Re: [PATCH] drivers/virt: vmgenid: add vm generation id driver

[Andy Lutomirski]

On Sun, Oct 18, 2020 at 8:52 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
>
> On Sat, Oct 17, 2020 at 03:24:08PM +0200, Jason A. Donenfeld wrote:
> > 4c. The guest kernel maintains an array of physical addresses that are
> > MADV_WIPEONFORK. The hypervisor knows about this array and its
> > location through whatever protocol, and before resuming a
> > moved/snapshotted/duplicated VM, it takes the responsibility for
> > memzeroing this memory. The huge pro here would be that this
> > eliminates all races, and reduces complexity quite a bit, because the
> > hypervisor can perfectly synchronize its bringup (and SMP bringup)
> > with this, and it can even optimize things like on-disk memory
> > snapshots to simply not write out those pages to disk.
> >
> > A 4c-like approach seems like it'd be a lot of bang for the buck -- we
> > reuse the existing mechanism (MADV_WIPEONFORK), so there's no new
> > userspace API to deal with, and it'd be race free, and eliminate a lot
> > of kernel complexity.
>
> Clearly this has a chance to break applications, right?
> If there's an app that uses this as a non-system-calls way
> to find out whether there was a fork, it will break
> when wipe triggers without a fork ...
> For example, imagine:
>
> MADV_WIPEONFORK
> copy secret data to MADV_DONTFORK
> fork
>
>
> used to work, with this change it gets 0s instead of the secret data.
>
>
> I am also not sure it's wise to expose each guest process
> to the hypervisor like this. E.g. each process needs a
> guest physical address of its own then. This is a finite resource.
>
>
> The mmap interface proposed here is somewhat baroque, but it is
> certainly simple to implement ...

Wipe of fork/vmgenid/whatever could end up being much more problematic
than it naively appears -- it could be wiped in the middle of a read.
Either the API needs to handle this cleanly, or we need something more
aggressive like signal-on-fork.

--Andy