Re: [RFC v4 0/2] WhiteEgret LSM module

[Steve Kemp]

This is an interesting idea, and an evolution since the initial
approach which was submitted based upon xattr attributes.  I still
find the idea of using attributes simpler to manage though, since
they're easy to add, and audit for.

I suspect the biggest objection to this module is that maintaining a
whitelist at all is often impractical.

My (trivial/toy/silly) module went the attribute-reading way:

https://github.com/skx/linux-security-modules/tree/master/security/whitelist

For a completely crazy take upon the idea of userspace decisions
though you can laugh at my attempt here:

https://github.com/skx/linux-security-modules/tree/master/security/can-exec

I callback to userspace for every decision, via
call_usermodehelper_setup.  The crazy part is that it actually works
at all!

Steve
On Fri, Oct 19, 2018 at 8:37 AM Shinya Takumi
<shinya1.takumi@xxxxxxxxxxxxx> wrote:
>
> WhiteEgret is an LSM to simply provide a whitelisting-type
> execution control.
>
> An execution-whitelist, simply called whitelist, is a list
> of executable components (e.g., applications, libraries)
> that are approved to run on a host. The whitelist is used
> to decide whether executable components are permitted to
> execute or not. This mechanism can stop an execution of
> unknown software, so it helps stop the execution of
> malicious code and other unauthorized software.
>
> It is important to maintain a whitelist properly according to
> the execution environments. Managing whitelists for information
> systems is a difficult task because their environments are
> changed frequently. On the other hand, for such devices that
> continue to do the same tasks for a certain period of time,
> we can use the same whitelist for the period once the whitelist
> is established. Thus the whitelisting-type execution control
> works best in such execution environments. Examples of the above
> execution environments include control devices in industrial
> control systems.
>
> Although the number of changing whitelists is not so large,
> it is necessary to change them according to a system life cycle
> or each phase of system operations. There is a requirement to
> change whitelists with the system operations continued because
> they often cannot easily be stopped. For example, such cases
> include temporarily allowing maintenance programs for maintenance
> or troubleshooting purposes while running the systems.
>
> WhiteEgret is aiming at satisfying the above requirement.
> WhiteEgret adopts a model that a whitelist is managed in user space.
> Namely, WhiteEgret assumes that a privileged user manages a whitelist
> in user space. This makes it possible to change the whitelist while
> running the systems.
>
> Mechanism of WhiteEgret
>
> WhiteEgret requires a user application called WhiteEgret User
> Application (WEUA, for short). WhiteEgret utilizes the
> bprm_check_security hook and the mmap_file hook.
> WhiteEgret asks WEUA whether an executable component hooked
> by the above hooks is permitted to execute or not.
> If the response from the WEUA is "permit", then WhiteEgret
> continues to process the executable component. If the response
> is "not permit", then WhiteEgret returns an error and blocks
> the execution of the executable component.
> The bprm_check_security hook is triggered by execve system
> call, so execution by almost all executable components are
> hooked by the hook. However, because shared objects do not
> invoke execve system call, WhiteEgret utilizes the mmap_file
> hook to hook the memory mapping by a shared object.
> Thus WhiteEgret ignores the mmap_file hook caused by
> non-executable and by executable which calls execve system call.
>
> To ask the permission to a WEUA, WhiteEgret sends the
> absolute path, the inode number and the device number of the
> executable component to the WEUA.
> Then the WEUA is expected to work as follows.
> The WEUA sees if the tuple of the absolute path and/or the inode
> number and/or the device number is contained in the whitelist.
> If it exists, the WEUA compares a hash value of the executable
> component indicated by the absolute path (and/or the inode number
> and/or device number) with that in the whitelist to see whether
> the executable component is changed or not after the whitelist is
> made. The WEUA returns "permit" if both tests are passed,
> otherwise returns "not permit".
>
> WhiteEgret v4 is also able to control for script execution. Some
> LSM hooks (file_open/file_permission/task_alloc/task_free) are
> added. Kernel configs are required to enable the hooks.
>
> Most of interpreters open script files to execute. Therefore
> WhiteEgret hooks for reading or opening a file. Then WhiteEgret
> asks the WEUA whether an execution of the script is permitted to
> execute or not. WhiteEgret is able to choose a hook entry for
> execution control between file_open or file_permission.
>
> Hook entries of task_alloc and task_free are used to control
> exections of script effectively. Some interpreters forks child
> processes to execte script files, so the WEUA managed a process
> family of an interpter by bprm_check_security, task_alloc and
> task_free.
>
> To use WhiteEgret
>
> Users have to prepare a whitelist and a WEUA to use WhiteEgret.
> A sample WEUA is involved in samples/whiteegret/.
>
> To enable WhiteEgret, you are required to build the kernel using
> normal procedures with CONFIG_SECURITY_WHITEEGRET=y.
>
> Additionally, SECURITY_WHITEEGRET_INTERPRETER=y option is
> required to enable to control script executions.
> And SECURITY_WHITEEGRET_WRITE=y option is also required to
> detect of writing script files.
>
> The patchset is also available in our github repo:
>   https://github.com/whiteegret/whiteegret
>
> ---
> Changes in v4:
> - Add LSM hooks (file_open/file_permission/task_alloc/task_free)
>   to control for script execution. Kernel configs are required
>     to enable the hooks.
>     - Modify the data struct for kernel space and user space
>       communication.
>
> Changes in v3:
> - Change to a minor LSM module.
>
> Changes in v2:
> - Change communication method between kernel space and user space
> from netlink to device driver, and device driver utilizes not LKM
> but securityfs.
> - Add inode number and device number to information of executable
> component sent from kernel space to user space.
> - Fix bugs regarding to locks during kernel space and user space
> communication.
> - Change return value from -EPERM to -EACCES when the execution of
> an executable component is denied.
>
> Shinya Takumi (2):
>   WhiteEgret: Add WhiteEgret core functions.
>   WhiteEgret: Add an example of user application.
>
>  samples/Kconfig                    |   6 +
>  samples/Makefile                   |   2 +-
>  samples/whiteegret/Makefile        |  14 ++
>  samples/whiteegret/checkwl.c       | 215 +++++++++++++++++
>  samples/whiteegret/checkwl.h       |  33 +++
>  samples/whiteegret/main.c          | 119 ++++++++++
>  security/Kconfig                   |   1 +
>  security/Makefile                  |   2 +
>  security/whiteegret/Kconfig        |  82 +++++++
>  security/whiteegret/Makefile       |   2 +
>  security/whiteegret/init.c         | 148 ++++++++++++
>  security/whiteegret/main.c         | 466 +++++++++++++++++++++++++++++++++++++
>  security/whiteegret/request.c      |  98 ++++++++
>  security/whiteegret/request.h      |  47 ++++
>  security/whiteegret/we.h           |  72 ++++++
>  security/whiteegret/we_fs.c        | 269 +++++++++++++++++++++
>  security/whiteegret/we_fs.h        |  23 ++
>  security/whiteegret/we_fs_common.h |  60 +++++
>  18 files changed, 1658 insertions(+), 1 deletion(-)
>  create mode 100644 samples/whiteegret/Makefile
>  create mode 100644 samples/whiteegret/checkwl.c
>  create mode 100644 samples/whiteegret/checkwl.h
>  create mode 100644 samples/whiteegret/main.c
>  create mode 100644 security/whiteegret/Kconfig
>  create mode 100644 security/whiteegret/Makefile
>  create mode 100644 security/whiteegret/init.c
>  create mode 100644 security/whiteegret/main.c
>  create mode 100644 security/whiteegret/request.c
>  create mode 100644 security/whiteegret/request.h
>  create mode 100644 security/whiteegret/we.h
>  create mode 100644 security/whiteegret/we_fs.c
>  create mode 100644 security/whiteegret/we_fs.h
>  create mode 100644 security/whiteegret/we_fs_common.h
>
> --
> 2.7.4
>