Re: [PATCH v2 0/5] mm/memfd: MFD_NOEXEC for memfd_create

From: Kees Cook <keescook@chromium.org>
To: Jeff Xu <jeffxu@chromium.org>
Cc: jeffxu@google.com, 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, mnissler@chromium.org, jannh@google.com,
	linux-hardening@vger.kernel.org, Aleksa Sarai <cyphar@cyphar.com>,
	dev@opencontainers.org, Christian Brauner <brauner@kernel.org>
Subject: Re: [PATCH v2 0/5] mm/memfd: MFD_NOEXEC for memfd_create
Date: Tue, 1 Nov 2022 19:45:42 -0700	[thread overview]
Message-ID: <202211011933.DD0979338@keescook> (raw)
In-Reply-To: <CABi2SkVXMUVhSTJezfHt_BKxyKP+x++9oveuB3qJZL7N672UKw@mail.gmail.com>

On Tue, Nov 01, 2022 at 04:14:39PM -0700, Jeff Xu wrote:
> Sorry for the long overdue reply.

No worries! I am a fan of thread necromancy. :)

> [...]
> 1> memfd_create:
> Add two flags:
> #define MFD_EXEC                      0x0008
> #define MFD_NOEXEC_SEAL    0x0010
> This lets application to set executable bit explicitly.
> (If application set both, it will be rejected)

So no MFD_NOEXEC without seal? (I'm fine with that.)

> 2> For old application that doesn't set executable bit:
> Add a pid name-spaced sysctl.kernel.pid_mfd_noexec, with:

bikeshed: vm.memfd_noexec
(doesn't belong in "kernel", and seems better suited to "vm" than "fs")

> value = 0: Default_EXEC
>      Honor MFD_EXEC and MFD_NOEXEC_SEAL
>      When none is set, will fall back to original behavior (EXEC)

Yeah. Rephrasing for myself to understand more clearly:

"memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL act like MFD_EXEC
was set."

> value = 1: Default_NOEXEC_SEAL
>       Honor MFD_EXEC and MFD_NOEXEC_SEAL
>       When none is set, will default to MFD_NOEXEC_SEAL

"memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL act like
MFD_NOEXEC_SEAL was set."

Also, I think there should be a pr_warn_ratelimited() when
memfd_create() is used without either bit, so that there is some
pressure to please adjust their API calls to explicitly set a bit.

> 3> Add a pid name-spaced sysctl kernel.pid_mfd_noexec_enforced: with:
> value = 0: default, not enforced.
> value = 1: enforce NOEXEC_SEAL (overwrite everything)

How about making this just mode "value 2" for the first sysctl?
"memfd_create() without MFD_NOEXEC_SEAL will be rejected."

-Kees

-- 
Kees Cook