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/