Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Michal Hocko]

On Thu 12-03-20 13:16:02, Minchan Kim wrote:
> On Thu, Mar 12, 2020 at 09:22:48AM +0100, Michal Hocko wrote:
[...]
> > From eca97990372679c097a88164ff4b3d7879b0e127 Mon Sep 17 00:00:00 2001
> > From: Michal Hocko <mhocko@xxxxxxxx>
> > Date: Thu, 12 Mar 2020 09:04:35 +0100
> > Subject: [PATCH] mm: do not allow MADV_PAGEOUT for CoW pages
> > 
> > Jann has brought up a very interesting point [1]. While shared pages are
> > excluded from MADV_PAGEOUT normally, CoW pages can be easily reclaimed
> > that way. This can lead to all sorts of hard to debug problems. E.g.
> > performance problems outlined by Daniel [2]. There are runtime
> > environments where there is a substantial memory shared among security
> > domains via CoW memory and a easy to reclaim way of that memory, which
> > MADV_{COLD,PAGEOUT} offers, can lead to either performance degradation
> > in for the parent process which might be more privileged or even open
> > side channel attacks. The feasibility of the later is not really clear
> 
> I am not sure it's a good idea to mention performance stuff because
> it's rather arguble. You and Johannes already pointed it out when I sbumit
> early draft which had shared page filtering out logic due to performance
> reason. You guys suggested the shared pages has higher chance to be touched
> so that if it's really hot pages, that whould keep in the memory. I agree.

Yes, the hot memory is likely to be referenced but the point was an
unexpected latency because of the major fault. I have to say that I have
underestimated the issue because I was not aware of runtimes mentioned
in the referenced links. Essentially a lot of CoW memory shared over
security domains.

> I think the only reason at this moment is just vulnerability.
> 
> > to me TBH but there is no real reason for exposure at this stage. It
> > seems there is no real use case to depend on reclaiming CoW memory via
> > madvise at this stage so it is much easier to simply disallow it and
> > this is what this patch does. Put it simply MADV_{PAGEOUT,COLD} can
> > operate only on the exclusively owned memory which is a straightforward
> > semantic.
> > 
> > [1] http://lkml.kernel.org/r/CAG48ez0G3JkMq61gUmyQAaCq=_TwHbi1XKzWRooxZkv08PQKuw@xxxxxxxxxxxxxx
> > [2] http://lkml.kernel.org/r/CAKOZueua_v8jHCpmEtTB6f3i9e2YnmX4mqdYVWhV4E=Z-n+zRQ@xxxxxxxxxxxxxx
> > 
> > Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> > ---
> >  mm/madvise.c | 12 +++++++++---
> >  1 file changed, 9 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 43b47d3fae02..4bb30ed6c8d2 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -335,12 +335,14 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >  		}
> >  
> >  		page = pmd_page(orig_pmd);
> > +
> > +		/* Do not interfere with other mappings of this page */
> 
> 
> How about this?
> /*
>  * paging out only single mapped private pages for anonymous mapping,
>  * otherwise, it opens a side channel.
>  */

I am not sure this is much more helpful without a larger context. I
would stick with the wording unless you insist.
 
> Otherwise, looks good to me.

Thanks for the review.

-- 
Michal Hocko
SUSE Labs