[v7,0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC

  From: Jeff Xu <jeffxu@google.com>

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, this set of patches add following:
1> Let memfd_create() set X bit at creation time.
2> Let memfd to be sealed for modifying X bit.
3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
   X bit.For example, if a container has vm.memfd_noexec=2, then
   memfd_create() without MFD_NOEXEC_SEAL will be rejected.
4> A new security hook in memfd_create(). This make it possible to a new
LSM, which rejects or allows executable memfd based on its security policy.

Change history:
v7:
- patch 2/6: remove #ifdef and MAX_PATH (memfd_test.c).
- patch 3/6: check capability (CAP_SYS_ADMIN) from userns instead of
		global ns (pid_sysctl.h). Add a tab (pid_namespace.h).
- patch 5/6: remove #ifdef (memfd_test.c)
- patch 6/6: remove unneeded security_move_mount(security.c).

v6:https://lore.kernel.org/lkml/20221206150233.1963717-1-jeffxu@google.com/
- Address comment and move "#ifdef CONFIG_" from .c file to pid_sysctl.h

v5:https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/
- Pass vm.memfd_noexec from current ns to child ns.
- Fix build issue detected by kernel test robot.
- Add missing security.c

v3:https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
- Address API design comments in v2.
- Let memfd_create() to set X bit at creation time.
- A new pid namespace sysctl: vm.memfd_noexec to control behavior of X bit.
- A new security hook in memfd_create().

v2:https://lore.kernel.org/lkml/20220805222126.142525-1-jeffxu@google.com/
- address comments in V1.
- add sysctl (vm.mfd_noexec) to set the default file permissions of
  memfd_create to be non-executable.

v1:https://lwn.net/Articles/890096/

[1] https://crbug.com/1305411
[2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20memfd%20escalation&can=1
[3] https://lwn.net/Articles/781013/

Daniel Verkamp (2):
  mm/memfd: add F_SEAL_EXEC
  selftests/memfd: add tests for F_SEAL_EXEC

Jeff Xu (4):
  mm/memfd: add MFD_NOEXEC_SEAL and MFD_EXEC
  mm/memfd: Add write seals when apply SEAL_EXEC to executable memfd
  selftests/memfd: add tests for MFD_NOEXEC_SEAL MFD_EXEC
  mm/memfd: security hook for memfd_create

 include/linux/lsm_hook_defs.h              |   1 +
 include/linux/lsm_hooks.h                  |   4 +
 include/linux/pid_namespace.h              |  19 ++
 include/linux/security.h                   |   6 +
 include/uapi/linux/fcntl.h                 |   1 +
 include/uapi/linux/memfd.h                 |   4 +
 kernel/pid_namespace.c                     |   5 +
 kernel/pid_sysctl.h                        |  59 ++++
 mm/memfd.c                                 |  61 +++-
 mm/shmem.c                                 |   6 +
 security/security.c                        |   5 +
 tools/testing/selftests/memfd/fuse_test.c  |   1 +
 tools/testing/selftests/memfd/memfd_test.c | 341 ++++++++++++++++++++-
 13 files changed, 510 insertions(+), 3 deletions(-)
 create mode 100644 kernel/pid_sysctl.h


base-commit: eb7081409f94a9a8608593d0fb63a1aa3d6f95d8
  

On Fri, Dec 9, 2022 at 11:05 AM <jeffxu@chromium.org> wrote:
> From: Jeff Xu <jeffxu@google.com>
>
> 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.

...

> [1] https://crbug.com/1305411

Can you make this accessible so those of us on the public lists can
view this bug?  If not, please remove it from future postings and
adjust your description accordingly.
  

On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@google.com>
> 
> 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, this set of patches add following:
> 1> Let memfd_create() set X bit at creation time.
> 2> Let memfd to be sealed for modifying X bit.
> 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
>    X bit.For example, if a container has vm.memfd_noexec=2, then
>    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> 4> A new security hook in memfd_create(). This make it possible to a new
> LSM, which rejects or allows executable memfd based on its security policy.

I think patch 1-5 look good to land. The LSM hook seems separable, and
could continue on its own. Thoughts?

(Which tree should memfd change go through?)

-Kees
  

On Wed, Dec 14, 2022 at 10:54 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@google.com>
> >
> > 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, this set of patches add following:
> > 1> Let memfd_create() set X bit at creation time.
> > 2> Let memfd to be sealed for modifying X bit.
> > 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
> >    X bit.For example, if a container has vm.memfd_noexec=2, then
> >    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> > 4> A new security hook in memfd_create(). This make it possible to a new
> > LSM, which rejects or allows executable memfd based on its security policy.
>
> I think patch 1-5 look good to land. The LSM hook seems separable, and
> could continue on its own. Thoughts?
>
Agreed.

> (Which tree should memfd change go through?)
>
I'm not sure, is there a recommendation ?

Thanks.
Jeff

> -Kees
>
> --
> Kees Cook
  

On Wed, Dec 14, 2022 at 03:32:16PM -0800, Jeff Xu wrote:
> On Wed, Dec 14, 2022 at 10:54 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@chromium.org wrote:
> > > From: Jeff Xu <jeffxu@google.com>
> > >
> > > 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, this set of patches add following:
> > > 1> Let memfd_create() set X bit at creation time.
> > > 2> Let memfd to be sealed for modifying X bit.
> > > 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
> > >    X bit.For example, if a container has vm.memfd_noexec=2, then
> > >    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> > > 4> A new security hook in memfd_create(). This make it possible to a new
> > > LSM, which rejects or allows executable memfd based on its security policy.
> >
> > I think patch 1-5 look good to land. The LSM hook seems separable, and
> > could continue on its own. Thoughts?
> >
> Agreed.
> 
> > (Which tree should memfd change go through?)
> >
> I'm not sure, is there a recommendation ?

It looks like it's traditionally through akpm's tree. Andrew, will you
carry patches 1-5?

Thanks!
  

On Wed, Dec 14, 2022 at 4:08 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Dec 14, 2022 at 03:32:16PM -0800, Jeff Xu wrote:
> > On Wed, Dec 14, 2022 at 10:54 AM Kees Cook <keescook@chromium.org> wrote:
> > >
> > > On Fri, Dec 09, 2022 at 04:04:47PM +0000, jeffxu@chromium.org wrote:
> > > > From: Jeff Xu <jeffxu@google.com>
> > > >
> > > > 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, this set of patches add following:
> > > > 1> Let memfd_create() set X bit at creation time.
> > > > 2> Let memfd to be sealed for modifying X bit.
> > > > 3> A new pid namespace sysctl: vm.memfd_noexec to control the behavior of
> > > >    X bit.For example, if a container has vm.memfd_noexec=2, then
> > > >    memfd_create() without MFD_NOEXEC_SEAL will be rejected.
> > > > 4> A new security hook in memfd_create(). This make it possible to a new
> > > > LSM, which rejects or allows executable memfd based on its security policy.
> > >
> > > I think patch 1-5 look good to land. The LSM hook seems separable, and
> > > could continue on its own. Thoughts?
> > >
> > Agreed.
> >
> > > (Which tree should memfd change go through?)
> > >
> > I'm not sure, is there a recommendation ?
>
> It looks like it's traditionally through akpm's tree. Andrew, will you
> carry patches 1-5?
>
Hi Andrew, if you are taking this, V8 is the latest that contains patch 1-5.

Thanks
Jeff

> Thanks!
>
> --
> Kees Cook