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
