Re: [PATCH 3/3] hw/9pfs/xen: drain in-flight PDUs before xen-9p disconnect

[Christian Schoenebeck <qemu_oss@crudebyte.com>]

Hi Stefano,

do you might have a chance to look at this Xen patch?

/Christian

On Thursday, 9 July 2026 15:50:36 CEST Christian Schoenebeck wrote:
> The xen-9p disconnect path has two issues:
> 
> 1. It frees the Xen9pfsRing structures while in-flight PDUs may still
>    reference them via pdu->tag to index rings[]. This causes a UAF
>    in xen_9pfs_push_and_notify() when worker threads resume after
>    completing filesystem operations.
> 
> 2. It never calls v9fs_device_unrealize_common(), which means server
>    state (struct LocalData, mountfd, FIDs) is never cleaned up on
>    disconnect, causing a resource leak on every guest-initiated
>    disconnect.
> 
> Fix both by draining in-flight PDUs via v9fs_reset() before tearing
> down rings, and calling v9fs_device_unrealize_common() to clean up
> server state.
> 
> Fixes: b37eeb0201 ("xen/9pfs: introduce Xen 9pfs backend")
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  hw/9pfs/xen-9p-backend.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> index 24c90d97ec..3b7a68779a 100644
> --- a/hw/9pfs/xen-9p-backend.c
> +++ b/hw/9pfs/xen-9p-backend.c
> @@ -368,10 +368,14 @@ static void xen_9pfs_evtchn_event(void *opaque)
>  static void xen_9pfs_disconnect(struct XenLegacyDevice *xendev)
>  {
>      Xen9pfsDev *xen_9pdev = container_of(xendev, Xen9pfsDev, xendev);
> +    V9fsState *s = &xen_9pdev->state;
>      int i;
> 
>      trace_xen_9pfs_disconnect(xendev->name);
> 
> +    v9fs_reset(s);
> +    v9fs_device_unrealize_common(s);
> +
>      for (i = 0; i < xen_9pdev->num_rings; i++) {
>          if (xen_9pdev->rings[i].evtchndev != NULL) {
>             
> qemu_set_fd_handler(qemu_xen_evtchn_fd(xen_9pdev->rings[i].evtchndev),