mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
authorLinus Torvalds <torvalds@linux-foundation.org>
Thu, 13 Oct 2016 20:07:36 +0000 (13:07 -0700)
committerWinnie Hsu <whsu@nvidia.com>
Wed, 26 Oct 2016 22:40:17 +0000 (15:40 -0700)
commitd0d666fcd21dea1424006c34fd158067ced90ce2
tree95fdbd4f5fbdaf2928790dd9ef36fbf6a2c67c7c
parent1b1420c3fd59880e0a9220bae507b2191d73e2b0
mm: remove gup_flags FOLL_WRITE games from __get_user_pages()

commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619 upstream.

This is an ancient bug that was actually attempted to be fixed once
(badly) by me eleven years ago in commit 4ceb5db9757a ("Fix
get_user_pages() race for write access") but that was then undone due to
problems on s390 by commit f33ea7f404e5 ("fix get_user_pages bug").

In the meantime, the s390 situation has long been fixed, and we can now
fix it by checking the pte_dirty() bit properly (and do it better).  The
s390 dirty bit was implemented in abf09bed3cce ("s390/mm: implement
software dirty bits") which made it into v3.9.  Earlier kernels will
have to look at the page state itself.

Also, the VM has become more scalable, and what used a purely
theoretical race back then has become easier to trigger.

To fix it, we introduce a new internal FOLL_COW flag to mark the "yes,
we already did a COW" rather than play racy games with FOLL_WRITE that
is very fundamental, and then use the pte dirty flag to validate that
the FOLL_COW flag is still valid.

Reported-and-tested-by: Phil "not Paul" Oester <kernel@linuxace.com>
Acked-by: Hugh Dickins <hughd@google.com>
Reviewed-by: Michal Hocko <mhocko@suse.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Willy Tarreau <w@1wt.eu>
Cc: Nick Piggin <npiggin@gmail.com>
Cc: Greg Thelen <gthelen@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[wt: s/gup.c/memory.c; s/follow_page_pte/follow_page_mask;
     s/faultin_page/__get_user_page]
Signed-off-by: Willy Tarreau <w@1wt.eu>

Change-Id: I6fbb1abf656ff7e05ec4c65f07dbbdd694546fb4
Signed-off-by: Krishna Reddy <vdumpa@nvidia.com>
Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
Reviewed-on: http://git-master/r/1241321
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bibek Basu <bbasu@nvidia.com>
Tested-by: Bibek Basu <bbasu@nvidia.com>
include/linux/mm.h
mm/memory.c