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.. 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/
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