[Overview] Fileless malware attacks are becoming more and more popular, and even ready-to-use frameworks are available [1], [2], [3]. They are based on running of the malware code from anonymous executable memory pages (which are not backed by an executable file or a library on a filesystem.) This allows effectively hiding malware presence in a system, making filesystem integrity checking tools unable to detect the intrusion. Typically, the malware first needs to intercept the execution flow (e.g., by the means of ROP-based exploit). Then it needs to download the main part (in the form of normal executable or library) from its server, because it is hard to implement the entire exploit in ROP-based form. There are a number of security mechanisms that can ensure the integrity of the file-system, but we need to ensure the integrity of the code in memory too, to be sure, that only authorized code is running in the system. The proposed LSM is preventing the creation of anonymous executable pages for the processes. The LSM intercepts mmap() and mprotect() system calls and handles it similarly to SELinux handlers. The module allows to block the violating system call or to kill the violating process, depending on the settings, along with rate-limited logging. Currently, the module restricts ether all processes or only the privileged processes, depending on the settings. The privileged process is a process for which any of the following is true: + uid == 0 && !issecure(SECURE_NOROOT) + euid == 0 && !issecure(SECURE_NOROOT) + suid == 0 && !issecure(SECURE_NOROOT) + cap_effective has any capability except of kernel.nax.allowed_caps + cap_permitted has any capability except of kernel.nax.allowed_caps Checking of uid/euid/suid is important because a process may call seteuid(0) to gain privileges (if SECURE_NO_SETUID_FIXUP secure bit is not set). The sysctl parameter kernel.nax.allowed_caps allows to define safe capabilities set for the privileged processes. [JIT] Because of blocked anonymous code execution, JIT-compiled code, some interpreters (which are using JIT) and libffi-based projects can be broken. Our observation shows that such processes are typically running by a user, so they will not be privileged, so they will be allowed to use anonymous executable pages. But for small embedded set-ups it could be possible to get rid of such processes at all, so the module could be enabled without further restrictions to protect both privileged and non-privileged processes. In addition, libffi can be modified not to use anonymous executable pages. [Similar implementations] Although SELinux could be used to enable similar functionality, this LSM is simpler. It could be used in set-ups, where SELinux would be overkill. There is also SARA LSM module, which solves similar task, but it is more complex. [Cooperation with other security mechanisms] NAX LSM is more useful in conjunction with IMA. IMA would be responsible for integrity checking of file-based executables and libraries, and NAX LSM would be responsible for preventing of anonymous code execution. Alternatively, NAX LSM can be used with read-only root file system, protected by dm-verity/fs-verity. [TODO] - Implement xattrs support for marking privileged binaries on a per-file basis. - Store NAX attributes in the per-task LSM blob to implement special launchers for the privileged processes, so all of the children processes of such a launcher would be allowed to have anonymous executable pages (but not to grandchildren). [Links] [1] https://blog.fbkcs.ru/elf-in-memory-execution/ [2] https://magisterquis.github.io/2018/03/31/in-memory-only-elf-execution.html [3] https://www.prodefence.org/fireelf-fileless-linux-malware-framework/ [Credits] Thanks to Mimi Zohar for consulting and to Simon Thoby for thorough review. [Changelog] V3 - Fix memory leak in allowed_caps assigning code. - Protect allowed_caps updating with a spinlock. - Fix Kconfig options description. - Add example for allowed_caps value. - Fix typo in documentation. V2 - Fixed typo in Kconfig. - Fixed "cap_effective" and "cap_permitted" parameters description in NAX.rst. - Added "nax_allowed_caps" setup parameter. Factored out capabilities parsing logic. - Added parameter for checking all processes (not only privileged). - Added Kconfig parameter for setting allowed capabilities. - Updated nax_file_mprotect() to avoid calling of nax_mmap_file() to avoid duplicated checks. - Protect allowed_caps with RCU. - Fixed all errors and most warning found by checkpatch.pl. - Updated the module documentation. Added description of the boot parameters to kernel-parameters. - Updated commit message. V1: - Initial implementation. Igor Zhbanov (1): NAX LSM: Add initial support Documentation/admin-guide/LSM/NAX.rst | 72 +++ Documentation/admin-guide/LSM/index.rst | 1 + .../admin-guide/kernel-parameters.rst | 1 + .../admin-guide/kernel-parameters.txt | 32 ++ security/Kconfig | 11 +- security/Makefile | 2 + security/nax/Kconfig | 114 +++++ security/nax/Makefile | 4 + security/nax/nax-lsm.c | 472 ++++++++++++++++++ 9 files changed, 704 insertions(+), 5 deletions(-) create mode 100644 Documentation/admin-guide/LSM/NAX.rst create mode 100644 security/nax/Kconfig create mode 100644 security/nax/Makefile create mode 100644 security/nax/nax-lsm.c -- 2.26.2
