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From: Jeff Xu <jeffxu@google.com>
Date: Thu, 23 May 2024 09:20:55 -0700
Message-ID: <CALmYWFv9dK5ZPzwx3WCLMXzuuDadvFxh84+8rrT7aL105+ZZAQ@mail.gmail.com>
Subject: Re: [PATCH v1] memfd: `MFD_NOEXEC_SEAL` should not imply `MFD_ALLOW_SEALING`
To: David Rheinsberg <david@readahead.eu>, Aleksa Sarai <cyphar@cyphar.com>
Cc: =?UTF-8?B?QmFybmFiw6FzIFDFkWN6ZQ==?= <pobrn@protonmail.com>, 
	Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, 
	linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, 
	dmitry.torokhov@gmail.com, Daniel Verkamp <dverkamp@chromium.org>, hughd@google.com, 
	jorgelo@chromium.org, skhan@linuxfoundation.org, 
	Kees Cook <keescook@chromium.org>
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On Thu, May 23, 2024 at 1:24=E2=80=AFAM David Rheinsberg <david@readahead.e=
u> wrote:
>
> Hi
>
> On Thu, May 23, 2024, at 4:25 AM, Barnab=C3=A1s P=C5=91cze wrote:
> > 2024. m=C3=A1jus 23., cs=C3=BCt=C3=B6rt=C3=B6k 1:23 keltez=C3=A9ssel, A=
ndrew Morton
> > <akpm@linux-foundation.org> =C3=ADrta:
> >> It's a change to a userspace API, yes?  Please let's have a detailed
> >> description of why this is OK.  Why it won't affect any existing users=
.
> >
> > Yes, it is a uAPI change. To trigger user visible change, a program has=
 to
> >
> >  - create a memfd
> >    - with MFD_NOEXEC_SEAL,
> >    - without MFD_ALLOW_SEALING;
> >  - try to add seals / check the seals.
> >
> > This change in essence reverts the kernel's behaviour to that of Linux
> > <6.3, where
> > only `MFD_ALLOW_SEALING` enabled sealing. If a program works correctly
> > on those
> > kernels, it will likely work correctly after this change.
> >
> > I have looked through Debian Code Search and GitHub, searching for
> > `MFD_NOEXEC_SEAL`.
> > And I could find only a single breakage that this change would case:
> > dbus-broker
> > has its own memfd_create() wrapper that is aware of this implicit
> > `MFD_ALLOW_SEALING`
> > behaviour[0], and tries to work around it. This workaround will break.
> > Luckily,
> > however, as far as I could tell this only affects the test suite of
> > dbus-broker,
> > not its normal operations, so I believe it should be fine. I have
> > prepared a PR
> > with a fix[1].
>
> We asked for exactly this fix before, so I very much support this. Our te=
st-suite in `dbus-broker` merely verifies what the current kernel behavior =
is (just like the kernel selftests). I am certainly ok if the kernel breaks=
 it. I will gladly adapt the test-suite.
>
> Previous discussion was in:
>
>     [PATCH] memfd: support MFD_NOEXEC alongside MFD_EXEC
>     https://lore.kernel.org/lkml/20230714114753.170814-1-david@readahead.=
eu/
>
> Note that this fix is particularly important in combination with `vm.memf=
d_noexec=3D2`, since this breaks existing user-space by enabling sealing on=
 all memfds unconditionally. I also encourage backporting to stable kernels=
.
>
Also with vm.memfd_noexec=3D1.
I think that problem must be addressed either with this patch, or with
a new flag.

Regarding vm.memfd_noexec, on another topic.
I think in addition to  vm.memfd_noexec =3D 1 and 2,  there still could
be another state: 3

=3D0. Do nothing.
=3D1. This will add MFD_NOEXEC_SEAL if application didn't set EXEC or
MFD_NOEXE_SEAL (to help with the migration)
=3D2: This will reject all calls without MFD_NOEXEC_SEAL (the whole
system doesn't allow executable memfd)
=3D3:  Application must set MFD_EXEC or MFD_NOEXEC_SEAL explicitly, or
else it will be rejected.

3 is useful because it lets applications choose what to use, and
forces applications to migrate to new semantics (this is what 2 did
before 9876cfe8).
The caveat is 3 is less restrictive than 2, so must document it clearly.

-Jeff

> Reviewed-by: David Rheinsberg <david@readahead.eu>
>
> Thanks
> David