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From: Michal Hocko <mhocko@kernel.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Jann Horn <jannh@google.com>, Linux-MM <linux-mm@kvack.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Daniel Colascione <dancol@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?
Date: Thu, 12 Mar 2020 21:41:55 +0100
Message-ID: <20200312204155.GE23944@dhcp22.suse.cz> (raw)
In-Reply-To: <20200312201602.GA68817@google.com>

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@suse.com>
> > 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@mail.gmail.com
> > [2] http://lkml.kernel.org/r/CAKOZueua_v8jHCpmEtTB6f3i9e2YnmX4mqdYVWhV4E=Z-n+zRQ@mail.gmail.com
> > 
> > Signed-off-by: Michal Hocko <mhocko@suse.com>
> > ---
> >  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


  parent reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-10 18:08 Jann Horn
2020-03-10 18:48 ` Michal Hocko
2020-03-10 19:11   ` Jann Horn
2020-03-10 21:09     ` Michal Hocko
2020-03-10 22:48       ` Dave Hansen
2020-03-11  8:45         ` Michal Hocko
2020-03-11 22:02           ` Minchan Kim
2020-03-11 23:53           ` Shakeel Butt
2020-03-12  0:18             ` Minchan Kim
2020-03-12  2:03               ` Daniel Colascione
2020-03-12 15:15                 ` Shakeel Butt
2020-03-10 20:19   ` Daniel Colascione
2020-03-10 21:40     ` Jann Horn
2020-03-10 21:52       ` Daniel Colascione
2020-03-10 22:14 ` Minchan Kim
2020-03-12  8:22 ` Michal Hocko
2020-03-12 15:40   ` Vlastimil Babka
2020-03-12 20:16   ` Minchan Kim
2020-03-12 20:26     ` Dave Hansen
2020-03-12 20:41     ` Michal Hocko [this message]
2020-03-13  2:08       ` Minchan Kim
2020-03-13  8:05         ` Michal Hocko
2020-03-13 20:59           ` Minchan Kim
2020-03-16  9:20             ` Michal Hocko
2020-03-17  1:43               ` Minchan Kim
2020-03-17  7:12                 ` Michal Hocko
2020-03-17 15:00                   ` Minchan Kim
2020-03-17 15:58                     ` Michal Hocko
2020-03-17 17:20                       ` Minchan Kim
2020-03-12 21:41     ` Dave Hansen
2020-03-13  2:00       ` Minchan Kim
2020-03-13 16:59         ` Dave Hansen
2020-03-13 21:13           ` Minchan Kim
2020-03-12 23:29     ` Jann Horn

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