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| 2 |
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| 3 | ==================================
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| 4 | Introduction of non-executable mfd
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| 5 | ==================================
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| 6 | :Author:
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| 7 | Daniel Verkamp <[email protected]>
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| 8 | Jeff Xu <[email protected]>
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| 9 |
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| 10 | :Contributor:
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| 11 | Aleksa Sarai <[email protected]>
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| 12 |
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| 13 | Since Linux introduced the memfd feature, memfds have always had their
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| 14 | execute bit set, and the memfd_create() syscall doesn't allow setting
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| 15 | it differently.
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| 16 |
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| 17 | However, in a secure-by-default system, such as ChromeOS, (where all
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| 18 | executables should come from the rootfs, which is protected by verified
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| 19 | boot), this executable nature of memfd opens a door for NoExec bypass
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| 20 | and enables “confused deputy attack”. E.g, in VRP bug [1]: cros_vm
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| 21 | process created a memfd to share the content with an external process,
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| 22 | however the memfd is overwritten and used for executing arbitrary code
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| 23 | and root escalation. [2] lists more VRP of this kind.
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| 24 |
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| 25 | On the other hand, executable memfd has its legit use: runc uses memfd’s
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| 26 | seal and executable feature to copy the contents of the binary then
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| 27 | execute them. For such a system, we need a solution to differentiate runc's
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| 28 | use of executable memfds and an attacker's [3].
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| 29 |
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| 30 | To address those above:
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| 31 | - Let memfd_create() set X bit at creation time.
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| 32 | - Let memfd be sealed for modifying X bit when NX is set.
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| 33 | - Add a new pid namespace sysctl: vm.memfd_noexec to help applications in
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| 34 | migrating and enforcing non-executable MFD.
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| 35 |
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| 36 | User API
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| 37 | ========
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| 38 | ``int memfd_create(const char *name, unsigned int flags)``
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| 39 |
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| 40 | ``MFD_NOEXEC_SEAL``
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| 41 | When MFD_NOEXEC_SEAL bit is set in the ``flags``, memfd is created
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| 42 | with NX. F_SEAL_EXEC is set and the memfd can't be modified to
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| 43 | add X later. MFD_ALLOW_SEALING is also implied.
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| 44 | This is the most common case for the application to use memfd.
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| 45 |
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| 46 | ``MFD_EXEC``
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| 47 | When MFD_EXEC bit is set in the ``flags``, memfd is created with X.
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| 48 |
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| 49 | Note:
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| 50 | ``MFD_NOEXEC_SEAL`` implies ``MFD_ALLOW_SEALING``. In case that
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| 51 | an app doesn't want sealing, it can add F_SEAL_SEAL after creation.
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| 52 |
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| 53 |
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| 54 | Sysctl:
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| 55 | ========
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| 56 | ``pid namespaced sysctl vm.memfd_noexec``
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| 57 |
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| 58 | The new pid namespaced sysctl vm.memfd_noexec has 3 values:
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| 59 |
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| 60 | - 0: MEMFD_NOEXEC_SCOPE_EXEC
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| 61 | memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
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| 62 | MFD_EXEC was set.
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| 63 |
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| 64 | - 1: MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL
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| 65 | memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like
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| 66 | MFD_NOEXEC_SEAL was set.
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| 67 |
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| 68 | - 2: MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED
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| 69 | memfd_create() without MFD_NOEXEC_SEAL will be rejected.
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| 70 |
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| 71 | The sysctl allows finer control of memfd_create for old software that
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| 72 | doesn't set the executable bit; for example, a container with
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| 73 | vm.memfd_noexec=1 means the old software will create non-executable memfd
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| 74 | by default while new software can create executable memfd by setting
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| 75 | MFD_EXEC.
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| 76 |
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| 77 | The value of vm.memfd_noexec is passed to child namespace at creation
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| 78 | time. In addition, the setting is hierarchical, i.e. during memfd_create,
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| 79 | we will search from current ns to root ns and use the most restrictive
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| 80 | setting.
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| 81 |
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| 82 | [1] https://crbug.com/1305267
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| 83 |
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| 84 | [2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20memfd%20escalation&can=1
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| 85 |
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| 86 | [3] https://lwn.net/Articles/781013/
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