Hi On Mon, Jun 10, 2024 at 7:20 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote: > > Hi-- > > On 6/7/24 1:35 PM, jeffxu@xxxxxxxxxxxx wrote: > > From: Jeff Xu <jeffxu@xxxxxxxxxxxx> > > > > Add documentation for memfd_create flags: FMD_NOEXEC_SEAL > > s/FMD/MFD/ > > > and MFD_EXEC > > > > 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..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@xxxxxxxxxxxx> > > + Jeff Xu <jeffxu@xxxxxxxxxxxx> > > + > > +:Contributor: > > + Aleksa Sarai <cyphar@xxxxxxxxxx> > > + > > +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
