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

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Message-ID: <YXV8Uq17fjBVvQNn@kernel.org>
Date:   Sun, 24 Oct 2021 18:31:30 +0300
From:   Mike Rapoport <rppt@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        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, Oct 24, 2021 at 11:57:02AM +0100, Matthew Wilcox wrote:
> On Sun, Oct 24, 2021 at 08:37:59AM +0300, Mike Rapoport 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?
> 
> On a sufficiently large machine, it is.  But then we have the same
> problem with other atomic_t.  If you really care, just check whether
> secretmem_users has gone negative, and return -ENFILE.  It doesn't
> even have to be all that exact; you've got 2 billion values of slop
> to use before you hit the wrap from negative to 0 which is the actual
> problem.
> 
> ie this:
> 
> +++ b/mm/secretmem.c
> @@ -203,6 +203,8 @@ SYSCALL_DEFINE1(memfd_secret, unsigned int, flags)
> 
>         if (flags & ~(SECRETMEM_FLAGS_MASK | O_CLOEXEC))
>                 return -EINVAL;
> +       if (atomic_read(&secretmem_users) < 0)
> +               return -ENFILE;

So you suggest to prevent creation of the file descriptor to ensure there
is no overflow of secretmem_users. I don't feel it's a clean and elegant
solution.

> 
>         fd = get_unused_fd_flags(flags & O_CLOEXEC);
>         if (fd < 0)
> 
> 
> Also, why does secretmem depend on !EMBEDDED?

There was a request from tiny-config maintainers to keep this code outside
tiny-config and the best option I could find to make secretmem depend on
!EMBEDDED.

-- 
Sincerely yours,
Mike.