Hi Aleksa On Thu, May 23, 2024 at 8:39 PM <jeffxu@xxxxxxxxxxxx> wrote: > > From: Jeff Xu <jeffxu@xxxxxxxxxx> > > Add documentation for MFD_NOEXEC_SEAL and MFD_EXEC > > Cc: stable@xxxxxxxxxxxxxxx > Signed-off-by: Jeff Xu <jeffxu@xxxxxxxxxx> > --- > Documentation/userspace-api/index.rst | 1 + > Documentation/userspace-api/mfd_noexec.rst | 90 ++++++++++++++++++++++ > 2 files changed, 91 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..6f11ad86b076 > --- /dev/null > +++ b/Documentation/userspace-api/mfd_noexec.rst > @@ -0,0 +1,90 @@ > +.. SPDX-License-Identifier: GPL-2.0 > + > +================================== > +Introduction of non executable mfd > +================================== > +:Author: > + Daniel Verkamp <dverkamp@xxxxxxxxxxxx> > + Jeff Xu <jeffxu@xxxxxxxxxx> > + > +:Contributor: > + Aleksa Sarai <cyphar@xxxxxxxxxx> > + Barnabás Pőcze <pobrn@xxxxxxxxxxxxxx> > + David Rheinsberg <david@xxxxxxxxxxxx> > + > +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. > + - 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 > + 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. > + 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`` and ``MFD_EXEC`` doesn't change the sealable > + characteristic of memfd, which is controlled by ``MFD_ALLOW_SEALING``. > + > + > +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 > +doesn't set the executable bit, for example, a container with > +vm.memfd_noexec=1 means the old-software will create non-executable memfd > +by default while new-software can create executable memfd by setting > +MFD_EXEC. > + > +The value of memfd_noexec is passed to child namespace at creation time, > +in addition, the setting is hierarchical, i.e. during memfd_create, > +we will search from current ns to root ns and use the most restrictive > +setting. > + Can you please help to review the sysctl part to check if I captured your change correctly ? Thanks -Jeff > +Reference: > +========== > +[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/ > -- > 2.45.1.288.g0e0cd299f1-goog >
