[PATCH] 9p/xen: fix refcount leak in p9_xen_response() on wrong tag

[Yifei Gao <gyf161023@xxxxxxxxx>]

p9_xen_response() looks up the request for an incoming reply with
p9_tag_lookup(), which takes a reference on the returned p9_req_t. When
the tag does not resolve to a request in REQ_STATUS_SENT, the function
warns and continues the loop without dropping that reference, permanently
leaking the p9_req_t and its msize buffers. The reply header, including
the tag, is supplied by the backend, so a malicious or buggy 9P backend
can leak kernel memory on every crafted response.

Drop the reference before continuing, mirroring the equivalent path in
trans_fd.c.

Fixes: f66c72bea129 ("xen/9pfs: receive responses")
Cc: stable@xxxxxxxxxxxxxxx
Assisted-by: Claude:claude-opus-4-8
Signed-off-by: Yifei Gao <gyf161023@xxxxxxxxx>
---
 net/9p/trans_xen.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index f9fb2db7a066..8eea0da8797f 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -203,6 +203,8 @@ static void p9_xen_response(struct work_struct *work)
                req = p9_tag_lookup(priv->client, h.tag);
                if (!req || req->status != REQ_STATUS_SENT) {
                        dev_warn(&priv->dev->dev, "Wrong req tag=%x\n", h.tag);
+                       if (req)
+                               p9_req_put(priv->client, req);
                        cons += h.size;
                        virt_mb();
                        ring->intf->in_cons = cons;
-- 
2.43.0