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

[jeffxu]

From: Jeff Xu <jeffxu@xxxxxxxxxxxx>

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.

MFD_NOEXEC_SEAL is a new flag, and applications must change their code
to use it. There is no backward compatibility problem.

When sysctl vm.noexec == 1 or 2, applications that don't set
MFD_NOEXEC_SEAL or MFD_EXEC will get MFD_NOEXEC_SEAL memfd. And
old-application might break, that is by-design, in such a system
vm.noexec = 0 shall be used. Also no backward compatibility problem.

I propose to include this documentation patch to assist in clarifying
the semantics of MFD_NOEXEC_SEAL, thereby preventing any potential
future confusion.

This patch supersede previous patch which is trying different
direction [3], and please remove [2] from mm-unstable branch when
applying this patch.

Finally, I would like to express my gratitude to David Rheinsberg and
Barnabás Pőcze for initiating the discussion on the topic of sealability.

[1]
https://lore.kernel.org/lkml/20230714114753.170814-1-david@xxxxxxxxxxxx/

[2] 
https://lore.kernel.org/lkml/20240513191544.94754-1-pobrn@xxxxxxxxxxxxxx/

[3]
https://lore.kernel.org/lkml/20240524033933.135049-1-jeffxu@xxxxxxxxxx/

Jeff Xu (1):
  mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

 Documentation/userspace-api/index.rst      |  1 +
 Documentation/userspace-api/mfd_noexec.rst | 86 ++++++++++++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 Documentation/userspace-api/mfd_noexec.rst

-- 
2.45.2.505.gda0bf45e8d-goog