linux-kernel - Re: [syzbot] WARNING: refcount bug in sys_memfd

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Message-ID: <CACT4Y+aqEOfSY+R1EkdNhCbnSCjWrLO4f0T9QxWG6-UV7z7+Fg@mail.gmail.com>
Date:   Sun, 24 Oct 2021 09:07:42 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Kees Cook <keescook@...omium.org>,
        syzbot <syzbot+b904a1de3ec43711eba5@...kaller.appspotmail.com>,
        Jordy Zomer <jordy@...ing.systems>, akpm@...ux-foundation.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] WARNING: refcount bug in sys_memfd_secret

On Sun, 24 Oct 2021 at 07:38, Mike Rapoport <rppt@...nel.org> 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@...nel.org> 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.