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