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. > > > - 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?
