mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

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], 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.

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/[email protected]/

[[email protected]: updates per Randy]
  Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: v3]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Jeff Xu <[email protected]>
Reviewed-by: Randy Dunlap <[email protected]>
Cc: Aleksa Sarai <[email protected]>
Cc: Barnabás Pőcze <[email protected]>
Cc: Daniel Verkamp <[email protected]>
Cc: David Rheinsberg <[email protected]>
Cc: Dmitry Torokhov <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Jorge Lucangeli Obes <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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