Re: [PATCH v3 1/2] 9p/trans_xen: make cleanup idempotent after dataring alloc errors

[Stefano Stabellini]

On Tue, 24 Mar 2026, Eric-Terminal wrote:
> From: Yufan Chen <ericterminal@xxxxxxxxx>
> 
> xen_9pfs_front_alloc_dataring() tears down resources on failure but
> leaves ring fields stale. If xen_9pfs_front_init() later jumps to the
> common error path, xen_9pfs_front_free() may touch the same resources
> again, causing duplicate/invalid gnttab_end_foreign_access() calls and
> potentially dereferencing a freed intf pointer.
> 
> Initialize dataring sentinels before allocation, gate teardown on those
> sentinels, and clear ref/intf/data/irq immediately after each release.
> 
> This keeps cleanup idempotent for partially initialized rings and
> prevents repeated teardown during init failure handling.
> 
> Signed-off-by: Yufan Chen <ericterminal@xxxxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> v3:
> - Split from mixed series into a dedicated 9p/trans_xen series.
> - No functional changes since v2.
> 
>  net/9p/trans_xen.c | 51 +++++++++++++++++++++++++++++++++-------------
> diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
> index 47af5a10e..85b9ebfaa 100644
> --- a/net/9p/trans_xen.c
> +++ b/net/9p/trans_xen.c
> @@ -283,25 +283,33 @@ static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
> 
>                         cancel_work_sync(&ring->work);
> 
> -                       if (!priv->rings[i].intf)
> +                       if (!ring->intf)
>                                 break;
> -                       if (priv->rings[i].irq > 0)
> -                               unbind_from_irqhandler(priv->rings[i].irq, ring);
> -                       if (priv->rings[i].data.in) {
> -                               for (j = 0;
> -                                    j < (1 << priv->rings[i].intf->ring_order);
> +                       if (ring->irq >= 0) {
> +                               unbind_from_irqhandler(ring->irq, ring);
> +                               ring->irq = -1;
> +                       }
> +                       if (ring->data.in) {
> +                               for (j = 0; j < (1 << ring->intf->ring_order);
>                                      j++) {
>                                         grant_ref_t ref;
> 
> -                                       ref = priv->rings[i].intf->ref[j];
> +                                       ref = ring->intf->ref[j];
>                                         gnttab_end_foreign_access(ref, NULL);
> +                                       ring->intf->ref[j] = INVALID_GRANT_REF;
>                                 }
> -                               free_pages_exact(priv->rings[i].data.in,
> -                                  1UL << (priv->rings[i].intf->ring_order +
> -                                          XEN_PAGE_SHIFT));
> +                               free_pages_exact(ring->data.in,
> +                                                1UL << (ring->intf->ring_order +
> +                                                        XEN_PAGE_SHIFT));
> +                               ring->data.in = NULL;
> +                               ring->data.out = NULL;
> +                       }
> +                       if (ring->ref != INVALID_GRANT_REF) {
> +                               gnttab_end_foreign_access(ring->ref, NULL);
> +                               ring->ref = INVALID_GRANT_REF;
>                         }
> -                       gnttab_end_foreign_access(priv->rings[i].ref, NULL);
> -                       free_page((unsigned long)priv->rings[i].intf);
> +                       free_page((unsigned long)ring->intf);
> +                       ring->intf = NULL;
>                 }
>                 kfree(priv->rings);
>         }
> @@ -334,6 +342,12 @@ static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
>         int ret = -ENOMEM;
>         void *bytes = NULL;
> 
> +       ring->intf = NULL;
> +       ring->data.in = NULL;
> +       ring->data.out = NULL;
> +       ring->ref = INVALID_GRANT_REF;
> +       ring->irq = -1;
> +
>         init_waitqueue_head(&ring->wq);
>         spin_lock_init(&ring->lock);
>         INIT_WORK(&ring->work, p9_xen_response);
> @@ -379,9 +393,18 @@ static int xen_9pfs_front_alloc_dataring(struct xenbus_device *dev,
>                 for (i--; i >= 0; i--)
>                         gnttab_end_foreign_access(ring->intf->ref[i], NULL);
>                 free_pages_exact(bytes, 1UL << (order + XEN_PAGE_SHIFT));
> +               ring->data.in = NULL;
> +               ring->data.out = NULL;
> +       }
> +       if (ring->ref != INVALID_GRANT_REF) {
> +               gnttab_end_foreign_access(ring->ref, NULL);
> +               ring->ref = INVALID_GRANT_REF;
> +       }
> +       if (ring->intf) {
> +               free_page((unsigned long)ring->intf);
> +               ring->intf = NULL;
>         }
> -       gnttab_end_foreign_access(ring->ref, NULL);
> -       free_page((unsigned long)ring->intf);
> +       ring->irq = -1;
>         return ret;
>  }
> 
> --
> 2.47.3
>