Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Vlastimil Babka]

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@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
> 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
> 

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

Maybe CC stable?

> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>

> ---
>  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)) {
>