ksm: prevent mremap move poisoning
authorHugh Dickins <hugh.dickins@tiscali.co.uk>
Tue, 22 Sep 2009 00:02:05 +0000 (17:02 -0700)
committerLinus Torvalds <torvalds@linux-foundation.org>
Tue, 22 Sep 2009 14:17:31 +0000 (07:17 -0700)
commit1ff829957316670af64be24192ef849e7253a509
tree9227f33bf6dd455edbada5153d1b72a1ca191590
parent31dbd01f314364b70c2e026a5793a29a4da8a9dc
ksm: prevent mremap move poisoning

KSM's scan allows for user pages to be COWed or unmapped at any time,
without requiring any notification.  But its stable tree does assume that
when it finds a KSM page where it placed a KSM page, then it is the same
KSM page that it placed there.

mremap move could break that assumption: if an area containing a KSM page
was unmapped, then an area containing a different KSM page was moved with
mremap into the place of the original, before KSM's scan came around to
notice.  That could then poison a node of the stable tree, so that memcmps
would "lie" and upset the ordering of the tree.

Probably noone will ever need mremap move on a VM_MERGEABLE area; except
that prohibiting it would make trouble for schemes in which we try making
everything VM_MERGEABLE e.g.  for testing: an mremap which normally works
would then fail mysteriously.

There's no need to go to any trouble, such as re-sorting KSM's list of
rmap_items to match the new layout: simply unmerge the area to COW all its
KSM pages before moving, but leave VM_MERGEABLE on so that they're
remerged later.

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Signed-off-by: Chris Wright <chrisw@redhat.com>
Signed-off-by: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Rik van Riel <riel@redhat.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Lee Schermerhorn <lee.schermerhorn@hp.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
mm/mremap.c