On Wed, Mar 21, 2018 at 04:22:49PM -0700, John Hubbard wrote: > On 03/21/2018 11:16 AM, [email protected] wrote: > > From: Jérôme Glisse <[email protected]> > > > > This code was lost in translation at one point. This properly call > > mmu_notifier_unregister_no_release() once last user is gone. This > > fix the zombie mm_struct as without this patch we do not drop the > > refcount we have on it. > > > > Changed since v1: > > - close race window between a last mirror unregistering and a new > > mirror registering, which could have lead to use after free() > > kind of bug > > > > Signed-off-by: Jérôme Glisse <[email protected]> > > Cc: Evgeny Baskakov <[email protected]> > > Cc: Ralph Campbell <[email protected]> > > Cc: Mark Hairgrove <[email protected]> > > Cc: John Hubbard <[email protected]> > > --- > > mm/hmm.c | 35 +++++++++++++++++++++++++++++++++-- > > 1 file changed, 33 insertions(+), 2 deletions(-) > > > > diff --git a/mm/hmm.c b/mm/hmm.c > > index 6088fa6ed137..f75aa8df6e97 100644 > > --- a/mm/hmm.c > > +++ b/mm/hmm.c > > @@ -222,13 +222,24 @@ int hmm_mirror_register(struct hmm_mirror *mirror, > > struct mm_struct *mm) > > if (!mm || !mirror || !mirror->ops) > > return -EINVAL; > > > > +again: > > mirror->hmm = hmm_register(mm); > > if (!mirror->hmm) > > return -ENOMEM; > > > > down_write(&mirror->hmm->mirrors_sem); > > - list_add(&mirror->list, &mirror->hmm->mirrors); > > - up_write(&mirror->hmm->mirrors_sem); > > + if (mirror->hmm->mm == NULL) { > > + /* > > + * A racing hmm_mirror_unregister() is about to destroy the hmm > > + * struct. Try again to allocate a new one. > > + */ > > + up_write(&mirror->hmm->mirrors_sem); > > + mirror->hmm = NULL; > > This is being set outside of locks, so now there is another race with > another hmm_mirror_register... > > I'll take a moment and draft up what I have in mind here, which is a more > symmetrical locking scheme for these routines. >
No this code is correct. hmm->mm is set after hmm struct is allocated and before it is public so no one can race with that. It is clear in hmm_mirror_unregister() under the write lock hence checking it here under that same lock is correct. Cheers, Jérôme
