Re: [libvirt PATCH v2] qemu: Enable unprivileged userfaultfd for post-copy migration

On Tue, Dec 07, 2021 at 10:19:42AM +0100, Jiri Denemark wrote: > Userfaultfd is by default allowed only for privileged processes. Since > libvirt runs QEMU unprivileged, we need to enable unprivileged access to > userfaultfd before starting post-copy migration. > > Rather than providing a static sysctl configuration file, we set the > sysctl knob in runtime once post-copy migration is requested. This way > unprivileged_userfaultfd is only enabled once actually used. I'm really not a fan of silently changing sysctl knobs on the fly like this, as it means the change is essentially invisible to the host admin. IIUC, the kernel change was made because of fear of risk this feature exposes to the kernel when combined with other flaws. Now I don't know how valid that fear is, but given that starting point, I think if we're going to change it, then the change ought to be visible to admins in a fairly obvious way. IOW, we something ought to be droppping a file into /etc/sysctl.d/ that enables it. The downside then is that it applies to all installs, even if they don't migrate. The flipside is that a default of 1 has been the historical value since postcopy first arrived, so all QEMU installs always had this behaviour. If we drop in a file 50-qemu-postcopy.conf, someone else can drop in a file 55-turn-it-off-again.conf to override our default. Stil this all feels so awful every way I look at it :-(