On Wed, Mar 21, 2018 at 04:22:49PM -0700, John Hubbard wrote:
> On 03/21/2018 11:16 AM, jglisse@xxxxxxxxxx wrote:
> > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> >
> > 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 <jglisse@xxxxxxxxxx>
> > Cc: Evgeny Baskakov <ebaskakov@xxxxxxxxxx>
> > Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > Cc: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> > 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
