UPSTREAM: mm: remove gup_flags FOLL_WRITE games from __get_user_pages()
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 <firstname.lastname@example.org>
Acked-by: Hugh Dickins <email@example.com>
Reviewed-by: Michal Hocko <firstname.lastname@example.org>
Cc: Andy Lutomirski <email@example.com>
Cc: Kees Cook <firstname.lastname@example.org>
Cc: Oleg Nesterov <email@example.com>
Cc: Willy Tarreau <firstname.lastname@example.org>
Cc: Nick Piggin <email@example.com>
Cc: Greg Thelen <firstname.lastname@example.org>
Signed-off-by: Linus Torvalds <email@example.com>
(cherry picked from commit 19be0eaffa3ac7d8eb6784ad9bdbc7d67ed8e619)
Reviewed-by: Guenter Roeck <firstname.lastname@example.org>
Commit-Queue: Andrey Ulanov <email@example.com>
Tested-by: Andrey Ulanov <firstname.lastname@example.org>
2 files changed