On 2020-05-06, Lev R. Oshvang . <levonshe@xxxxxxxxx> wrote: > On Tue, May 5, 2020 at 6:36 PM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > > > On 05/05/2020 17:31, Mickaël Salaün wrote: > > > Hi, > > > > > > This fifth patch series add new kernel configurations (OMAYEXEC_STATIC, > > > OMAYEXEC_ENFORCE_MOUNT, and OMAYEXEC_ENFORCE_FILE) to enable to > > > configure the security policy at kernel build time. As requested by > > > Mimi Zohar, I completed the series with one of her patches for IMA. > > > > > > The goal of this patch series is to enable to control script execution > > > with interpreters help. A new O_MAYEXEC flag, usable through > > > openat2(2), is added to enable userspace script interpreter 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_OPENEXEC flag closes 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 openat2(2) [2], SGX integration > > > [3], bpffs [4] or IPE [5]. > > > > > > Userspace 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 > > > withou -c, stdin piping of code) are on their way. > > > > > > The initial idea come from CLIP OS 4 and the original implementation has > > > been used for more than 12 years: > > > https://github.com/clipos-archive/clipos4_doc > > > > > > 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 > > > > > > This patch series can be applied on top of v5.7-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/20200428175129.634352-1-mic@xxxxxxxxxxx/ > > > > The previous version (v4) is > > https://lore.kernel.org/lkml/20200430132320.699508-1-mic@xxxxxxxxxxx/ > > > Hi Michael > > I have couple of question > 1. Why you did not add O_MAYEXEC to open()? > Some time ago (around v4.14) open() did not return EINVAL when > VALID_OPEN_FLAGS check failed. > Now it does, so I do not see a problem that interpreter will use > simple open(), ( Although that path might be manipulated, but file > contents will be verified by IMA) You don't get -EINVAL from open() in the case of unknown flags, that's something only openat2() does in the open*() family. Hence why it's only introduced for openat2(). > 2. When you apply a new flag to mount, it means that IMA will check > all files under this mount and it does not matter whether the file in > question is a script or not. > IMHO it is too hard overhead for performance reasons. > > Regards, > LEv -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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