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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 4ceb5db ("Fix get_user_pages() race for write access") but that was then undone due to problems on s390 by commit f33ea7f ("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 abf09be ("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> Cc: email@example.com Signed-off-by: Linus Torvalds <firstname.lastname@example.org>
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