From: Dmitry Vyukov <dvyukov@google.com>
To: Mike Rapoport <rppt@kernel.org>
Cc: Kees Cook <keescook@chromium.org>,
Jordy Zomer <jordy@pwning.systems>,
linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, James Bottomley <jejb@linux.ibm.com>
Subject: Re: [PATCH] mm/secretmem: use refcount_t instead of atomic_t
Date: Thu, 21 Oct 2021 11:00:09 +0200 [thread overview]
Message-ID: <CACT4Y+b1sW6-Hkn8HQYw_SsT7X3tp-CJNh2ci0wG3ZnQz9jjig@mail.gmail.com> (raw)
In-Reply-To: <YST8vi6J1NlCdirU@kernel.org>
On Tue, 24 Aug 2021 at 16:06, Mike Rapoport <rppt@kernel.org> wrote:
>
> On Thu, Aug 19, 2021 at 10:33:49PM -0700, Kees Cook wrote:
> > On Fri, Aug 20, 2021 at 06:33:38AM +0200, Jordy Zomer wrote:
> > > When a secret memory region is active, memfd_secret disables
> > > hibernation. One of the goals is to keep the secret data from being
> > > written to persistent-storage.
> > >
> > > It accomplishes this by maintaining a reference count to
> > > `secretmem_users`. Once this reference is held your system can not be
> > > hibernated due to the check in `hibernation_available()`. However,
> > > because `secretmem_users` is of type `atomic_t`, reference counter
> > > overflows are possible.
> >
> > It's an unlikely condition to hit given max-open-fds, etc, but there's
> > no reason to leave this weakness. Changing this to refcount_t is easy
> > and better than using atomic_t.
> >
> > Reviewed-by: Kees Cook <keescook@chromium.org>
> >
> > > 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.
> >
> > IMO, this hibernation check is also buggy, since it looks to be
> > vulnerable to ToCToU: processes aren't frozen when
> > hibernation_available() checks secretmem_users(), so a process could add
> > one and fill it before the process freezer stops it.
> >
> > And of course, there's still the ptrace hole[1], which is think is quite
> > serious as it renders the entire defense moot.
>
> I thought about what can be done here and could not come up with anything
> better that prevent PTRACE on a process with secretmem, but this seems to
> me too much from usability vs security POV.
>
> Protecting against root is always hard and secretmem anyway does not
> provide 100% guarantee by itself but rather makes an accidental data leak
> or non-target attack much harder.
>
> To be effective it also presumes that other hardening features are turned
> on by the system administrator on production systems, so it's not
> unrealistic to rely on ptrace being disabled.
Hi,
The issue existed before this change, but I think refcount_inc needs
to be done before fd_install. After fd_install finishes, the fd can be
used by userspace and we can have secret data in memory before the
refcount_inc.
A straightforward mis-use where a user will predict the returned fd in
another thread before the syscall returns and will use it to store
secret data is somewhat dubious because such a user just shoots
themself in the foot.
But a more interesting mis-used would be to close the predicted fd and
decrement the refcount before the corresponding refcount_inc, this way
one can briefly drop the refcount to zero while there are other users
of secretmem.
next prev parent reply other threads:[~2021-10-21 9:00 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 [this message]
2021-08-20 14:57 ` James Bottomley
2021-08-20 16:05 ` Kees Cook
2021-08-20 16:38 ` Jordy Zomer
2021-08-20 19:40 ` James Bottomley
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