocation under our spinlock.
> + * Get the user-visible handle using idr as the _last_ step.
> + * Preload and perform allocation under our spinlock.
> */
> idr_preload(GFP_KERNEL);
> spin_lock(&file_priv->table_lock);
> -
> ret = idr_alloc(&file_priv->object_idr, obj, 1, 0, GFP_NOWAIT);
> -
> spin_unlock(&file_priv->table_lock);
> idr_preload_end();
>
> - mutex_unlock(&dev->object_name_lock);
> if (ret < 0)
> - goto err_unref;
> -
> - handle = ret;
> + goto err_close;
>
> - ret = drm_vma_node_allow(&obj->vma_node, file_priv);
> - if (ret)
> - goto err_remove;
> + mutex_unlock(&dev->object_name_lock);
>
> - if (obj->funcs->open) {
> - ret = obj->funcs->open(obj, file_priv);
> - if (ret)
> - goto err_revoke;
> - }
> + *handlep = ret;
>
> - *handlep = handle;
> return 0;
>
> +err_close:
> + if (obj->funcs->close)
> + obj->funcs->close(obj, file_priv);
> err_revoke:
> drm_vma_node_revoke(&obj->vma_node, file_priv);
> -err_remove:
> - spin_lock(&file_priv->table_lock);
> - idr_remove(&file_priv->object_idr, handle);
> - spin_unlock(&file_priv->table_lock);
> -err_unref:
> - drm_gem_object_handle_put_unlocked(obj);
> +err_put:
> + if (--obj->handle_count == 0)
> + drm_gem_object_put(obj);
> +
> + mutex_unlock(&dev->object_name_lock);
> +
> return ret;
> }
>
