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