On Sun, Oct 18, 2020 at 8:52 AM Michael S. Tsirkin [off-list ref] wrote:

On Sat, Oct 17, 2020 at 03:24:08PM +0200, Jason A. Donenfeld wrote:

quoted

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