@@ -32,6 +32,7 @@ Security-related interfaces
seccomp_filter
landlock
lsm
+ mfd_noexec
spec_ctrl
tee
new file mode 100644
@@ -0,0 +1,86 @@
+.. 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 their
+execute bit set, and the memfd_create() syscall doesn't allow setting
+it differently.
+
+However, in a secure-by-default system, such as ChromeOS, (where all
+executables should come from the rootfs, which is protected by verified
+boot), this executable nature of memfd opens a door for NoExec bypass
+and enables “confused deputy attack”. E.g, in VRP bug [1]: cros_vm
+process created a memfd to share the content with an external process,
+however the memfd is overwritten and used for executing arbitrary code
+and root escalation. [2] lists more VRP of this kind.
+
+On the other hand, executable memfd has its legit use: runc uses memfd’s
+seal and executable feature to copy the contents of the binary then
+execute them. For such a system, we need a solution to differentiate runc's
+use 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 that
+doesn't set the executable bit; for example, a container with
+vm.memfd_noexec=1 means the old software will create non-executable memfd
+by default while new software can create executable memfd by setting
+MFD_EXEC.
+
+The value of vm.memfd_noexec is passed to child namespace at creation
+time. In addition, the setting is hierarchical, i.e. during memfd_create,
+we will search from current ns to root ns and use the most restrictive
+setting.
+
+[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/