linux-kernel - Re: [PATCH v1 1/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [<thread-prev] [day] [month] [year] [list]
Message-ID: <CALmYWFvuRsTZSx3E1BhnwxHL3Qn-wQF9th2JkXwpFcO9at_9vw@mail.gmail.com>
Date: Mon, 10 Jun 2024 20:32:34 -0700
From: Jeff Xu <jeffxu@...gle.com>
To: Randy Dunlap <rdunlap@...radead.org>
Cc: jeffxu@...omium.org, akpm@...ux-foundation.org, cyphar@...har.com, 
	david@...dahead.eu, dmitry.torokhov@...il.com, dverkamp@...omium.org, 
	hughd@...gle.com, jorgelo@...omium.org, keescook@...omium.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-mm@...ck.org, pobrn@...tonmail.com, skhan@...uxfoundation.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH v1 1/1] mm/memfd: add documentation for MFD_NOEXEC_SEAL MFD_EXEC

Hi

On Mon, Jun 10, 2024 at 7:20 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>
> Hi--
>
> On 6/7/24 1:35 PM, jeffxu@...omium.org wrote:
> > From: Jeff Xu <jeffxu@...omium.org>
> >
> > Add documentation for memfd_create flags: FMD_NOEXEC_SEAL
>
> s/FMD/MFD/
>
> > and MFD_EXEC
> >
> > Signed-off-by: Jeff Xu <jeffxu@...omium.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@...omium.org>
> > +    Jeff Xu <jeffxu@...omium.org>
> > +
> > +:Contributor:
> > +     Aleksa Sarai <cyphar@...har.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,
>
Updated in V2.
Thanks!
-Jeff

> > +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