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Date:   Fri, 20 Aug 2021 09:05:05 -0700
From:   Kees Cook <keescook@chromium.org>
To:     James Bottomley <James.Bottomley@hansenpartnership.com>
Cc:     Jordy Zomer <jordy@pwning.systems>, 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
Message-ID: <202108200904.81ED4AA52@keescook>
References: <20210820043339.2151352-1-jordy@pwning.systems>
 <0874a50b61cfaf7c817cab7344c49c1641c1fd10.camel@HansenPartnership.com>
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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