Check whether the 'res' has already been attach_backing to avoid
memory leak.

Fixes: CVE-2021-3544
Reported-by: Li Qiang <liq3ea@163.com>
virtio-gpu fix: 204f01b309 ("virtio-gpu: fix memory leak
in resource attach backing")

Signed-off-by: Li Qiang <liq3ea@163.com>
---
 contrib/vhost-user-gpu/vhost-user-gpu.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/contrib/vhost-user-gpu/vhost-user-gpu.c b/contrib/vhost-user-gpu/vhost-user-gpu.c
index b5e153d0d6..0437e52b64 100644
--- a/contrib/vhost-user-gpu/vhost-user-gpu.c
+++ b/contrib/vhost-user-gpu/vhost-user-gpu.c
@@ -489,6 +489,11 @@ vg_resource_attach_backing(VuGpu *g,
         return;
     }
 
+    if (res->iov) {
+        cmd->error = VIRTIO_GPU_RESP_ERR_UNSPEC;
+        return;
+    }
+
     ret = vg_create_mapping_iov(g, &ab, cmd, &res->iov);
     if (ret != 0) {
         cmd->error = VIRTIO_GPU_RESP_ERR_UNSPEC;
-- 
2.25.1