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

[Michael S. Tsirkin]

On Sun, Oct 18, 2020 at 09:14:00AM -0700, Andy Lutomirski wrote:
> On Sun, Oct 18, 2020 at 8:59 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote:
> >
> > On Sun, Oct 18, 2020 at 08:54:36AM -0700, Andy Lutomirski wrote:
> > > 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
> >
> >
> > Right, it's not on fork, it's actually when process is snapshotted.
> >
> > If we assume it's CRIU we care about, then I
> > wonder what's wrong with something like
> > MADV_CHANGEONPTRACE_SEIZE
> > and basically say it's X bytes which change the value...
> 
> I feel like we may be approaching this from the wrong end.  Rather
> than saying "what data structure can the kernel expose that might
> plausibly be useful", how about we try identifying some specific
> userspace needs and see what a good solution could look like.  I can
> identify two major cryptographic use cases:

Well, I'm aware of a non-cryptographic use-case:
https://bugzilla.redhat.com/show_bug.cgi?id=1118834

this seems to just ask for the guest to have a way to detect that
a VM cloning triggered.


-- 
MST