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

[Eric Biggers <ebiggers@kernel.org>]

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().

- Eric