Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Michal Hocko]

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.

[1] http://lkml.kernel.org/r/20190619132450.GQ2968@xxxxxxxxxxxxxx

-- 
Michal Hocko
SUSE Labs