[PATCH v2] drm/virtio: drop the dedicated vbuf kmem_cache

[Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>]

virtio_gpu_alloc_vbufs() creates a kmem_cache with the fixed name
"virtio-gpu-vbufs" for every device and destroys it from
virtio_gpu_release(), which only runs once the last drm_device
reference is dropped. If a reference outlives the PCI removal (for
instance an open /dev/fbN), a rescan probes the device again while the
old cache still exists:

  kmem_cache of name 'virtio-gpu-vbufs' already exists
  WARNING: mm/slab_common.c:111 at __kmem_cache_create_args
  Call Trace:
   virtio_gpu_alloc_vbufs
   virtio_gpu_init
   virtio_gpu_probe
   ...
   pci_rescan_bus
   rescan_store

kmem_cache_create() then fails and the new device does not probe.

A vbuf is 216 bytes and lands in kmalloc-256 anyway, so the dedicated
cache buys nothing. Use kzalloc()/kfree() and drop the per-device cache.

Reported-by: syzbot+1b129b44597a126d2d79@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1b129b44597a126d2d79
Fixes: dc5698e80cf7 ("Add virtio gpu driver.")
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@gmail.com>
---
v2: (Hillf Danton) drop the dedicated cache and use kzalloc()/kfree()
    instead of sharing one cache across devices.
    v1: https://lore.kernel.org/r/20260926162452.136863-1-ngocthang2710.1999@gmail.com

 drivers/gpu/drm/virtio/virtgpu_drv.h |  3 ---
 drivers/gpu/drm/virtio/virtgpu_kms.c |  8 --------
 drivers/gpu/drm/virtio/virtgpu_vq.c  | 23 +++--------------------
 3 files changed, 3 insertions(+), 31 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
index 9df4c7117341..2639d9ec0470 100644
--- a/drivers/gpu/drm/virtio/virtgpu_drv.h
+++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
@@ -261,7 +261,6 @@ struct virtio_gpu_device {
 	struct virtio_gpu_queue ctrlq;
 	struct virtio_gpu_queue cursorq;
 	bool vqs_released;
-	struct kmem_cache *vbufs;
 
 	atomic_t pending_commands;
 
@@ -361,8 +360,6 @@ void virtio_gpu_array_put_free_delayed(struct virtio_gpu_device *vgdev,
 void virtio_gpu_array_put_free_work(struct work_struct *work);
 
 /* virtgpu_vq.c */
-int virtio_gpu_alloc_vbufs(struct virtio_gpu_device *vgdev);
-void virtio_gpu_free_vbufs(struct virtio_gpu_device *vgdev);
 void virtio_gpu_reclaim_vbufs(struct virtio_gpu_device *vgdev);
 void virtio_gpu_cmd_create_resource(struct virtio_gpu_device *vgdev,
 				    struct virtio_gpu_object *bo,
diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
index 1d4d3bf46a20..047b591b5da2 100644
--- a/drivers/gpu/drm/virtio/virtgpu_kms.c
+++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
@@ -264,11 +264,6 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
 		DRM_ERROR("failed to find virt queues\n");
 		goto err_vqs;
 	}
-	ret = virtio_gpu_alloc_vbufs(vgdev);
-	if (ret) {
-		DRM_ERROR("failed to alloc vbufs\n");
-		goto err_vbufs;
-	}
 
 	/* get display info */
 	virtio_cread_le(vgdev->vdev, struct virtio_gpu_config,
@@ -324,8 +319,6 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
 	virtio_reset_device(vgdev->vdev);
 	virtio_gpu_modeset_fini(vgdev);
 err_scanouts:
-	virtio_gpu_free_vbufs(vgdev);
-err_vbufs:
 	vgdev->vdev->config->del_vqs(vgdev->vdev);
 err_vqs:
 	dev->dev_private = NULL;
@@ -365,7 +358,6 @@ void virtio_gpu_release(struct drm_device *dev)
 		return;
 
 	virtio_gpu_modeset_fini(vgdev);
-	virtio_gpu_free_vbufs(vgdev);
 	virtio_gpu_cleanup_cap_cache(vgdev);
 
 	if (vgdev->has_host_visible)
diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index c02c03c10d92..5eb78d69f561 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -70,30 +70,13 @@ void virtio_gpu_cursor_ack(struct virtqueue *vq)
 	schedule_work(&vgdev->cursorq.dequeue_work);
 }
 
-int virtio_gpu_alloc_vbufs(struct virtio_gpu_device *vgdev)
-{
-	vgdev->vbufs = kmem_cache_create("virtio-gpu-vbufs",
-					 VBUFFER_SIZE,
-					 __alignof__(struct virtio_gpu_vbuffer),
-					 0, NULL);
-	if (!vgdev->vbufs)
-		return -ENOMEM;
-	return 0;
-}
-
-void virtio_gpu_free_vbufs(struct virtio_gpu_device *vgdev)
-{
-	kmem_cache_destroy(vgdev->vbufs);
-	vgdev->vbufs = NULL;
-}
-
 /* For drm_panic */
 static struct virtio_gpu_vbuffer*
 virtio_gpu_panic_get_vbuf(struct virtio_gpu_device *vgdev, int size)
 {
 	struct virtio_gpu_vbuffer *vbuf;
 
-	vbuf = kmem_cache_zalloc(vgdev->vbufs, GFP_ATOMIC);
+	vbuf = kzalloc(VBUFFER_SIZE, GFP_ATOMIC);
 
 	vbuf->buf = (void *)vbuf + sizeof(*vbuf);
 	vbuf->size = size;
@@ -110,7 +93,7 @@ virtio_gpu_get_vbuf(struct virtio_gpu_device *vgdev,
 {
 	struct virtio_gpu_vbuffer *vbuf;
 
-	vbuf = kmem_cache_zalloc(vgdev->vbufs, GFP_KERNEL | __GFP_NOFAIL);
+	vbuf = kzalloc(VBUFFER_SIZE, GFP_KERNEL | __GFP_NOFAIL);
 
 	BUG_ON(size > MAX_INLINE_CMD_SIZE ||
 	       size < sizeof(struct virtio_gpu_ctrl_hdr));
@@ -205,7 +188,7 @@ static void free_vbuf(struct virtio_gpu_device *vgdev,
 	if (vbuf->resp_size > MAX_INLINE_RESP_SIZE)
 		kfree(vbuf->resp_buf);
 	kvfree(vbuf->data_buf);
-	kmem_cache_free(vgdev->vbufs, vbuf);
+	kfree(vbuf);
 }
 
 void virtio_gpu_reclaim_vbufs(struct virtio_gpu_device *vgdev)
-- 
2.43.0