Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Minchan Kim]

On Thu, Mar 12, 2020 at 09:41:55PM +0100, Michal Hocko wrote:
> 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

I don't understand your point here. If it's likely to be referenced
among several processes, it doesn't have the major fault latency.
What's your point here?

> 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 tend to agree about security part in the description, but still don't
agree with performance concern in the description so I'd like to remove
it in the description. Current situation is caused by security concern
unfortunately, not performance reason.

> 
> > 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.

The comment you provides explain what code does, not *why*.
Comment is always lack of explaining the whole story but we try to
demonstrate a certain context to make reader careful.
Havind said, I will not insist on it.

Thanks.