Hi, This ninth patch series rework the previous AT_INTERPRETED and O_MAYEXEC series with a new syscall: introspect_access(2) . Access check are now only possible on a file descriptor, which enable to avoid possible race conditions in user space. For now, the only LSM hook triggered by introspect_access(2) is inode_permission() which takes a struct inode as argument. However, struct path is still available in this syscall, which enables to add a new hook to fit the needs of IMA and other path-based LSMs. Goal of introspect_access(2) ============================ The goal of this patch series is to enable to control script execution with interpreters help. A new introspect_access() system call is added to enable user space script interpreters to delegate to the kernel (and thus the system security policy) the permission to interpret/execute scripts or other files containing what can be seen as commands. A simple system-wide security policy can be enforced by the system administrator through a sysctl configuration consistent with the mount points or the file access rights. The documentation patch explains the prerequisites. Furthermore, the security policy can also be delegated to an LSM, either a MAC system or an integrity system. For instance, the new kernel MAY_INTROSPECTION_EXEC flag is required to close a major IMA measurement/appraisal interpreter integrity gap by bringing the ability to check the use of scripts [1]. Other uses are expected, such as for magic-links [2], SGX integration [3], bpffs [4] or IPE [5]. Possible extended usage ======================= For now, only the X_OK mode is compatible with introspect_access(2). This enables to restrict the addition of new control flows in a process. Using R_OK or W_OK with introspect_access(2) returns -EINVAL. Possible future use-cases for R_OK with introspect_access(2) may be to check configuration files that may impact the behavior of applications (i.e. influence critical part of the current control flow). Those should then be trusted as well. The W_OK with introspect_access(2) could be used to check that a file descriptor is allowed to receive sensitive data such as debug logs. Prerequisite of its use ======================= User space needs to adapt to take advantage of this new feature. For example, the PEP 578 [6] (Runtime Audit Hooks) enables Python 3.8 to be extended with policy enforcement points related to code interpretation, which can be used to align with the PowerShell audit features. Additional Python security improvements (e.g. a limited interpreter without -c, stdin piping of code) are on their way [7]. Examples ======== The initial idea comes from CLIP OS 4 and the original implementation has been used for more than 12 years: https://github.com/clipos-archive/clipos4_doc Chrome OS has a similar approach: https://chromium.googlesource.com/chromiumos/docs/+/master/security/noexec_shell_scripts.md Userland patches can be found here: https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC Actually, there is more than the O_MAYEXEC changes (which matches this search) e.g., to prevent Python interactive execution. There are patches for Bash, Wine, Java (Icedtea), Busybox's ash, Perl and Python. There are also some related patches which do not directly rely on O_MAYEXEC but which restrict the use of browser plugins and extensions, which may be seen as scripts too: https://github.com/clipos-archive/clipos4_portage-overlay/tree/master/www-client An introduction to O_MAYEXEC was given at the Linux Security Summit Europe 2018 - Linux Kernel Security Contributions by ANSSI: https://www.youtube.com/watch?v=chNjCRtPKQY&t=17m15s The "write xor execute" principle was explained at Kernel Recipes 2018 - CLIP OS: a defense-in-depth OS: https://www.youtube.com/watch?v=PjRE0uBtkHU&t=11m14s See also an overview article: https://lwn.net/Articles/820000/ This patch series can be applied on top of v5.9-rc4 . This can be tested with CONFIG_SYSCTL. I would really appreciate constructive comments on this patch series. Previous version: https://lore.kernel.org/lkml/20200908075956.1069018-1-mic@xxxxxxxxxxx/ [1] https://lore.kernel.org/lkml/1544647356.4028.105.camel@xxxxxxxxxxxxx/ [2] https://lore.kernel.org/lkml/20190904201933.10736-6-cyphar@xxxxxxxxxx/ [3] https://lore.kernel.org/lkml/CALCETrVovr8XNZSroey7pHF46O=kj_c5D9K8h=z2T_cNrpvMig@xxxxxxxxxxxxxx/ [4] https://lore.kernel.org/lkml/CALCETrVeZ0eufFXwfhtaG_j+AdvbzEWE0M3wjXMWVEO7pj+xkw@xxxxxxxxxxxxxx/ [5] https://lore.kernel.org/lkml/20200406221439.1469862-12-deven.desai@xxxxxxxxxxxxxxxxxxx/ [6] https://www.python.org/dev/peps/pep-0578/ [7] https://lore.kernel.org/lkml/0c70debd-e79e-d514-06c6-4cd1e021fa8b@xxxxxxxxxx/ Regards, Mickaël Salaün (3): fs: Add introspect_access(2) syscall implementation and related sysctl arch: Wire up introspect_access(2) selftest/interpreter: Add tests for introspect_access(2) policies Documentation/admin-guide/sysctl/fs.rst | 50 +++ arch/alpha/kernel/syscalls/syscall.tbl | 1 + arch/arm/tools/syscall.tbl | 1 + arch/arm64/include/asm/unistd.h | 2 +- arch/arm64/include/asm/unistd32.h | 2 + arch/ia64/kernel/syscalls/syscall.tbl | 1 + arch/m68k/kernel/syscalls/syscall.tbl | 1 + arch/microblaze/kernel/syscalls/syscall.tbl | 1 + arch/mips/kernel/syscalls/syscall_n32.tbl | 1 + arch/mips/kernel/syscalls/syscall_n64.tbl | 1 + arch/mips/kernel/syscalls/syscall_o32.tbl | 1 + arch/parisc/kernel/syscalls/syscall.tbl | 1 + arch/powerpc/kernel/syscalls/syscall.tbl | 1 + arch/s390/kernel/syscalls/syscall.tbl | 1 + arch/sh/kernel/syscalls/syscall.tbl | 1 + arch/sparc/kernel/syscalls/syscall.tbl | 1 + arch/x86/entry/syscalls/syscall_32.tbl | 1 + arch/x86/entry/syscalls/syscall_64.tbl | 1 + arch/xtensa/kernel/syscalls/syscall.tbl | 1 + fs/open.c | 79 ++++ include/linux/fs.h | 3 + include/linux/syscalls.h | 1 + include/uapi/asm-generic/unistd.h | 4 +- kernel/sysctl.c | 12 +- .../testing/selftests/interpreter/.gitignore | 2 + tools/testing/selftests/interpreter/Makefile | 18 + tools/testing/selftests/interpreter/config | 1 + .../interpreter/introspection_policy_test.c | 361 ++++++++++++++++++ 28 files changed, 547 insertions(+), 4 deletions(-) create mode 100644 tools/testing/selftests/interpreter/.gitignore create mode 100644 tools/testing/selftests/interpreter/Makefile create mode 100644 tools/testing/selftests/interpreter/config create mode 100644 tools/testing/selftests/interpreter/introspection_policy_test.c -- 2.28.0
