On Mon, Jul 08, 2024 at 04:35:38PM -0400, Mimi Zohar wrote: > Hi Mickaël, > > On Thu, 2024-07-04 at 21:01 +0200, Mickaël Salaün wrote: > > Hi, > > > > The ultimate goal of this patch series is to be able to ensure that > > direct file execution (e.g. ./script.sh) and indirect file execution > > (e.g. sh script.sh) lead to the same result, especially from a security > > point of view. > > > > Overview > > -------- > > > > This patch series is a new approach of the initial O_MAYEXEC feature, > > and a revamp of the previous patch series. Taking into account the last > > reviews [1], we now stick to the kernel semantic for file executability. > > One major change is the clear split between access check and policy > > management. > > > > The first patch brings the AT_CHECK flag to execveat(2). The goal is to > > enable user space to check if a file could be executed (by the kernel). > > Unlike stat(2) that only checks file permissions, execveat2(2) + > > AT_CHECK take into account the full context, including mount points > > (noexec), caller's limits, and all potential LSM extra checks (e.g. > > argv, envp, credentials). > > > > The second patch brings two new securebits used to set or get a security > > policy for a set of processes. For this to be meaningful, all > > executable code needs to be trusted. In practice, this means that > > (malicious) users can be restricted to only run scripts provided (and > > trusted) by the system. > > > > [1] https://lore.kernel.org/r/CAHk-=wjPGNLyzeBMWdQu+kUdQLHQugznwY7CvWjmvNW47D5sog@xxxxxxxxxxxxxx > > > > Script execution > > ---------------- > > > > One important thing to keep in mind is that the goal of this patch > > series is to get the same security restrictions with these commands: > > * ./script.py > > * python script.py > > * python < script.py > > * python -m script.pyT > > This is really needed, but is it the "only" purpose of this patch set or can it > be used to also monitor files the script opens (for read) with the intention of > executing. This feature can indeed also be used to monitor files requested by scripts to be executed e.g. using https://docs.python.org/3/library/io.html#io.open_code IMA/EVM can include this check in its logs. > > > > > However, on secure systems, we should be able to forbid these commands > > because there is no way to reliably identify the origin of the script: > > * xargs -a script.py -d '\r' -- python -c > > * cat script.py | python > > * python > > > > Background > > ---------- > > > > Compared to the previous patch series, there is no more dedicated > > syscall nor sysctl configuration. This new patch series only add new > > flags: one for execveat(2) and four for prctl(2). > > > > This kind of script interpreter restriction may already be used in > > hardened systems, which may need to fork interpreters and install > > different versions of the binaries. This mechanism should enable to > > avoid the use of duplicate binaries (and potential forked source code) > > for secure interpreters (e.g. secure Python [2]) by making it possible > > to dynamically enforce restrictions or not. > > > > The ability to control script execution is also required to close a > > major IMA measurement/appraisal interpreter integrity [3]. > > Definitely. But it isn't limited to controlling script execution, but also > measuring the script. Will it be possible to measure and appraise the indirect > script calls with this patch set? Yes. You should only need to implement security_bprm_creds_for_exec() for IMA/EVM. BTW, I noticed that IMA only uses the security_bprm_check() hook (which can be called several times for one execve), but security_bprm_creds_for_exec() might be more appropriate. > > Mimi > > > This new execveat + AT_CHECK should not be confused with the O_EXEC flag > > (for open) which is intended for execute-only, which obviously doesn't > > work for scripts. > > > > I gave a talk about controlling script execution where I explain the > > previous approaches [4]. The design of the WIP RFC I talked about > > changed quite a bit since then. > > > > [2] https://github.com/zooba/spython > > [3] https://lore.kernel.org/lkml/20211014130125.6991-1-zohar@xxxxxxxxxxxxx/ > > [4] https://lssna2023.sched.com/event/1K7bO > > > >
