drm/vmwgfx: Fix possible invalid drm gem put calls

vmw_bo_unreference sets the input buffer to null on exit, resulting in
null ptr deref's on the subsequent drm gem put calls.

This went unnoticed because only very old userspace would be exercising
those paths but it wouldn't be hard to hit on old distros with brand
new kernels.

Introduce a new function that abstracts unrefing of user bo's to make
the code cleaner and more explicit.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Reported-by: Ian Forbes <iforbes@vmware.com>
Fixes: 9ef8d83 ("drm/vmwgfx: Do not drop the reference to the handle too soon")
Cc: <stable@vger.kernel.org> # v6.4+
Reviewed-by: Maaz Mombasawala<mombasawalam@vmware.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230818041301.407636-1-zack@kde.org

drivers/gpu/drm/vmwgfx/vmwgfx_bo.c

@@ -497,10 +497,9 @@ static int vmw_user_bo_synccpu_release(struct drm_file *filp,
 		if (!(flags & drm_vmw_synccpu_allow_cs)) {
 			atomic_dec(&vmw_bo->cpu_writers);
 		}
-		ttm_bo_put(&vmw_bo->tbo);
+		vmw_user_bo_unref(vmw_bo);
 	}
 
-	drm_gem_object_put(&vmw_bo->tbo.base);
 	return ret;
 }
 
@@ -540,8 +539,7 @@ int vmw_user_bo_synccpu_ioctl(struct drm_device *dev, void *data,
 			return ret;
 
 		ret = vmw_user_bo_synccpu_grab(vbo, arg->flags);
-		vmw_bo_unreference(&vbo);
-		drm_gem_object_put(&vbo->tbo.base);
+		vmw_user_bo_unref(vbo);
 		if (unlikely(ret != 0)) {
 			if (ret == -ERESTARTSYS || ret == -EBUSY)
 				return -EBUSY;

drivers/gpu/drm/vmwgfx/vmwgfx_bo.h

@@ -195,6 +195,14 @@ static inline struct vmw_bo *vmw_bo_reference(struct vmw_bo *buf)
 	return buf;
 }
 
+static inline void vmw_user_bo_unref(struct vmw_bo *vbo)
+{
+	if (vbo) {
+		ttm_bo_put(&vbo->tbo);
+		drm_gem_object_put(&vbo->tbo.base);
+	}
+}
+
 static inline struct vmw_bo *to_vmw_bo(struct drm_gem_object *gobj)
 {
 	return container_of((gobj), struct vmw_bo, tbo.base);

drivers/gpu/drm/vmwgfx/vmwgfx_execbuf.c

@@ -1164,8 +1164,7 @@ static int vmw_translate_mob_ptr(struct vmw_private *dev_priv,
 	}
 	vmw_bo_placement_set(vmw_bo, VMW_BO_DOMAIN_MOB, VMW_BO_DOMAIN_MOB);
 	ret = vmw_validation_add_bo(sw_context->ctx, vmw_bo);
-	ttm_bo_put(&vmw_bo->tbo);
-	drm_gem_object_put(&vmw_bo->tbo.base);
+	vmw_user_bo_unref(vmw_bo);
 	if (unlikely(ret != 0))
 		return ret;
 
@@ -1221,8 +1220,7 @@ static int vmw_translate_guest_ptr(struct vmw_private *dev_priv,
 	vmw_bo_placement_set(vmw_bo, VMW_BO_DOMAIN_GMR | VMW_BO_DOMAIN_VRAM,
 			     VMW_BO_DOMAIN_GMR | VMW_BO_DOMAIN_VRAM);
 	ret = vmw_validation_add_bo(sw_context->ctx, vmw_bo);
-	ttm_bo_put(&vmw_bo->tbo);
-	drm_gem_object_put(&vmw_bo->tbo.base);
+	vmw_user_bo_unref(vmw_bo);
 	if (unlikely(ret != 0))
 		return ret;
 

drivers/gpu/drm/vmwgfx/vmwgfx_kms.c

@@ -1665,10 +1665,8 @@ static struct drm_framebuffer *vmw_kms_fb_create(struct drm_device *dev,
 
 err_out:
 	/* vmw_user_lookup_handle takes one ref so does new_fb */
-	if (bo) {
-		vmw_bo_unreference(&bo);
-		drm_gem_object_put(&bo->tbo.base);
-	}
+	if (bo)
+		vmw_user_bo_unref(bo);
 	if (surface)
 		vmw_surface_unreference(&surface);
 

drivers/gpu/drm/vmwgfx/vmwgfx_overlay.c

@@ -451,8 +451,7 @@ int vmw_overlay_ioctl(struct drm_device *dev, void *data,
 
 	ret = vmw_overlay_update_stream(dev_priv, buf, arg, true);
 
-	vmw_bo_unreference(&buf);
-	drm_gem_object_put(&buf->tbo.base);
+	vmw_user_bo_unref(buf);
 
 out_unlock:
 	mutex_unlock(&overlay->mutex);

drivers/gpu/drm/vmwgfx/vmwgfx_shader.c

@@ -809,8 +809,7 @@ static int vmw_shader_define(struct drm_device *dev, struct drm_file *file_priv,
 				    shader_type, num_input_sig,
 				    num_output_sig, tfile, shader_handle);
 out_bad_arg:
-	vmw_bo_unreference(&buffer);
-	drm_gem_object_put(&buffer->tbo.base);
+	vmw_user_bo_unref(buffer);
 	return ret;
 }