Re: [PATCH v7 0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC

[Jeff Xu]

On Wed, Dec 14, 2022 at 10:54 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote:
>
> On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@xxxxxxxxxxxx wrote:
> > From: Jeff Xu <jeffxu@xxxxxxxxxx>
> >
> > Since Linux introduced the memfd feature, memfd 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 in 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 system, we need a solution to differentiate runc's
> > use of  executable memfds and an attacker's [3].
> >
> > To address those above, this set of patches add following:
> > 1> Let memfd_create() set X bit at creation time.
> > 2> Let memfd to be sealed for modifying X bit.
> > 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
> >    X bit.For example, if a container has vm.memfd_noexec=2, then
> >    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> > 4> A new security hook in memfd_create(). This make it possible to a new
> > LSM, which rejects or allows executable memfd based on its security policy.
>
> I think patch 1-5 look good to land. The LSM hook seems separable, and
> could continue on its own. Thoughts?
>
Agreed.

> (Which tree should memfd change go through?)
>
I'm not sure, is there a recommendation ?

Thanks.
Jeff

> -Kees
>
> --
> Kees Cook