Re: [syzbot] WARNING: refcount bug in sys_memfd_secret

[Dmitry Vyukov]

On Sun, 24 Oct 2021 at 07:38, Mike Rapoport <rppt@xxxxxxxxxx> wrote:
>
> On Sat, Oct 23, 2021 at 11:46:18PM +0100, Matthew Wilcox wrote:
> > On Sat, Oct 23, 2021 at 10:03:11AM -0700, Kees Cook wrote:
> > > On October 23, 2021 8:27:28 AM PDT, Mike Rapoport <rppt@xxxxxxxxxx> wrote:
> > > >and my first reaction was to send a revert the untested commit 110860541f44
> > > >("mm/secretmem: use refcount_t instead of atomic_t").
> >
> > I think you should.  This isn't a real problem.
>
> Do you mean that creation of 4 billion of file descriptors is not feasible?

FWIW I think refcount is at least capable of catching the issue I
described with the counter temporarily going below its true value.
With refcount it can be caught during fuzzing as refcount reaching 0
and then being incremented again. Basically this warning, but a true
positive.