linux-kernel - Re: [PATCH] mm/secretmem: use refcount_t instead of atomic

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Date:   Thu, 19 Aug 2021 22:33:49 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Jordy Zomer <jordy@...ing.systems>
Cc:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Mike Rapoport <rppt@...nel.org>,
        James Bottomley <jejb@...ux.ibm.com>
Subject: Re: [PATCH] mm/secretmem: use refcount_t instead of atomic_t

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@...omium.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