From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Morton Subject: + mm-do-not-allow-madv_pageout-for-cow-pages.patch added to -mm tree Date: Thu, 12 Mar 2020 17:26:32 -0700 Message-ID: <20200313002632.K95DZ6F32%akpm@linux-foundation.org> References: <20200305222751.6d781a3f2802d79510941e4e@linux-foundation.org> Reply-To: linux-kernel@vger.kernel.org Return-path: Received: from mail.kernel.org ([198.145.29.99]:56650 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726830AbgCMA0e (ORCPT ); Thu, 12 Mar 2020 20:26:34 -0400 In-Reply-To: <20200305222751.6d781a3f2802d79510941e4e@linux-foundation.org> Sender: mm-commits-owner@vger.kernel.org List-Id: mm-commits@vger.kernel.org To: dancol@google.com, dave.hansen@intel.com, jannh@google.com, joel@joelfernandes.org, mhocko@suse.com, minchan@kernel.org, mm-commits@vger.kernel.org, stable@vger.kernel.org, vbabka@suse.cz The patch titled Subject: mm: do not allow MADV_PAGEOUT for CoW pages has been added to the -mm tree. Its filename is mm-do-not-allow-madv_pageout-for-cow-pages.patch This patch should soon appear at http://ozlabs.org/~akpm/mmots/broken-out/mm-do-not-allow-madv_pageout-for-cow-pages.patch and later at http://ozlabs.org/~akpm/mmotm/broken-out/mm-do-not-allow-madv_pageout-for-cow-pages.patch Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next and is updated there every 3-4 working days ------------------------------------------------------ From: Michal Hocko Subject: 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 latter 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 Link: http://lkml.kernel.org/r/20200312082248.GS23944@dhcp22.suse.cz Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD") Signed-off-by: Michal Hocko Reported-by: Jann Horn Acked-by: Vlastimil Babka Cc: Minchan Kim Cc: Daniel Colascione Cc: Dave Hansen Cc: "Joel Fernandes (Google)" Cc: Signed-off-by: Andrew Morton --- mm/madvise.c | 12 +++++++++--- 1 file changed, 9 insertions(+), 3 deletions(-) --- a/mm/madvise.c~mm-do-not-allow-madv_pageout-for-cow-pages +++ a/mm/madvise.c @@ -335,12 +335,14 @@ static int madvise_cold_or_pageout_pte_r } 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;