hw/9pfs/xen: drain in-flight PDUs before xen-9p disconnect

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.

Additionally, explicit calls of xen_9pfs_disconnect() in the error
paths of xen_9pfs_pdu_vmarshal() and xen_9pfs_pdu_vunmarshal() must
be deferred (via aio_bh_schedule_oneshot()), because
xen_9pfs_pdu_v(un)marshal() are running within a coroutine context
which makes them unsafe [1] for calling v9fs_reset() directly, as
the latter e.g. has a loop like:

    while (!QLIST_EMPTY(&s->active_list)) {
        aio_poll(qemu_get_aio_context(), true);
    }

which would a) never terminate (as the coroutine is on the
active_list) and b) aio_poll() is marked as no_coroutine_fn.

[1] https://lore.kernel.org/qemu-devel/3351181.5fSG56mABF@weasel/

And finally, add an idempotent guard to xen_9pfs_disconnect()
for the v9fs_reset(s) and v9fs_device_unrealize_common(s) calls
specifically [2], just to be sure.

[2] https://lore.kernel.org/qemu-devel/alpine.DEB.2.22.394.2607221815520.5295@ubuntu-linux-20-04-desktop/

Fixes: b37eeb0201 ("xen/9pfs: introduce Xen 9pfs backend")
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Link: https://lore.kernel.org/qemu-devel/82bc736158e827e05d4b55da27c39d42e2062e96.1784809978.git.qemu_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>

diff --git a/hw/9pfs/9p.c b/hw/9pfs/9p.c
index 474ac6cc4008226eb352b3cbe16197ae356cb154..1c61e6b3e7731a9654bb6789f9c88c99caa569f3 100644 (file)
--- a/hw/9pfs/9p.c
+++ b/hw/9pfs/9p.c
@@ -4530,6 +4530,7 @@ void v9fs_device_unrealize_common(V9fsState *s)
     qp_table_destroy(&s->qpp_table);
     qp_table_destroy(&s->qpf_table);
     g_free(s->ctx.fs_root);
+    s->transport = NULL;
 }
 
 typedef struct VirtfsCoResetData {

diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
index 24c90d97ec94e404aad8845826c5b477cb22a618..d44fa8d61e2ed5a0c66295f05206531ef4bf0b33 100644 (file)
--- a/hw/9pfs/xen-9p-backend.c
+++ b/hw/9pfs/xen-9p-backend.c
@@ -68,6 +68,11 @@ typedef struct Xen9pfsDev {
 
 static void xen_9pfs_disconnect(struct XenLegacyDevice *xendev);
 
+static void xen_9pfs_disconnect_bh(void *opaque)
+{
+    xen_9pfs_disconnect(opaque);
+}
+
 static void xen_9pfs_in_sg(Xen9pfsRing *ring,
                            struct iovec *in_sg,
                            int *num,
@@ -150,7 +155,8 @@ static ssize_t xen_9pfs_pdu_vmarshal(V9fsPDU *pdu,
                       "Failed to encode VirtFS reply type %d\n",
                       pdu->id + 1);
         xen_be_set_state(&xen_9pfs->xendev, XenbusStateClosing);
-        xen_9pfs_disconnect(&xen_9pfs->xendev);
+        aio_bh_schedule_oneshot(qemu_get_aio_context(),
+                                xen_9pfs_disconnect_bh, &xen_9pfs->xendev);
     }
     return ret;
 }
@@ -173,7 +179,8 @@ static ssize_t xen_9pfs_pdu_vunmarshal(V9fsPDU *pdu,
         xen_pv_printf(&xen_9pfs->xendev, 0,
                       "Failed to decode VirtFS request type %d\n", pdu->id);
         xen_be_set_state(&xen_9pfs->xendev, XenbusStateClosing);
-        xen_9pfs_disconnect(&xen_9pfs->xendev);
+        aio_bh_schedule_oneshot(qemu_get_aio_context(),
+                                xen_9pfs_disconnect_bh, &xen_9pfs->xendev);
     }
     return ret;
 }
@@ -368,10 +375,16 @@ 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);
 
+    if (s->transport) {
+        v9fs_reset(s); /* cancel all in-flight PDUs to prevent UAF */
+        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),