Re: [PATCH] secretmem: Prevent secretmem_users from wrapping to zero

[Matthew Wilcox <willy@infradead.org>]

On Mon, Oct 25, 2021 at 12:29:46PM -0700, Kees Cook wrote:
> On Mon, Oct 25, 2021 at 07:16:34PM +0100, Matthew Wilcox (Oracle) wrote:
> > Commit 110860541f44 ("mm/secretmem: use refcount_t instead of atomic_t")
> > attempted to fix the problem of secretmem_users wrapping to zero and
> > allowing suspend once again.  Prevent secretmem_users from wrapping to
> > zero by forbidding new users if the number of users has wrapped from
> > positive to negative.  This stops a long way short of reaching the
> > necessary 4 billion users, so there's no need to be clever with special
> > anti-wrap types or checking the return value from atomic_inc().
> 
> I still prefer refcount_t here because it provides deterministic
> saturation, but the risk right now is so narrow ("don't hibernate"),
> I'm not going to fight for it. I think it'd be fine to use it initialized
> to 1, and have the removal check for == 0 as a failure state, which would
> deterministically cover the underflow case too.

I still think that's abusing the refcount_t pattern.  refcount_t should
be for ... reference counts.  Not these other things.