vhost-user-gpu: fix OOB write in 'virgl_cmd_get_capset' (CVE-2021-3546)

If 'virgl_cmd_get_capset' set 'max_size' to 0,
the 'virgl_renderer_fill_caps' will write the data after the 'resp'.
This patch avoid this by checking the returned 'max_size'.

virtio-gpu fix: abd7f08b

 ("display: virtio-gpu-3d: check
virgl capabilities max_size")

Fixes: CVE-2021-3546
Reported-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Li Qiang <liq3ea@163.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20210516030403.107723-8-liq3ea@163.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

contrib/vhost-user-gpu/virgl.c

@@ -177,6 +177,10 @@ virgl_cmd_get_capset(VuGpu *g,
 
     virgl_renderer_get_cap_set(gc.capset_id, &max_ver,
                                &max_size);
+    if (!max_size) {
+        cmd->error = VIRTIO_GPU_RESP_ERR_INVALID_PARAMETER;
+        return;
+    }
     resp = g_malloc0(sizeof(*resp) + max_size);
 
     resp->hdr.type = VIRTIO_GPU_RESP_OK_CAPSET;