Re: [Qemu-devel] [RFC] memory-backend-file/nvdimm: support read-only files as memory-backends

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

Cc'ing Igor & Xiao.

On 7/8/19 11:19 PM, Julio Montes wrote:
> Currently is not possible to use a file that is part of a read-only
> filesystem as memory backend for nvdimm devices, even if this is not modified
> in the guest. In order to improve the security of Virtual Machines that share
> and do not modify the memory-backend-file, QEMU should support
> read-only memory-backeds.
> 
> Use case:
> * Kata Containers use a memory-backed-file as read-only rootfs, and this
>   file is used to start all the virtual machines in the node.
>   It would be really bad if somehow a malicious container modified it.
> 
> Signed-off-by: Julio Montes <julio.montes@intel.com>
> ---
>  exec.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/exec.c b/exec.c
> index 50ea9c5aaa..1eb170b55a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1852,6 +1852,12 @@ static int file_ram_open(const char *path,
>                  break;
>              }
>              g_free(filename);
> +        } else if (errno == EROFS) {
> +            fd = open(path, O_RDONLY);

While I can understand your use case, I'm not sure we want this silenced
as default. I'd expect an explicit command line option for that backend,
but I don't know well this area so let's wait for other to review.

> +            if (fd >= 0) {
> +                /* @path names an existing read-only file, use it */
> +                break;
> +            }
>          }
>          if (errno != EEXIST && errno != EINTR) {
>              error_setg_errno(errp, errno,
> --
> 2.17.2
>