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From:   Jeff Xu <jeffxu@google.com>
Date:   Wed, 14 Dec 2022 15:32:16 -0800
Message-ID: <CALmYWFss4hGOgJaeah8p7q86xmE7AOwOazxggGCuY=A+ZUVWhQ@mail.gmail.com>
Subject: Re: [PATCH v7 0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC
To:     Kees Cook <keescook@chromium.org>
Cc:     jeffxu@chromium.org, skhan@linuxfoundation.org,
        akpm@linux-foundation.org, dmitry.torokhov@gmail.com,
        dverkamp@chromium.org, hughd@google.com, jorgelo@chromium.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-mm@kvack.org, jannh@google.com,
        linux-hardening@vger.kernel.org,
        linux-security-module@vger.kernel.org
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On Wed, Dec 14, 2022 at 10:54 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@google.com>
> >
> > Since Linux introduced the memfd feature, memfd have always had their
> > execute bit set, and the memfd_create() syscall doesn't allow setting
> > it differently.
> >
> > However, in a secure by default system, such as ChromeOS, (where all
> > executables should come from the rootfs, which is protected by Verified
> > boot), this executable nature of memfd opens a door for NoExec bypass
> > and enables =E2=80=9Cconfused deputy attack=E2=80=9D.  E.g, in VRP bug =
[1]: cros_vm
> > process created a memfd to share the content with an external process,
> > however the memfd is overwritten and used for executing arbitrary code
> > and root escalation. [2] lists more VRP in this kind.
> >
> > On the other hand, executable memfd has its legit use, runc uses memfd=
=E2=80=99s
> > seal and executable feature to copy the contents of the binary then
> > execute them, for such system, we need a solution to differentiate runc=
's
> > use of  executable memfds and an attacker's [3].
> >
> > To address those above, this set of patches add following:
> > 1> Let memfd_create() set X bit at creation time.
> > 2> Let memfd to be sealed for modifying X bit.
> > 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior =
of
> >    X bit.For example, if a container has vm.memfd_noexec=3D2, then
> >    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> > 4> A new security hook in memfd_create(). This make it possible to a ne=
w
> > LSM, which rejects or allows executable memfd based on its security pol=
icy.
>
> I think patch 1-5 look good to land. The LSM hook seems separable, and
> could continue on its own. Thoughts?
>
Agreed.

> (Which tree should memfd change go through?)
>
I'm not sure, is there a recommendation ?

Thanks.
Jeff

> -Kees
>
> --
> Kees Cook