Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Daniel Colascione]

On Tue, Mar 10, 2020 at 11:48 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Tue 10-03-20 19:08:28, Jann Horn wrote:
> > Hi!
> >
> > >From looking at the source code, it looks to me as if using
> > MADV_PAGEOUT on a CoW anonymous mapping will page out the page if
> > possible, even if other processes still have the same page mapped. Is
> > that correct?
> >
> > If so, that's probably bad in environments where many processes (with
> > different privileges) are forked from a single zygote process (like
> > Android and Chrome), I think? If you accidentally call it on a CoW
> > anonymous mapping with shared pages, you'll degrade the performance of
> > other processes. And if an attacker does it intentionally, they could
> > use that to aid with exploiting race conditions or weird
> > microarchitectural stuff (e.g. the new https://lviattack.eu/lvi.pdf
> > talks about "the assumption that attackers can provoke page faults or
> > microcode assists for (arbitrary) load operations in the victim
> > domain").
> >
> > Should madvise_cold_or_pageout_pte_range() maybe refuse to operate on
> > pages with mapcount>1, or something like that? Or does it already do
> > that, and I just missed the check?
>
> I have brought up side channel attacks earlier [1] but only in the
> context of shared page cache pages. I didn't really consider shared
> anonymous pages to be a real problem. I was under impression that CoW
> pages shouldn't be a real problem because any security sensible
> applications shouldn't allow untrusted code to be forked and CoW
> anything really important. I believe we have made this assumption
> in other places - IIRC on gup with FOLL_FORCE but I admit I have
> very happily forgot most details.

I'm more worried about the performance implications. Consider
libc.so's data section: that's a COW mapping, and we COW it during
zygote initialization as we load and relocate libc.so. Child processes
shouldn't be dirtying and re-COWing those relocated pages. If I
understand Jann's message correctly, MADV_PAGEOUT would force the
pages corresponding to the libc.so data segment out to zram just
because we MADV_PAGEOUT-ed a single process that happened to use libc.
We should leave those pages in memory, IMHO.