linux-hardening - Re: [PATCH] mm/secretmem: Avoid letting secretmem

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Date:   Thu, 21 Oct 2021 20:39:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Mike Rapoport <rppt@...nel.org>,
        Jordy Zomer <jordy@...dyzomer.github.io>, linux-mm@...ck.org,
        Dmitry Vyukov <dvyukov@...gle.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        David Hildenbrand <david@...hat.com>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mm/secretmem: Avoid letting secretmem_users drop to zero

On Thu, Oct 21, 2021 at 07:53:11PM -0700, Andrew Morton wrote:
> On Thu, 21 Oct 2021 08:40:46 -0700 Kees Cook <keescook@...omium.org> wrote:
> 
> > Quoting Dmitry: "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-use 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."
> > 
> > Move fd_install() after refcount_inc().
> 
> I added cc:stable.  Or doesn't the benefit/risk ratio justify that?

I hadn't because commit 110860541f44 ("mm/secretmem: use refcount_t
instead of atomic_t") wasn't, and this would build on top of it.

I think the exposure is very small in both places, so probably best to
avoid the churn, but I'm not _opposed_ to it.

-- 
Kees Cook