Re: [PATCH v2 1/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

[Randy Dunlap]

On 6/10/24 8:49 PM, jeffxu@xxxxxxxxxxxx wrote:
> From: Jeff Xu <jeffxu@xxxxxxxxxxxx>
> 
> Add documentation for memfd_create flags: MFD_NOEXEC_SEAL
> and MFD_EXEC
> 
> Cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxxxx>
> 
> ---
>  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..ec6e3560fbff
> --- /dev/null
> +++ b/Documentation/userspace-api/mfd_noexec.rst
> @@ -0,0 +1,86 @@
> +.. SPDX-License-Identifier: GPL-2.0
> +
> +==================================
> +Introduction of non executable mfd

Missed:
                   non-executable

> +==================================
> +:Author:
> +    Daniel Verkamp <dverkamp@xxxxxxxxxxxx>
> +    Jeff Xu <jeffxu@xxxxxxxxxxxx>
> +
> +:Contributor:
> +	Aleksa Sarai <cyphar@xxxxxxxxxx>
> +
> +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 to

                                                         help applications in

> +   migrating and enforcing non-executable MFD.
> +
> +User API
> +========

The rest looks good. Thanks.

-- 
~Randy