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: v8: - Update ref bug in cover letter. - Add Reviewed-by field. - Remove security hook (security_memfd_create) patch, which will have its own patch set in future.
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/1305267 [2] https://bugs.chromium.org/p/chromium/issues/list?q=type%3Dbug-security%20mem... [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 (3): 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
include/linux/pid_namespace.h | 19 ++ 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 | 56 +++- mm/shmem.c | 6 + tools/testing/selftests/memfd/fuse_test.c | 1 + tools/testing/selftests/memfd/memfd_test.c | 341 ++++++++++++++++++++- 9 files changed, 489 insertions(+), 3 deletions(-) create mode 100644 kernel/pid_sysctl.h
base-commit: eb7081409f94a9a8608593d0fb63a1aa3d6f95d8