From: Michal Hocko mhocko@suse.com 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... [2] http://lkml.kernel.org/r/CAKOZueua_v8jHCpmEtTB6f3i9e2YnmX4mqdYVWhV4E=Z-n+zRQ...

Link: http://lkml.kernel.org/r/20200312082248.GS23944@dhcp22.suse.cz Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD") Signed-off-by: Michal Hocko mhocko@suse.com Reported-by: Jann Horn jannh@google.com Acked-by: Vlastimil Babka vbabka@suse.cz Cc: Minchan Kim minchan@kernel.org Cc: Daniel Colascione dancol@google.com Cc: Dave Hansen dave.hansen@intel.com Cc: "Joel Fernandes (Google)" joel@joelfernandes.org Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

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; - get_page(page); spin_unlock(ptl); lock_page(page); @@ -426,6 +428,10 @@ regular_page: 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)) { _