From: Kees Cook <keescook@chromium.org>
To: Jordy Zomer <jordy@pwning.systems>
Cc: linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
linux-mm@kvack.org, Mike Rapoport <rppt@kernel.org>,
James Bottomley <jejb@linux.ibm.com>
Subject: Re: [PATCH] mm/secretmem: use refcount_t instead of atomic_t
Date: Thu, 19 Aug 2021 22:33:49 -0700 [thread overview]
Message-ID: <202108192227.8BE02F1C@keescook> (raw)
In-Reply-To: <20210820043339.2151352-1-jordy@pwning.systems>
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.
-Kees
[1] https://lore.kernel.org/lkml/202105071620.E834B1FA92@keescook/
--
Kees Cook
next prev parent reply other threads:[~2021-08-20 5:33 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 [this message]
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
2021-08-20 19:40 ` James Bottomley
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