| Message ID | 20240611034903.3456796-2-jeffxu@chromium.org (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | mm/memfd: add documentation for MFD_NOEXEC_SEAL | expand |
On 6/10/24 8:49 PM, jeffxu@chromium.org wrote: > From: Jeff Xu <jeffxu@chromium.org> > > Add documentation for memfd_create flags: MFD_NOEXEC_SEAL > and MFD_EXEC > > Cc: stable@vger.kernel.org > Signed-off-by: Jeff Xu <jeffxu@chromium.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..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@chromium.org> > + Jeff Xu <jeffxu@chromium.org> > + > +:Contributor: > + Aleksa Sarai <cyphar@cyphar.com> > + > +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.
On Tue, Jun 11, 2024 at 3:41 PM Randy Dunlap <rdunlap@infradead.org> wrote: > > > > On 6/10/24 8:49 PM, jeffxu@chromium.org wrote: > > From: Jeff Xu <jeffxu@chromium.org> > > > > Add documentation for memfd_create flags: MFD_NOEXEC_SEAL > > and MFD_EXEC > > > > Cc: stable@vger.kernel.org > > Signed-off-by: Jeff Xu <jeffxu@chromium.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..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@chromium.org> > > + Jeff Xu <jeffxu@chromium.org> > > + > > +:Contributor: > > + Aleksa Sarai <cyphar@cyphar.com> > > + > > +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
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 +================================== +:Author: + Daniel Verkamp <dverkamp@chromium.org> + Jeff Xu <jeffxu@chromium.org> + +:Contributor: + Aleksa Sarai <cyphar@cyphar.com> + +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 + 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 + an app doesn't want sealing, it can add F_SEAL_SEAL after creation. + + +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 vm.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. + +[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/