linux-kernel - Re: [PATCH v7 0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD

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Date:   Wed, 14 Dec 2022 10:54:40 -0800
From:   Kees Cook <keescook@...omium.org>
To:     jeffxu@...omium.org
Cc:     skhan@...uxfoundation.org, akpm@...ux-foundation.org,
        dmitry.torokhov@...il.com, dverkamp@...omium.org, hughd@...gle.com,
        jeffxu@...gle.com, jorgelo@...omium.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-mm@...ck.org, jannh@...gle.com,
        linux-hardening@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v7 0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC

On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@...omium.org wrote:
> From: Jeff Xu <jeffxu@...gle.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 “confused deputy attack”.  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’s
> 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=2, then
>    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> 4> A new security hook in memfd_create(). This make it possible to a new
> LSM, which rejects or allows executable memfd based on its security policy.

I think patch 1-5 look good to land. The LSM hook seems separable, and
could continue on its own. Thoughts?

(Which tree should memfd change go through?)

-Kees

-- 
Kees Cook