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authorMichal Hocko <mhocko@suse.com>2020-03-21 18:22:26 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2020-03-21 18:56:06 -0700
commit12e967fd8e4e6c3d275b4c69c890adc838891300 (patch)
tree222c48dd30ba11c9ede8ed90ac494dbd0c11c4a8
parente26733e0d0ec6798eca93daa300bc3f43616127f (diff)
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 Fixes: 9c276cc65a58 ("mm: introduce MADV_COLD") Reported-by: Jann Horn <jannh@google.com> Signed-off-by: Michal Hocko <mhocko@suse.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> 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> Link: http://lkml.kernel.org/r/20200312082248.GS23944@dhcp22.suse.cz Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--mm/madvise.c12
1 files 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 @@ 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)) {