From: Jordy Zomer <jordy@pwning.systems>
To: Kees Cook <keescook@chromium.org>,
James Bottomley <James.Bottomley@hansenpartnership.com>
Cc: linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Mike Rapoport <rppt@linux.ibm.com>
Subject: Re: [PATCH] mm/secretmem: use refcount_t instead of atomic_t
Date: Fri, 20 Aug 2021 12:38:40 -0400 (EDT) [thread overview]
Message-ID: <209705133.1285234.1629477520318@privateemail.com> (raw)
In-Reply-To: <202108200904.81ED4AA52@keescook>
Hi There!
Because this is a global variable, it appears to be exploitable. Either we generate a sufficient number of processes to achieve this counter, or you increase the open file limit with ulimit or sysctl. Unless the kernel has a hard restriction on the number of potential file descriptors that I'm not aware of.
In any case, it's probably a good idea to patch this to make it explicitly secure. If you discover a hard-limit in the kernel for open file descriptors, please let me know. I'm genuinely interested :D!
Best Regards,
Jordy
> On 08/20/2021 12:05 PM Kees Cook <keescook@chromium.org> wrote:
>
>
> On Fri, Aug 20, 2021 at 07:57:25AM -0700, James Bottomley wrote:
> > On Fri, 2021-08-20 at 06:33 +0200, Jordy Zomer wrote:
> > > As you can see there's an `atomic_inc` for each `memfd` that is
> > > opened in the `memfd_secret` syscall. If a local attacker succeeds to
> > > open 2^32 memfd's, the counter will wrap around to 0. This implies
> > > that you may hibernate again, even though there are still regions of
> > > this secret memory, thereby bypassing the security check.
> >
> > This isn't a possible attack, is it? secret memory is per process and
> > each process usually has an open fd limit of 1024. That's not to say
> > we shouldn't have overflow protection just in case, but I think today
> > we don't have a problem.
>
> But it's a _global_ setting, so it's still possible, though likely
> impractical today. But refcount_t mitigates it and is a trivial change.
> :)
>
> --
> Kees Cook
next prev parent reply other threads:[~2021-08-20 16:38 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-08-20 4:33 [PATCH] mm/secretmem: use refcount_t instead of atomic_t Jordy Zomer
2021-08-20 5:33 ` Kees Cook
2021-08-24 14:05 ` Mike Rapoport
2021-10-21 9:00 ` Dmitry Vyukov
2021-08-20 14:57 ` James Bottomley
2021-08-20 16:05 ` Kees Cook
2021-08-20 16:38 ` Jordy Zomer [this message]
2021-08-20 19:40 ` James Bottomley
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