[PATCH v2 2/2] memfd:add MEMFD_NOEXEC_SEAL documentation

[jeffxu]

From: Jeff Xu <jeffxu@xxxxxxxxxx>

Add documentation for MFD_NOEXEC_SEAL and MFD_EXEC

Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxx>
---
 Documentation/userspace-api/index.rst      |  1 +
 Documentation/userspace-api/mfd_noexec.rst | 90 ++++++++++++++++++++++
 2 files changed, 91 insertions(+)
 create mode 100644 Documentation/userspace-api/mfd_noexec.rst

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
 
diff --git a/Documentation/userspace-api/mfd_noexec.rst b/Documentation/userspace-api/mfd_noexec.rst
new file mode 100644
index 000000000000..6f11ad86b076
--- /dev/null
+++ b/Documentation/userspace-api/mfd_noexec.rst
@@ -0,0 +1,90 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Introduction of non executable mfd
+==================================
+:Author:
+    Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
+    Jeff Xu <jeffxu@xxxxxxxxxx>
+
+:Contributor:
+	Aleksa Sarai <cyphar@xxxxxxxxxx>
+	Barnabás Pőcze <pobrn@xxxxxxxxxxxxxx>
+	David Rheinsberg <david@xxxxxxxxxxxx>
+
+Since Linux introduced the memfd feature, memfd 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 in 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 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.
+ - A new pid namespace sysctl: vm.memfd_noexec to help applications to
+   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.
+	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`` and ``MFD_EXEC`` doesn't change the sealable
+	characteristic of memfd, which is controlled by ``MFD_ALLOW_SEALING``.
+
+
+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 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.
+
+Reference:
+==========
+[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/
-- 
2.45.1.288.g0e0cd299f1-goog