Re: [RFC PATCH v19 0/5] Script execution control (was O_MAYEXEC)

Roberto Sassu
2024-07-16T15:58:21Z

On Tue, 2024-07-09 at 22:43 +0200, Mickaël Salaün wrote:
> 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@mail.gmail.com
> > >
> > > 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.

Ok, I tried a trivial modification to have this working:

diff --git a/security/integrity/ima/ima_main.c b/security/integrity/ima/ima_main.c
index f04f43af651c..2a6b04c91601 100644
--- a/security/integrity/ima/ima_main.c
+++ b/security/integrity/ima/ima_main.c
@@ -554,6 +554,14 @@ static int ima_bprm_check(struct linux_binprm *bprm)
MAY_EXEC, CREDS_CHECK);
}

+static int ima_bprm_creds_for_exec(struct linux_binprm *bprm)
+{
+ if (!bprm->is_check)
+ return 0;
+
+ return ima_bprm_check(bprm);
+}
+
/**
* ima_file_check - based on policy, collect/store measurement.
* @file: pointer to the file to be measured
@@ -1177,6 +1185,7 @@ static int __init init_ima(void)

static struct security_hook_list ima_hooks[] __ro_after_init = {
LSM_HOOK_INIT(bprm_check_security, ima_bprm_check),
+ LSM_HOOK_INIT(bprm_creds_for_exec, ima_bprm_creds_for_exec),
LSM_HOOK_INIT(file_post_open, ima_file_check),
LSM_HOOK_INIT(inode_post_create_tmpfile, ima_post_create_tmpfile),
LSM_HOOK_INIT(file_release, ima_file_free),

I also adapted the Clip OS 4 patch for bash.

The result seems good so far:

# echo "measure fowner=2000 func=BPRM_CHECK" > /sys/kernel/security/ima/policy

# ./bash /root/test.sh
Hello World

# cat /sys/kernel/security/ima/ascii_runtime_measurements
10 35435d0858d895b90097306171a2e5fcc7f5da9e ima-ng sha256:0e4acf326a82c6bded9d86f48d272d7a036b6490081bb6466ecc2a0e416b244a boot_aggregate
10 4cd9df168a2cf8d18be46543e66c76a53ca6a03d ima-ng sha256:e7f3c2dab66f56fef963fbab55fc6d64bc22a5f900c29042e6ecd87e08f2b535 /root/test.sh

So, it is there.

It works only with +x permission. If not, I get:

# ./bash /root/test.sh
./bash: /root/test.sh: Permission denied

But the Clip OS 4 patch does not cover the redirection case:

# ./bash < /root/test.sh
Hello World

Do you have a more recent patch for that?

Thanks

Roberto

> >
> > 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@linux.ibm.com/
> > > [4] https://lssna2023.sched.com/event/1K7bO
> > >
> >
> >