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 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. > - 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! -- Kees Cook
