From: Mike Rapoport <rppt@kernel.org>
To: Matthew Wilcox <willy@infradead.org>
Cc: Kees Cook <keescook@chromium.org>,
Dmitry Vyukov <dvyukov@google.com>,
syzbot <syzbot+b904a1de3ec43711eba5@syzkaller.appspotmail.com>,
Jordy Zomer <jordy@pwning.systems>,
akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
linux-mm@kvack.org, syzkaller-bugs@googlegroups.com
Subject: Re: [syzbot] WARNING: refcount bug in sys_memfd_secret
Date: Sun, 24 Oct 2021 18:31:30 +0300 [thread overview]
Message-ID: <YXV8Uq17fjBVvQNn@kernel.org> (raw)
In-Reply-To: <YXU7/iRjf9v77gon@casper.infradead.org>
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@kernel.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.
next prev parent reply other threads:[~2021-10-24 15:31 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-22 15:02 [syzbot] WARNING: refcount bug in sys_memfd_secret syzbot
2021-10-22 15:07 ` Dmitry Vyukov
2021-10-22 16:25 ` Jordy Zomer
2021-10-22 16:29 ` Dmitry Vyukov
2021-10-22 16:39 ` Jordy Zomer
2021-10-23 15:27 ` Mike Rapoport
2021-10-23 17:03 ` Kees Cook
2021-10-23 22:46 ` Matthew Wilcox
2021-10-24 5:37 ` Mike Rapoport
2021-10-24 7:07 ` Dmitry Vyukov
2021-10-24 10:57 ` Matthew Wilcox
2021-10-24 15:31 ` Mike Rapoport [this message]
2021-10-23 12:02 ` syzbot
2021-10-23 22:31 ` syzbot
2021-11-24 13:47 ` syzbot
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