[PATCH v3 0/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL

[PATCH v3 0/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL

[jeffxu]

From: Jeff Xu <jeffxu@chromium.org>

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] [2],
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.

This patch supersede previous patch which is trying different
direction [3], and please remove [2] from mm-unstable branch when
applying this patch.

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/

[2] 
https://lore.kernel.org/lkml/20240513191544.94754-1-pobrn@protonmail.com/

[3]
https://lore.kernel.org/lkml/20240524033933.135049-1-jeffxu@google.com/

v3:
Additional Randy Dunlap' comments.

v2:
Update according to Randy Dunlap' comments.
https://lore.kernel.org/linux-mm/20240611034903.3456796-1-jeffxu@chromium.org/

v1:
https://lore.kernel.org/linux-mm/20240607203543.2151433-1-jeffxu@google.com/

Jeff Xu (1):
  mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

 Documentation/userspace-api/index.rst      |  1 +
 Documentation/userspace-api/mfd_noexec.rst | 86 ++++++++++++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 Documentation/userspace-api/mfd_noexec.rst

-- 
2.45.2.505.gda0bf45e8d-goog

[PATCH v3 1/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

[jeffxu]

From: Jeff Xu <jeffxu@chromium.org>

Add documentation for memfd_create flags: MFD_NOEXEC_SEAL
and MFD_EXEC

Cc: stable@vger.kernel.org
Signed-off-by: Jeff Xu <jeffxu@chromium.org>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>

---
 Documentation/userspace-api/index.rst      |  1 +
 Documentation/userspace-api/mfd_noexec.rst | 86 ++++++++++++++++++++++
 2 files changed, 87 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..7afcc480e38f
--- /dev/null
+++ b/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@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/
-- 
2.45.2.505.gda0bf45e8d-goog