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| author | Jeff Xu <jeffxu@chromium.org> | 2024-06-07 23:35:41 +0300 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2024-06-15 20:43:07 +0300 |
| commit | 653c5c75115c1e23b8393c1cb1ad2d6f6712742f (patch) | |
| tree | 019b5e0a8f256986a35c668b6015822fc5aedf54 /Documentation/userspace-api/index.rst | |
| parent | 3afb76a66b5559a7b595155803ce23801558a7a9 (diff) | |
| download | linux-653c5c75115c1e23b8393c1cb1ad2d6f6712742f.tar.xz | |
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/20230714114753.170814-1-david@readahead.eu/
[jeffxu@chromium.org: updates per Randy]
Link: https://lkml.kernel.org/r/20240611034903.3456796-2-jeffxu@chromium.org
[jeffxu@chromium.org: v3]
Link: https://lkml.kernel.org/r/20240611231409.3899809-2-jeffxu@chromium.org
Link: https://lkml.kernel.org/r/20240607203543.2151433-2-jeffxu@google.com
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Aleksa Sarai <cyphar@cyphar.com>
Cc: Barnabás Pőcze <pobrn@protonmail.com>
Cc: Daniel Verkamp <dverkamp@chromium.org>
Cc: David Rheinsberg <david@readahead.eu>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jorge Lucangeli Obes <jorgelo@chromium.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'Documentation/userspace-api/index.rst')
| -rw-r--r-- | Documentation/userspace-api/index.rst | 1 |
1 files changed, 1 insertions, 0 deletions
diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst index 5926115ec0ed..8a251d71fa6e 100644 --- a/Documentation/userspace-api/index.rst +++ b/Documentation/userspace-api/index.rst @@ -32,6 +32,7 @@ Security-related interfaces seccomp_filter landlock lsm + mfd_noexec spec_ctrl tee |