[PATCH v2] drm/vmwgfx: Do not drop the reference to the handle too soon

[Zack Rusin]

From: Zack Rusin <zackr at vmware.com>

v2: Update the commit message to include note describing why the second
usag of vmw_gem_object_create_with_handle in vmwgfx_surface.c wasn't
changed

It is possible for userspace to predict the next buffer handle and
to destroy the buffer while it's still used by the kernel. Delay
dropping the internal reference on the buffers until kernel is done
with them.

Also fixes the second usage of vmw_gem_object_create_with_handle in
vmwgfx_surface.c which wasn't grabbing an explicit reference
to the gem object which could have been destroyed by the userspace
on the owning surface at any point.

Signed-off-by: Zack Rusin <zackr at vmware.com>
Fixes: 8afa13a0583f ("drm/vmwgfx: Implement DRIVER_GEM")
Cc: <stable at vger.kernel.org> # v5.17+
---
 drivers/gpu/drm/vmwgfx/vmwgfx_bo.c      | 3 ++-
 drivers/gpu/drm/vmwgfx/vmwgfx_gem.c     | 4 ++--
 drivers/gpu/drm/vmwgfx/vmwgfx_surface.c | 1 -
 3 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
index 43ffa5c7acbd..65bd88c8fef9 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_bo.c
@@ -708,7 +708,8 @@ int vmw_dumb_create(struct drm_file *file_priv,
 	ret = vmw_gem_object_create_with_handle(dev_priv, file_priv,
 						args->size, &args->handle,
 						&vbo);
-
+	/* drop reference from allocate - handle holds it now */
+	drm_gem_object_put(&vbo->tbo.base);
 	return ret;
 }
 
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c b/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
index 51bd1f8c5cc4..d6baf73a6458 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_gem.c
@@ -133,8 +133,6 @@ int vmw_gem_object_create_with_handle(struct vmw_private *dev_priv,
 	(*p_vbo)->tbo.base.funcs = &vmw_gem_object_funcs;
 
 	ret = drm_gem_handle_create(filp, &(*p_vbo)->tbo.base, handle);
-	/* drop reference from allocate - handle holds it now */
-	drm_gem_object_put(&(*p_vbo)->tbo.base);
 out_no_bo:
 	return ret;
 }
@@ -161,6 +159,8 @@ int vmw_gem_object_create_ioctl(struct drm_device *dev, void *data,
 	rep->map_handle = drm_vma_node_offset_addr(&vbo->tbo.base.vma_node);
 	rep->cur_gmr_id = handle;
 	rep->cur_gmr_offset = 0;
+	/* drop reference from allocate - handle holds it now */
+	drm_gem_object_put(&vbo->tbo.base);
 out_no_bo:
 	return ret;
 }
diff --git a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
index 9d4ae9623a00..d18fec953fa7 100644
--- a/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
+++ b/drivers/gpu/drm/vmwgfx/vmwgfx_surface.c
@@ -867,7 +867,6 @@ int vmw_surface_define_ioctl(struct drm_device *dev, void *data,
 			goto out_unlock;
 		}
 		vmw_bo_reference(res->guest_memory_bo);
-		drm_gem_object_get(&res->guest_memory_bo->tbo.base);
 	}
 
 	tmp = vmw_resource_reference(&srf->res);
-- 
2.38.1