On Tue, Jun 11, 2024 at 3:41 PM Randy Dunlap <rdunlap@xxxxxxxxxxxxx> wrote: > > > > On 6/10/24 8:49 PM, jeffxu@xxxxxxxxxxxx wrote: > > From: Jeff Xu <jeffxu@xxxxxxxxxxxx> > > > > Add documentation for memfd_create flags: MFD_NOEXEC_SEAL > > and MFD_EXEC > > > > Cc: stable@xxxxxxxxxxxxxxx > > 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..ec6e3560fbff > > --- /dev/null > > +++ b/Documentation/userspace-api/mfd_noexec.rst > > @@ -0,0 +1,86 @@ > > +.. SPDX-License-Identifier: GPL-2.0 > > + > > +================================== > > +Introduction of non executable mfd > > Missed: > non-executable > > > +================================== > > +:Author: > > + Daniel Verkamp <dverkamp@xxxxxxxxxxxx> > > + Jeff Xu <jeffxu@xxxxxxxxxxxx> > > + > > +:Contributor: > > + Aleksa Sarai <cyphar@xxxxxxxxxx> > > + > > +Since Linux introduced the memfd feature, memfds 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 of 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 a 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. > > + - Add a new pid namespace sysctl: vm.memfd_noexec to help applications to > > help applications in > > > + migrating and enforcing non-executable MFD. > > + > > +User API > > +======== > > The rest looks good. Thanks. > Thanks for your review! > -- > ~Randy
