Re: [PATCH v7 5/7] fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC

Kees Cook
2020-07-24T19:03:50Z

On Thu, Jul 23, 2020 at 07:12:25PM +0200, Mickaël Salaün wrote:
> Allow for the enforcement of the O_MAYEXEC openat2(2) flag. Thanks to
> the noexec option from the underlying VFS mount, or to the file execute
> permission, userspace can enforce these execution policies. This may
> allow script interpreters to check execution permission before reading
> commands from a file, or dynamic linkers to allow shared object loading.
>
> Add a new sysctl fs.open_mayexec_enforce to enable system administrators
> to enforce two complementary security policies according to the
> installed system: enforce the noexec mount option, and enforce
> executable file permission. Indeed, because of compatibility with
> installed systems, only system administrators are able to check that
> this new enforcement is in line with the system mount points and file
> permissions. A following patch adds documentation.
>
> Being able to restrict execution also enables to protect the kernel by
> restricting arbitrary syscalls that an attacker could perform with a
> crafted binary or certain script languages. It also improves multilevel
> isolation by reducing the ability of an attacker to use side channels
> with specific code. These restrictions can natively be enforced for ELF
> binaries (with the noexec mount option) but require this kernel
> extension to properly handle scripts (e.g., Python, Perl). To get a
> consistent execution policy, additional memory restrictions should also
> be enforced (e.g. thanks to SELinux).
>
> Because the O_MAYEXEC flag is a meant to enforce a system-wide security
> policy (but not application-centric policies), it does not make sense
> for userland to check the sysctl value. Indeed, this new flag only
> enables to extend the system ability to enforce a policy thanks to (some
> trusted) userland collaboration. Moreover, additional security policies
> could be managed by LSMs. This is a best-effort approach from the
> application developer point of view:
> https://lore.kernel.org/lkml/1477d3d7-4b36-afad-7077-a38f42322238@digikod.net/
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>

Acked-by: Kees Cook <keescook@chromium.org>

--
Kees Cook