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

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Date:   Fri, 20 Aug 2021 07:57:25 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Jordy Zomer <jordy@...ing.systems>, linux-kernel@...r.kernel.org
Cc:     Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Mike Rapoport <rppt@...ux.ibm.com>
Subject: Re: [PATCH] mm/secretmem: use refcount_t instead of atomic_t

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.

James