9p/trans_xen: make cleanup idempotent after dataring alloc errors

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@gmail.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Message-ID: <20260324153023.86853-2-ericterminal@gmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: Eric-Terminal

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;
 }