On Tue, Jul 16, 2024 at 08:02:37AM -0700, Jeff Xu wrote: > On Thu, Jul 11, 2024 at 1:57 AM Mickaël Salaün <mic@xxxxxxxxxxx> wrote: > > > > On Wed, Jul 10, 2024 at 09:26:14AM -0700, Kees Cook wrote: > > > On Wed, Jul 10, 2024 at 11:58:25AM +0200, Mickaël Salaün wrote: > > > > Here is another proposal: > > > > > > > > We can change a bit the semantic by making it the norm to always check > > > > file executability with AT_CHECK, and using the securebits to restrict > > > > file interpretation and/or command injection (e.g. user supplied shell > > > > commands). Non-executable checked files can be reported/logged at the > > > > kernel level, with audit, configured by sysadmins. > > > > > > > > New securebits (feel free to propose better names): > > > > > > > > - SECBIT_EXEC_RESTRICT_FILE: requires AT_CHECK to pass. > > > > > > Would you want the enforcement of this bit done by userspace or the > > > kernel? > > > > > > IIUC, userspace would always perform AT_CHECK regardless of > > > SECBIT_EXEC_RESTRICT_FILE, and then which would happen? > > > > > > 1) userspace would ignore errors from AT_CHECK when > > > SECBIT_EXEC_RESTRICT_FILE is unset > > > > Yes, that's the idea. > > > > > > > > or > > > > > > 2) kernel would allow all AT_CHECK when SECBIT_EXEC_RESTRICT_FILE is > > > unset > > > > > > I suspect 1 is best and what you intend, given that > > > SECBIT_EXEC_DENY_INTERACTIVE can only be enforced by userspace. > > > > Indeed. We don't want AT_CHECK's behavior to change according to > > securebits. > > > One bit is good. > > > > > > > > - SECBIT_EXEC_DENY_INTERACTIVE: deny any command injection via > > > > command line arguments, environment variables, or configuration files. > > > > This should be ignored by dynamic linkers. We could also have an > > > > allow-list of shells for which this bit is not set, managed by an > > > > LSM's policy, if the native securebits scoping approach is not enough. > > > > > > > > Different modes for script interpreters: > > > > > > > > 1. RESTRICT_FILE=0 DENY_INTERACTIVE=0 (default) > > > > Always interpret scripts, and allow arbitrary user commands. > > > > => No threat, everyone and everything is trusted, but we can get > > > > ahead of potential issues with logs to prepare for a migration to a > > > > restrictive mode. > > > > > > > > 2. RESTRICT_FILE=1 DENY_INTERACTIVE=0 > > > > Deny script interpretation if they are not executable, and allow > > > > arbitrary user commands. > > > > => Threat: (potential) malicious scripts run by trusted (and not > > > > fooled) users. That could protect against unintended script > > > > executions (e.g. sh /tmp/*.sh). > > > > ==> Makes sense for (semi-restricted) user sessions. > > > > > > > > 3. RESTRICT_FILE=1 DENY_INTERACTIVE=1 > > > > Deny script interpretation if they are not executable, and also deny > > > > any arbitrary user commands. > > > > => Threat: malicious scripts run by untrusted users. > > > > ==> Makes sense for system services executing scripts. > > > > > > > > 4. RESTRICT_FILE=0 DENY_INTERACTIVE=1 > > > > Always interpret scripts, but deny arbitrary user commands. > > > > => Goal: monitor/measure/assess script content (e.g. with IMA/EVM) in > > > > a system where the access rights are not (yet) ready. Arbitrary > > > > user commands would be much more difficult to monitor. > > > > ==> First step of restricting system services that should not > > > > directly pass arbitrary commands to shells. > > > > > > I like these bits! > > > > Good! Jeff, Steve, Florian, Matt, others, what do you think? > > For below two cases: will they be restricted by one (or some) mode above ? > > 1> cat /tmp/a.sh | sh > > 2> sh -c "$(cat /tmp/a.sh)" Yes, DENY_INTERACTIVE=1 is to deny both of these cases (i.e. arbitrary user command). These other examples should be allowed with AT_CHECK and RESTRICT_FILE=1 if a.sh is executable though: * sh /tmp/a.sh * sh < /tmp/a.sh
