mbox series

[v6,0/6] mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC

Message ID 20221207154939.2532830-1-jeffxu@google.com
Headers
Series mm/memfd: introduce MFD_NOEXEC_SEAL and MFD_EXEC |

Message

Jeff Xu Dec. 7, 2022, 3:49 p.m. UTC 
  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.

This is V6 version of patch: see [4] [5] [6] [7] for previous versions.

[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/
[4] https://lwn.net/Articles/890096/
[5] https://lore.kernel.org/lkml/20220805222126.142525-1-jeffxu@google.com/
[6] https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
[7] https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/

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                        |  13 +
 tools/testing/selftests/memfd/fuse_test.c  |   1 +
 tools/testing/selftests/memfd/memfd_test.c | 348 ++++++++++++++++++++-
 13 files changed, 525 insertions(+), 3 deletions(-)
 create mode 100644 kernel/pid_sysctl.h


base-commit: eb7081409f94a9a8608593d0fb63a1aa3d6f95d8

Comments

Kees Cook Dec. 8, 2022, 4:13 p.m. UTC | #1 
On Wed, Dec 07, 2022 at 03:49:33PM +0000, jeffxu@chromium.org wrote:
> This is V6 version of patch: see [4] [5] [6] [7] for previous versions.

When sending a new version, can you include an overview of what changed
between this version and the prior version? This helps reviewers who are
following along, so it's easier to focus our attention on the
differences. Also, it's helpful to version the links:

> [4] https://lwn.net/Articles/890096/
> [5] https://lore.kernel.org/lkml/20220805222126.142525-1-jeffxu@google.com/
> [6] https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
> [7] https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/

e.g.:

v6:
- moved foo to bar
- improve comments for baz
v5: https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/
v3: https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
v2: ...etc

-Kees
Jeff Xu Dec. 8, 2022, 6:33 p.m. UTC | #2 
On Thu, Dec 8, 2022 at 8:13 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Wed, Dec 07, 2022 at 03:49:33PM +0000, jeffxu@chromium.org wrote:
> > This is V6 version of patch: see [4] [5] [6] [7] for previous versions.
>
> When sending a new version, can you include an overview of what changed
> between this version and the prior version? This helps reviewers who are
> following along, so it's easier to focus our attention on the
> differences. Also, it's helpful to version the links:
>
> > [4] https://lwn.net/Articles/890096/
> > [5] https://lore.kernel.org/lkml/20220805222126.142525-1-jeffxu@google.com/
> > [6] https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
> > [7] https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/
>
> e.g.:
>
> v6:
> - moved foo to bar
> - improve comments for baz
> v5: https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/
> v3: https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
> v2: ...etc
>
Will do!
Much appreciated for helping me  through the process of my first patch
in the kernel.

Jeff

> -Kees
>
> --
> Kees Cook
Kees Cook Dec. 8, 2022, 8:55 p.m. UTC | #3 
On Thu, Dec 08, 2022 at 10:33:19AM -0800, Jeff Xu wrote:
> On Thu, Dec 8, 2022 at 8:13 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Wed, Dec 07, 2022 at 03:49:33PM +0000, jeffxu@chromium.org wrote:
> > > This is V6 version of patch: see [4] [5] [6] [7] for previous versions.
> >
> > When sending a new version, can you include an overview of what changed
> > between this version and the prior version? This helps reviewers who are
> > following along, so it's easier to focus our attention on the
> > differences. Also, it's helpful to version the links:
> >
> > > [4] https://lwn.net/Articles/890096/
> > > [5] https://lore.kernel.org/lkml/20220805222126.142525-1-jeffxu@google.com/
> > > [6] https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
> > > [7] https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/
> >
> > e.g.:
> >
> > v6:
> > - moved foo to bar
> > - improve comments for baz
> > v5: https://lore.kernel.org/lkml/20221206152358.1966099-1-jeffxu@google.com/
> > v3: https://lore.kernel.org/lkml/20221202013404.163143-1-jeffxu@google.com/
> > v2: ...etc
> >
> Will do!
> Much appreciated for helping me  through the process of my first patch
> in the kernel.

Happy to help! I'm excited to see this gap in memfd security closed. :)