Re: [PATCH] Add mmap_assert_locked() annotations to find_vma*()

[Liam Howlett]

* Jann Horn <jannh@xxxxxxxxxx> [210804 10:42]:
> On Wed, Aug 4, 2021 at 1:07 AM Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
> > * Luigi Rizzo <lrizzo@xxxxxxxxxx> [210803 17:49]:
> > > On Tue, Aug 3, 2021 at 6:08 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> > > >
> > > > On Sat, Jul 31, 2021 at 10:53:41AM -0700, Luigi Rizzo wrote:
> > > > > find_vma() and variants need protection when used.
> > > > > This patch adds mmap_assert_lock() calls in the functions.
> > > > >
> > > > > To make sure the invariant is satisfied, we also need to add a
> > > > > mmap_read_loc() around the get_user_pages_remote() call in
> > > > > get_arg_page(). The lock is not strictly necessary because the mm
> > > > > has been newly created, but the extra cost is limited because
> > > > > the same mutex was also acquired shortly before in __bprm_mm_init(),
> > > > > so it is hot and uncontended.
> > > > >
> > > > > Signed-off-by: Luigi Rizzo <lrizzo@xxxxxxxxxx>
> > > > >  fs/exec.c | 2 ++
> > > > >  mm/mmap.c | 2 ++
> > > > >  2 files changed, 4 insertions(+)
> > > > >
> > > > > diff --git a/fs/exec.c b/fs/exec.c
> > > > > index 38f63451b928..ac7603e985b4 100644
> > > > > +++ b/fs/exec.c
> > > > > @@ -217,8 +217,10 @@ static struct page *get_arg_page(struct linux_binprm *bprm, unsigned long pos,
> > > > >        * We are doing an exec().  'current' is the process
> > > > >        * doing the exec and bprm->mm is the new process's mm.
> > > > >        */
> > > > > +     mmap_read_lock(bprm->mm);
> > > > >       ret = get_user_pages_remote(bprm->mm, pos, 1, gup_flags,
> > > > >                       &page, NULL, NULL);
> > > > > +     mmap_read_unlock(bprm->mm);
> > > > >       if (ret <= 0)
> > > > >               return NULL;
> > > >
> > > > Wasn't Jann Horn working on something like this too?
> > > >
> > > > https://lore.kernel.org/linux-mm/20201016225713.1971256-1-jannh@xxxxxxxxxx/
> > > >
> > > > IIRC it was very tricky here, are you sure it is OK to obtain this lock
> > > > here?
> > >
> > > I cannot comment on Jann's patch series but no other thread knows
> > > about this mm at this point in the code so the lock is definitely
> > > safe to acquire (shortly before there was also a write lock acquired
> > > on the same mm, in the same conditions).
> >
> > If there is no other code that knows about this mm, then does one need
> > the lock at all?  Is this just to satisfy the new check you added?
> >
> > If you want to make this change, I would suggest writing it in a way to
> > ensure the call to expand_downwards() in the same function also holds
> > the lock.  I believe this is technically required as well?  What do you
> > think?
> 
> The call to expand_downwards() takes a VMA pointer as argument, and
> the mmap lock is the only thing that normally prevents concurrent
> freeing of VMA structs. Taking a lock there would be of limited utility - either
> the lock is not necessary because nobody else can access the MM, or
> the lock is insufficient because someone could have freed the VMA
> pointer before the lock was taken. So I think that taking a lock
> around the expand_downwards() call would just be obfuscating things,
> unless you specifically want to prevent concurrent *reads* while
> concurrent *writes* are impossible.

Good point on the VMA being passed in, that certainly points to your
previous patch being a better approach.  That resolves my questions
around the patch.

> 
> Since I haven't sent a new version of my old series for almost a year,
> I think it'd be fine to take Luigi's patch for now, and undo it at a
> later point when/if we want to actually use proper locking here
> because we're worried about concurrent access to the MM.