Re: [PATCH v1 1/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

[Randy Dunlap <rdunlap@infradead.org>]

Hi--

On 6/7/24 1:35 PM, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@chromium.org>
> 
> Add documentation for memfd_create flags: FMD_NOEXEC_SEAL

s/FMD/MFD/

> and MFD_EXEC
> 
> Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> ---
>  Documentation/userspace-api/index.rst      |  1 +
>  Documentation/userspace-api/mfd_noexec.rst | 86 ++++++++++++++++++++++
>  2 files changed, 87 insertions(+)
>  create mode 100644 Documentation/userspace-api/mfd_noexec.rst
> 
> diff --git a/Documentation/userspace-api/index.rst b/Documentation/userspace-api/index.rst
> index 5926115ec0ed..8a251d71fa6e 100644
> --- a/Documentation/userspace-api/index.rst
> +++ b/Documentation/userspace-api/index.rst
> @@ -32,6 +32,7 @@ Security-related interfaces
>     seccomp_filter
>     landlock
>     lsm
> +   mfd_noexec
>     spec_ctrl
>     tee
>  
> diff --git a/Documentation/userspace-api/mfd_noexec.rst b/Documentation/userspace-api/mfd_noexec.rst
> new file mode 100644
> index 000000000000..0d2c840f37e1
> --- /dev/null
> +++ b/Documentation/userspace-api/mfd_noexec.rst
> @@ -0,0 +1,86 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==================================
> +Introduction of non executable mfd

                   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, memfd have always had their

                                             memfds
i.e., plural

> +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

                 secure-by-default

> +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 in this kind.

                                           of this kind.

> +
> +On the other hand, executable memfd has its legit use, runc uses memfd’s

                                                     use:

> +seal and executable feature to copy the contents of the binary then
> +execute them, for such system, we need a solution to differentiate runc's

           them. For such a system,

> +use of  executable memfds and an attacker's [3].
> +
> +To address those above.

                    above:

> + - Let memfd_create() set X bit at creation time.
> + - Let memfd be sealed for modifying X bit when NX is set.
> + - A new pid namespace sysctl: vm.memfd_noexec to help applications to

    - Add a new                                           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
> +	app doesn't want sealing, it can add F_SEAL_SEAL after creation.

        an app

> +
> +
> +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

                                                       old software

> +doesn't set the executable bit, for example, a container with

                              bit;

> +vm.memfd_noexec=1 means the old-software will create non-executable memfd

                               old software

> +by default while new-software can create executable memfd by setting

                    new software

> +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,

   time. In addition,

> +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/

-- 
~Randy