The quilt patch titled
Subject: mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC
has been removed from the -mm tree. Its filename was
mm-memfd-add-documentation-for-mfd_noexec_seal-mfd_exec.patch
This patch was dropped because it was merged into the mm-hotfixes-stable branch
of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
------------------------------------------------------
From: Jeff Xu <jeffxu@xxxxxxxxxxxx>
Subject: mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC
Date: Fri, 7 Jun 2024 20:35:41 +0000
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 PÅ?cze for initiating the discussion on the topic of sealability.
[1]
https://lore.kernel.org/lkml/20230714114753.170814-1-david@xxxxxxxxxxxx/
[jeffxu@xxxxxxxxxxxx: updates per Randy]
Link: https://lkml.kernel.org/r/20240611034903.3456796-2-jeffxu@xxxxxxxxxxxx
[jeffxu@xxxxxxxxxxxx: v3]
Link: https://lkml.kernel.org/r/20240611231409.3899809-2-jeffxu@xxxxxxxxxxxx
Link: https://lkml.kernel.org/r/20240607203543.2151433-2-jeffxu@xxxxxxxxxx
Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx>
Reviewed-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Aleksa Sarai <cyphar@xxxxxxxxxx>
Cc: Barnabás PÅ?cze <pobrn@xxxxxxxxxxxxxx>
Cc: Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
Cc: David Rheinsberg <david@xxxxxxxxxxxx>
Cc: Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx>
Cc: Hugh Dickins <hughd@xxxxxxxxxx>
Cc: Jorge Lucangeli Obes <jorgelo@xxxxxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Shuah Khan <skhan@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/userspace-api/index.rst | 1
Documentation/userspace-api/mfd_noexec.rst | 86 +++++++++++++++++++
2 files changed, 87 insertions(+)
--- a/Documentation/userspace-api/index.rst~mm-memfd-add-documentation-for-mfd_noexec_seal-mfd_exec
+++ a/Documentation/userspace-api/index.rst
@@ -32,6 +32,7 @@ Security-related interfaces
seccomp_filter
landlock
lsm
+ mfd_noexec
spec_ctrl
tee
--- /dev/null
+++ a/Documentation/userspace-api/mfd_noexec.rst
@@ -0,0 +1,86 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+==================================
+Introduction of non-executable mfd
+==================================
+:Author:
+ Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
+ Jeff Xu <jeffxu@xxxxxxxxxxxx>
+
+:Contributor:
+ Aleksa Sarai <cyphar@xxxxxxxxxx>
+
+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 â??confused deputy attackâ??. 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 memfdâ??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/
_
Patches currently in -mm which might be from jeffxu@xxxxxxxxxxxx are
proc-pid-smaps-add-mseal-info-for-vma.patch
