Re: [PATCH] mm, swap: Potential NULL dereference in get_swap_page_of_type()

Daniel Jordan
2019-01-14T22:25:29Z

On Sat, Jan 12, 2019 at 12:20:07AM +0100, Andrea Parri wrote:
> On Fri, Jan 11, 2019 at 09:41:28AM -0800, Daniel Jordan wrote:
> > On Fri, Jan 11, 2019 at 12:59:19PM +0300, Dan Carpenter wrote:
> > > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > > index f0edf7244256..21e92c757205 100644
> > > --- a/mm/swapfile.c
> > > +++ b/mm/swapfile.c
> > > @@ -1048,9 +1048,12 @@ swp_entry_t get_swap_page_of_type(int type)
> > > struct swap_info_struct *si;
> > > pgoff_t offset;
> > >
> > > + if (type >= nr_swapfiles)
> > > + goto fail;
> > > +
> >
> > As long as we're worrying about NULL, I think there should be an smp_rmb here
> > to ensure swap_info[type] isn't NULL in case of an (admittedly unlikely) racing
> > swapon that increments nr_swapfiles. See smp_wmb in alloc_swap_info and the
> > matching smp_rmb's in the file. And READ_ONCE's on either side of the barrier
> > per LKMM.
> >
> > I'm adding Andrea (randomly selected from the many LKMM folks to avoid spamming
> > all) who can correct me if I'm wrong about any of this.
>
> This is to confirm that your analysis seems correct to me: the barriers
> should guarantee that get_swap_page_of_type() will observe the store to
> swap_info[type] performed by alloc_swap_info() (or a "co"-later store),
> provided get_swap_page_of_type() observes the increment of nr_swapfiles
> performed by the (same instance of) alloc_swap_info().

That's good to hear, thanks for looking into it.

> One clarification about the READ_ONCE() matter: the LKMM cannot handle
> plain or unmarked (shared memory) accesses in their generality at the
> moment (patches providing support for these accesses are in the making,
> but they will take some time); IAC, I'm confident to anticipate that,
> for the particular pattern in question (aka, MP), marking the accesses
> to nr_swapfiles will be "LKMM-sane" (one way to achieve this would be
> to convert nr_swapfiles to an atomic_t type...).

I guess you mean we could either use READ_ONCE or make nr_swapfiles atomic,
they're different ways of achieving the same thing.

> I take the liberty of adding other LKMM folks (so that they can blame
> me for "the spam"! ;-) ): I've learnt from experience that four or more
> eyes are better than two when it comes to discuss these matters... ;-)

Ok, it's fine with me as long as they blame you :)

> > > si = swap_info[type];