• Linus Torvalds's avatar
    mm: remove gup_flags FOLL_WRITE games from __get_user_pages() · 19be0eaf
    Linus Torvalds authored
    This is an ancient bug that was actually attempted to be fixed once
    (badly) by me eleven years ago in commit 4ceb5db9 ("Fix
    get_user_pages() race for write access") but that was then undone due to
    problems on s390 by commit f33ea7f4 ("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 abf09bed
    
     ("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: default avatarPhil "not Paul" Oester <kernel@linuxace.com>
    Acked-by: default avatarHugh Dickins <hughd@google.com>
    Reviewed-by: default avatarMichal 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: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    19be0eaf
mm.h 76.7 KB