Re: [PATCH 0/3] Postcopy migration: Add userfaultfd- user-mode-only capability

[Peter Xu <peterx@redhat.com>]

On Fri, Oct 15, 2021 at 04:16:15PM +0800, lma wrote:
> 在 2021-10-15 14:12，Peter Xu 写道：
> > On Fri, Oct 15, 2021 at 01:38:06PM +0800, lma wrote:
> > > 在 2021-10-15 07:43，Peter Xu 写道：
> > > > On Thu, Oct 14, 2021 at 05:15:48PM +0800, Lin Ma wrote:
> > > > > Since kernel v5.11, Unprivileged user (without SYS_CAP_PTRACE
> > > > > capability)
> > > > > must pass UFFD_USER_MODE_ONLY to userfaultd in case
> > > > > unprivileged_userfaultfd
> > > > > sysctl knob is 0.
> > > > > Please refer to https://lwn.net/Articles/819834/ and the kernel
> > > > > commits:
> > > > > 37cd0575 userfaultfd: add UFFD_USER_MODE_ONLY
> > > > > d0d4730a userfaultfd: add user-mode only option to
> > > > > unprivileged_userfaultfd sysctl knob
> > > > >
> > > > > This patch set adds a migration capability to pass UFFD_USER_MODE_ONLY
> > > > > for postcopy migration.
> > > >
> > > > Then it's at least no KVM, no vhost, am I right?  Could I ask is there a
> > > > real
> > > > user behind this?  Thanks,
> > > 
> > > Well, The "user-mode-only" has nothing to do with qemu's user-mode
> > > emulation.
> > 
> > I didn't follow why you thought my question was about "user-mode
> > emulation"..
> Sorry about the misunderstanding.

No worry. :)

> 
> > To ask in another way: after this new cap set, qemu will get a SIGBUS
> > and VM
> > will crash during postcopy migrating as long as either KVM or
> > vhost-kernel
> > faulted on any of the missing pages, am I right?
> 
> Oops...Yes, you're right. It indeed casues qemu crash on destination due to
> fault on missing pages.
> This patch set and my thought about introducing this cap to qemu are wrong.

I can't say it's wrong, it's just that it may need some more thoughts on how to
make it applicable.

We'll need to make sure no kernel module will access guest pages, however I
think it'll be so hard to guarantee.  For example, there can be some read()
syscall from qemu initiated with guest pages passed in as the buffer (so the
kernel will fill up the buffer when syscall returns), then if that page is
missing on dst then that'll also trigger a kernel page fault and it'll crash
qemu too even if no kvm/vhost-kernel is used.  We'll need to dig out everything
like that.

The other thing is about my original question on whether it'll be useful in any
way, and I just worry it won't help anyone, because afaiu any real user of
migration (I believe it's majorly public/private cloud) will definitely at
least be kvm based as tcg could be too slow.  Then they'll simply enable the
unprivileged uffd on the hosts, since even if it's unsafe it'll be at least as
unsafe as before unprivileged_userfaultfd is introduced.

Thanks,

-- 
Peter Xu