87 lines
3.1 KiB
ReStructuredText
87 lines
3.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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==================================
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Introduction of non-executable mfd
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==================================
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:Author:
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Daniel Verkamp <dverkamp@chromium.org>
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Jeff Xu <jeffxu@chromium.org>
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:Contributor:
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Aleksa Sarai <cyphar@cyphar.com>
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Since Linux introduced the memfd feature, memfds have always had their
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execute bit set, and the memfd_create() syscall doesn't allow setting
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it differently.
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However, in a secure-by-default system, such as ChromeOS, (where all
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executables should come from the rootfs, which is protected by verified
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boot), this executable nature of memfd opens a door for NoExec bypass
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and enables “confused deputy attack”. E.g, in VRP bug [1]: cros_vm
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process created a memfd to share the content with an external process,
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however the memfd is overwritten and used for executing arbitrary code
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and root escalation. [2] lists more VRP of this kind.
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On the other hand, executable memfd has its legit use: runc uses memfd’s
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seal and executable feature to copy the contents of the binary then
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execute them. For such a system, we need a solution to differentiate runc's
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use of executable memfds and an attacker's [3].
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To address those above:
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- Let memfd_create() set X bit at creation time.
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- Let memfd be sealed for modifying X bit when NX is set.
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- Add a new pid namespace sysctl: vm.memfd_noexec to help applications in
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migrating and enforcing non-executable MFD.
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User API
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========
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``int memfd_create(const char *name, unsigned int flags)``
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``MFD_NOEXEC_SEAL``
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When MFD_NOEXEC_SEAL bit is set in the ``flags``, memfd is created
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with NX. F_SEAL_EXEC is set and the memfd can't be modified to
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add X later. MFD_ALLOW_SEALING is also implied.
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This is the most common case for the application to use memfd.
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``MFD_EXEC``
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When MFD_EXEC bit is set in the ``flags``, memfd is created with X.
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Note:
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``MFD_NOEXEC_SEAL`` implies ``MFD_ALLOW_SEALING``. In case that
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an app doesn't want sealing, it can add F_SEAL_SEAL after creation.
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Sysctl:
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========
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``pid namespaced sysctl vm.memfd_noexec``
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The new pid namespaced sysctl vm.memfd_noexec has 3 values:
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- 0: MEMFD_NOEXEC_SCOPE_EXEC
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memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
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MFD_EXEC was set.
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- 1: MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL
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memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
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MFD_NOEXEC_SEAL was set.
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- 2: MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED
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memfd_create() without MFD_NOEXEC_SEAL will be rejected.
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The sysctl allows finer control of memfd_create for old software that
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doesn't set the executable bit; for example, a container with
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vm.memfd_noexec=1 means the old software will create non-executable memfd
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by default while new software can create executable memfd by setting
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MFD_EXEC.
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The value of vm.memfd_noexec is passed to child namespace at creation
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time. In addition, the setting is hierarchical, i.e. during memfd_create,
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we will search from current ns to root ns and use the most restrictive
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setting.
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[1] https://crbug.com/1305267
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[2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20memfd%20escalation&can=1
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[3] https://lwn.net/Articles/781013/
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