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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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.. SPDX-License-Identifier: GPL-2.0
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==================================
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Introduction of non-executable mfd
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==================================
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:Author:
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Daniel Verkamp <dverkamp@chromium.org>
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Jeff Xu <jeffxu@chromium.org>
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:Contributor:
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Aleksa Sarai <cyphar@cyphar.com>
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Since Linux introduced the memfd feature, memfds have always had their
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execute bit set, and the memfd_create() syscall doesn't allow setting
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it differently.
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However, in a secure-by-default system, such as ChromeOS, (where all
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executables should come from the rootfs, which is protected by verified
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boot), this executable nature of memfd opens a door for NoExec bypass
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and enables “confused deputy attack”. E.g, in VRP bug [1]: cros_vm
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process created a memfd to share the content with an external process,
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however the memfd is overwritten and used for executing arbitrary code
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and root escalation. [2] lists more VRP of this kind.
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On the other hand, executable memfd has its legit use: runc uses memfd’s
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seal and executable feature to copy the contents of the binary then
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execute them. For such a system, we need a solution to differentiate runc's
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use of executable memfds and an attacker's [3].
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To address those above:
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- Let memfd_create() set X bit at creation time.
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- Let memfd be sealed for modifying X bit when NX is set.
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- Add a new pid namespace sysctl: vm.memfd_noexec to help applications in
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migrating and enforcing non-executable MFD.
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User API
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========
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``int memfd_create(const char *name, unsigned int flags)``
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``MFD_NOEXEC_SEAL``
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When MFD_NOEXEC_SEAL bit is set in the ``flags``, memfd is created
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with NX. F_SEAL_EXEC is set and the memfd can't be modified to
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add X later. MFD_ALLOW_SEALING is also implied.
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This is the most common case for the application to use memfd.
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``MFD_EXEC``
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When MFD_EXEC bit is set in the ``flags``, memfd is created with X.
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Note:
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``MFD_NOEXEC_SEAL`` implies ``MFD_ALLOW_SEALING``. In case that
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an app doesn't want sealing, it can add F_SEAL_SEAL after creation.
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Sysctl:
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========
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``pid namespaced sysctl vm.memfd_noexec``
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The new pid namespaced sysctl vm.memfd_noexec has 3 values:
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- 0: MEMFD_NOEXEC_SCOPE_EXEC
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memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
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MFD_EXEC was set.
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- 1: MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL
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memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
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MFD_NOEXEC_SEAL was set.
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- 2: MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED
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memfd_create() without MFD_NOEXEC_SEAL will be rejected.
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The sysctl allows finer control of memfd_create for old software that
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doesn't set the executable bit; for example, a container with
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vm.memfd_noexec=1 means the old software will create non-executable memfd
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by default while new software can create executable memfd by setting
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MFD_EXEC.
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The value of vm.memfd_noexec is passed to child namespace at creation
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time. In addition, the setting is hierarchical, i.e. during memfd_create,
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we will search from current ns to root ns and use the most restrictive
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setting.
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[1] https://crbug.com/1305267
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[2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20memfd%20escalation&can=1
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[3] https://lwn.net/Articles/781013/
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