linux-kernel - Re: [RFC PATCH v1 1/2] mm/memfd: Add support for F_SEAL_FUTURE

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Message-ID: <202501061643.986D9453@keescook>
Date: Mon, 6 Jan 2025 16:44:33 -0800
From: Kees Cook <kees@...nel.org>
To: Jeff Xu <jeffxu@...omium.org>
Cc: Jann Horn <jannh@...gle.com>,
	Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
	"Isaac J. Manjarres" <isaacmanjarres@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jeff Layton <jlayton@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>,
	Alexander Aring <alex.aring@...il.com>,
	"Liam R. Howlett" <Liam.Howlett@...cle.com>,
	Vlastimil Babka <vbabka@...e.cz>, Shuah Khan <shuah@...nel.org>,
	kernel-team@...roid.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	Suren Baghdasaryan <surenb@...gle.com>,
	Kalesh Singh <kaleshsingh@...gle.com>,
	John Stultz <jstultz@...gle.com>
Subject: Re: [RFC PATCH v1 1/2] mm/memfd: Add support for F_SEAL_FUTURE_EXEC
 to memfd

On Mon, Jan 06, 2025 at 10:26:27AM -0800, Jeff Xu wrote:
> + Kees because this is related to W^X memfd and security.
> 
> On Fri, Jan 3, 2025 at 7:14 AM Jann Horn <jannh@...gle.com> wrote:
> >
> > On Fri, Dec 6, 2024 at 7:19 PM Lorenzo Stoakes
> > <lorenzo.stoakes@...cle.com> wrote:
> > > On Thu, Dec 05, 2024 at 05:09:22PM -0800, Isaac J. Manjarres wrote:
> > > > +             if (is_exec_sealed(seals)) {
> > >
> > > Are we intentionally disallowing a MAP_PRIVATE memfd's mapping's execution?
> > > I've not tested this scenario so don't know if we somehow disallow this in
> > > another way but note on write checks we only care about shared mappings.
> > >
> > > I mean one could argue that a MAP_PRIVATE situation is the same as copying
> > > the data into an anon buffer and doing what you want with it, here you
> > > could argue the same...
> > >
> > > So probably we should only care about VM_SHARED?
> >
> > FWIW I think it doesn't make sense to distinguish between
> > shared/private mappings here - in the scenario described in the cover
> > letter, it wouldn't matter that much to an attacker whether the
> > mapping is shared or private (as long as the VMA contents haven't been
> > CoWed already).
> +1 on this.
> The concept of blocking this for only shared mapping is questionable.

Right -- why does sharedness matter? It seems more robust to me to not
create a corner case but rather apply the flag/behavior universally?

-- 
Kees Cook