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653c5c7511
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> |
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