Re: [PATCH v2 1/1] psi: Fix uaf issue when psi trigger is destroyed while being polled

[Suren Baghdasaryan <surenb@google.com>]

On Tue, Jan 11, 2022 at 11:41 AM Eric Biggers <ebiggers@kernel.org> wrote:
>
> On Tue, Jan 11, 2022 at 11:11:32AM -0800, Linus Torvalds wrote:
> > On Tue, Jan 11, 2022 at 10:48 AM Eric Biggers <ebiggers@kernel.org> wrote:
> > >
> > > The write here needs to use smp_store_release(), since it is paired with the
> > > concurrent READ_ONCE() in psi_trigger_poll().
> >
> > A smp_store_release() doesn't make sense pairing with a READ_ONCE().
> >
> > Any memory ordering that the smp_store_release() does on the writing
> > side is entirely irrelevant, since the READ_ONCE() doesn't imply any
> > ordering on the reading side. Ordering one but not the other is
> > nonsensical.
> >
> > So the proper pattern is to use a WRITE_ONCE() to pair with a
> > READ_ONCE() (when you don't care about memory ordering, or you handle
> > it explicitly), or a smp_load_acquire() with a smp_store_release() (in
> > which case writes before the smp_store_release() on the writing side
> > will be ordered wrt accesses after smp_load_acquire() on the reading
> > side).
> >
> > Of course, in practice, for pointers, the whole "dereference off a
> > pointer" on the read side *does* imply a barrier in all relevant
> > situations. So yes, a smp_store_release() -> READ_ONCE() does work in
> > practice, although it's technically wrong (in particular, it's wrong
> > on alpha, because of the completely broken memory ordering that alpha
> > has that doesn't even honor data dependencies as read-side orderings)
> >
> > But in this case, I do think that since there's some setup involved
> > with the trigger pointer, the proper serialization is to use
> > smp_store_release() to set the pointer, and then smp_load_acquire() on
> > the reading side.
> >
> > Or just use the RCU primitives - they are even better optimized, and
> > handle exactly that case, and can be more efficient on some
> > architectures if release->acquire isn't already cheap.
> >
> > That said, we've pretty much always accepted that normal word writes
> > are not going to tear, so we *have* also accepted just
> >
> >  - do any normal store of a value on the write side
> >
> >  - do a READ_ONCE() on the reading side
> >
> > where the reading side doesn't actually care *what* value it gets, it
> > only cares that the value it gets is *stable* (ie no compiler reloads
> > that might show up as two different values on the reading side).
> >
> > Of course, that has the same issue as WRITE_ONCE/READ_ONCE - you need
> > to worry about memory ordering separately.
> >
> > > > +     seq->private = new;
> > >
> > > Likewise here.
> >
> > Yeah, same deal, except here you can't even use the RCU ones, because
> > 'seq->private' isn't annotated for RCU.
> >
> > Or you'd do the casting, of course.
> >
>
> This is yet another case of "one time init".  There have been long discussions
> on this topic before:
> * https://lore.kernel.org/linux-fsdevel/20200713033330.205104-1-ebiggers@kernel.org/T/#u
> * https://lore.kernel.org/lkml/20200916233042.51634-1-ebiggers@kernel.org/T/#u
> * https://lwn.net/Articles/827180/
>
> I even attempted to document the best practices:
> * https://lore.kernel.org/linux-fsdevel/20200717044427.68747-1-ebiggers@kernel.org/T/#u
>
> However, no one could agree on whether READ_ONCE() or smp_load_acquire() should
> be used.  smp_load_acquire() is always correct, so it remains my preference.
> However, READ_ONCE() is correct in some cases, and some people (including the
> primary LKMM maintainer) insist that it be used in all such cases, as well as in
> rcu_dereference() even though this places difficult-to-understand constraints on
> how rcu_dereference() can be used.
>
> My preference is that smp_load_acquire() be used.  But be aware that this risks
> the READ_ONCE() people coming out of the woodwork and arguing for READ_ONCE().

I like my chances here (I believe we do need memory ordering in this
case). I'll post a fix with smp_load_acquire/smp_store_release shortly
after I run my tests. Thanks for the guidance!

>
> - Eric