Re: [PATCH v1 0/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL

[Jeff Xu <jeffxu@chromium.org>]

Resent, (previous email is not plain text)

Hi

On Fri, Jun 7, 2024 at 2:41 PM Barnabás Pőcze <pobrn@protonmail.com> wrote:
>
> Hi
>
>
> 2024. június 7., péntek 22:35 keltezéssel, jeffxu@chromium.org <jeffxu@chromium.org> írta:
>
> > From: Jeff Xu <jeffxu@chromium.org>
> >
> > When MFD_NOEXEC_SEAL was introduced, there was one big mistake: it
> > didn't have proper documentation. This led to a lot of confusion,
> > especially about whether or not memfd created with the MFD_NOEXEC_SEAL
> > flag is sealable. Before MFD_NOEXEC_SEAL, memfd had to explicitly set
> > MFD_ALLOW_SEALING to be sealable, so it's a fair question.
> >
> > As one might have noticed, unlike other flags in memfd_create,
> > MFD_NOEXEC_SEAL is actually a combination of multiple flags. The idea
> > is to make it easier to use memfd in the most common way, which is
> > NOEXEC + F_SEAL_EXEC + MFD_ALLOW_SEALING. This works with sysctl
> > vm.noexec to help existing applications move to a more secure way of
> > using memfd.
> >
> > Proposals have been made to put MFD_NOEXEC_SEAL non-sealable, unless
> > MFD_ALLOW_SEALING is set, to be consistent with other flags [1] [2],
> > Those are based on the viewpoint that each flag is an atomic unit,
> > which is a reasonable assumption. However, MFD_NOEXEC_SEAL was
> > designed with the intent of promoting the most secure method of using
> > memfd, therefore a combination of multiple functionalities into one
> > bit.
> >
> > Furthermore, the MFD_NOEXEC_SEAL has been added for more than one
> > year, and multiple applications and distributions have backported and
> > utilized it. Altering ABI now presents a degree of risk and may lead
> > to disruption.
>
> I feel compelled to mention again that based on my investigation the risk is
> minimal. Not to mention that it can easily be reverted if need be.
>
The risk is not zero. If we changed the ABI it would be propagated to
early kernel stable versions. Various Linux distributions also
backported the patch to earlier kernels such as 5.4. If it needs a
revert, then everyone has to do it again.

> In my view, it is better to fix the inconsistency than to document it. I would
> argue that "`MFD_ALLOW_SEALING` is needed to enable sealing except that XYZ"
> is unintuitive and confusing for a non-significant amount of people.
>
I understand,  documentation helps resolve the confusion, the next
step  is to  update the man page for memfd.

Thanks
-Jeff