1.. SPDX-License-Identifier: GPL-2.0 2 3================================== 4Introduction of non-executable mfd 5================================== 6:Author: 7 Daniel Verkamp <dverkamp@chromium.org> 8 Jeff Xu <jeffxu@chromium.org> 9 10:Contributor: 11 Aleksa Sarai <cyphar@cyphar.com> 12 13Since Linux introduced the memfd feature, memfds have always had their 14execute bit set, and the memfd_create() syscall doesn't allow setting 15it differently. 16 17However, in a secure-by-default system, such as ChromeOS, (where all 18executables should come from the rootfs, which is protected by verified 19boot), this executable nature of memfd opens a door for NoExec bypass 20and enables “confused deputy attack”. E.g, in VRP bug [1]: cros_vm 21process created a memfd to share the content with an external process, 22however the memfd is overwritten and used for executing arbitrary code 23and root escalation. [2] lists more VRP of this kind. 24 25On the other hand, executable memfd has its legit use: runc uses memfd’s 26seal and executable feature to copy the contents of the binary then 27execute them. For such a system, we need a solution to differentiate runc's 28use of executable memfds and an attacker's [3]. 29 30To address those above: 31 - Let memfd_create() set X bit at creation time. 32 - Let memfd be sealed for modifying X bit when NX is set. 33 - Add a new pid namespace sysctl: vm.memfd_noexec to help applications in 34 migrating and enforcing non-executable MFD. 35 36User API 37======== 38``int memfd_create(const char *name, unsigned int flags)`` 39 40``MFD_NOEXEC_SEAL`` 41 When MFD_NOEXEC_SEAL bit is set in the ``flags``, memfd is created 42 with NX. F_SEAL_EXEC is set and the memfd can't be modified to 43 add X later. MFD_ALLOW_SEALING is also implied. 44 This is the most common case for the application to use memfd. 45 46``MFD_EXEC`` 47 When MFD_EXEC bit is set in the ``flags``, memfd is created with X. 48 49Note: 50 ``MFD_NOEXEC_SEAL`` implies ``MFD_ALLOW_SEALING``. In case that 51 an app doesn't want sealing, it can add F_SEAL_SEAL after creation. 52 53 54Sysctl: 55======== 56``pid namespaced sysctl vm.memfd_noexec`` 57 58The new pid namespaced sysctl vm.memfd_noexec has 3 values: 59 60 - 0: MEMFD_NOEXEC_SCOPE_EXEC 61 memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like 62 MFD_EXEC was set. 63 64 - 1: MEMFD_NOEXEC_SCOPE_NOEXEC_SEAL 65 memfd_create() without MFD_EXEC nor MFD_NOEXEC_SEAL acts like 66 MFD_NOEXEC_SEAL was set. 67 68 - 2: MEMFD_NOEXEC_SCOPE_NOEXEC_ENFORCED 69 memfd_create() without MFD_NOEXEC_SEAL will be rejected. 70 71The sysctl allows finer control of memfd_create for old software that 72doesn't set the executable bit; for example, a container with 73vm.memfd_noexec=1 means the old software will create non-executable memfd 74by default while new software can create executable memfd by setting 75MFD_EXEC. 76 77The value of vm.memfd_noexec is passed to child namespace at creation 78time. In addition, the setting is hierarchical, i.e. during memfd_create, 79we will search from current ns to root ns and use the most restrictive 80setting. 81 82[1] https://crbug.com/1305267 83 84[2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20memfd%20escalation&can=1 85 86[3] https://lwn.net/Articles/781013/ 87