[PATCH 1/2] qemu: Move pid file of pr-helper to stateDir

[Michal Prívozník]

On 10/11/21 2:11 PM, Peng Liang wrote:
> Libvirt will put the pid file of pr-helper to per-domain directory.
> However, the ownership of the per-domain directory is the user to run
> the QEMU process and the user has the write permission of the directory.
> If VM escape occurs, the attacker can
> 1. write arbitrary content to the pid file (if running QEMU using root),
>    then the attacker can kill any process by writing appropriate pid to
>    the pid file;
> 2. spoof the pid file (if running QEMU using a regular user), then the
>    pr-helper process will never be cleared even if the VM is destroyed.
> 
> So, move the pid file of pr-helper from per-domain directory to
> stateDir just like the pid file of the domain.
> 
> Signed-off-by: Peng Liang <liangpeng10 at huawei.com>
> ---
>  src/qemu/qemu_process.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
> index 1d0165af6daa..583f3ec76c7b 100644
> --- a/src/qemu/qemu_process.c
> +++ b/src/qemu/qemu_process.c
> @@ -2859,9 +2859,11 @@ static char *
>  qemuProcessBuildPRHelperPidfilePath(virDomainObj *vm)
>  {
>      qemuDomainObjPrivate *priv = vm->privateData;
> -    const char *prdAlias = qemuDomainGetManagedPRAlias();
> +    g_autofree char *domname = virDomainDefGetShortName(vm->def);
> +    g_autofree char *prdName = g_strdup_printf("%s-%s", domname, qemuDomainGetManagedPRAlias());
> +    g_autoptr(virQEMUDriverConfig) cfg = virQEMUDriverGetConfig(priv->driver);
>  
> -    return virPidFileBuildPath(priv->libDir, prdAlias);
> +    return virPidFileBuildPath(cfg->stateDir, prdName);
>  }

I don't think it is this simple. The pidfile path is not stored in
status XML but generated when starting/stopping PR helper process.
Therefore, if there is PR helper that was started with older libvirt its
pidfile is /var/lib/libvirt/qemu/domain-1-fedora/pr-helper0.pid  but
after your change a different path is generated
/var/run/libvirt/qemu/1-fedora-pr-helper0.pid  and thus
qemuProcessKillManagedPRDaemon() does not kill the PR helper process and
leaves it behind.

One solution would be to start recording the pidfile path in status XML
and then when no path is found during parse we know that the old path
was used.

Michal