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From: Vlastimil Babka <vbabka@suse.cz>
To: Michal Hocko <mhocko@kernel.org>, Jann Horn <jannh@google.com>
Cc: Minchan Kim <minchan@kernel.org>, 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 16:40:44 +0100
Message-ID: <86dba6eb-9c08-695a-aafd-75260bff1767@suse.cz> (raw)
In-Reply-To: <20200312082248.GS23944@dhcp22.suse.cz>

On 3/12/20 9:22 AM, Michal Hocko wrote:
> [Cc akpm]
> 
> So what about this?
> 
> 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
> 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
> 

Reported-by: Jann Horn <jannh@google.com>
Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD")

Maybe CC stable?

> Signed-off-by: Michal Hocko <mhocko@suse.com>

Acked-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  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 */
> +		if (page_mapcount(page) != 1)
> +			goto huge_unlock;
> +
>  		if (next - addr != HPAGE_PMD_SIZE) {
>  			int err;
>  
> -			if (page_mapcount(page) != 1)
> -				goto huge_unlock;
> -
>  			get_page(page);
>  			spin_unlock(ptl);
>  			lock_page(page);
> @@ -426,6 +428,10 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
>  			continue;
>  		}
>  
> +		/* Do not interfere with other mappings of this page */
> +		if (page_mapcount(page) != 1)
> +			continue;
> +
>  		VM_BUG_ON_PAGE(PageTransCompound(page), page);
>  
>  		if (pte_young(ptent)) {
> 



  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 [this message]
2020-03-12 20:16   ` Minchan Kim
2020-03-12 20:26     ` Dave Hansen
2020-03-12 20:41     ` Michal Hocko
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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