On Fri, Jul 05, 2024 at 07:53:10PM +0200, Mickaël Salaün wrote: > On Thu, Jul 04, 2024 at 05:04:03PM -0700, Kees Cook wrote: > > On Thu, Jul 04, 2024 at 09:01:33PM +0200, Mickaël Salaün wrote: > > > Add a new AT_CHECK flag to execveat(2) to check if a file would be > > > allowed for execution. The main use case is for script interpreters and > > > dynamic linkers to check execution permission according to the kernel's > > > security policy. Another use case is to add context to access logs e.g., > > > which script (instead of interpreter) accessed a file. As any > > > executable code, scripts could also use this check [1]. > > > > > > This is different than faccessat(2) which only checks file access > > > rights, but not the full context e.g. mount point's noexec, stack limit, > > > and all potential LSM extra checks (e.g. argv, envp, credentials). > > > Since the use of AT_CHECK follows the exact kernel semantic as for a > > > real execution, user space gets the same error codes. > > > > Nice! I much prefer this method of going through the exec machinery so > > we always have a single code path for these kinds of checks. > > > > > Because AT_CHECK is dedicated to user space interpreters, it doesn't > > > make sense for the kernel to parse the checked files, look for > > > interpreters known to the kernel (e.g. ELF, shebang), and return ENOEXEC > > > if the format is unknown. Because of that, security_bprm_check() is > > > never called when AT_CHECK is used. > > > > I'd like some additional comments in the code that reminds us that > > access control checks have finished past a certain point. > > Where in the code? Just before the bprm->is_check assignment? Yeah, that's what I was thinking. -- Kees Cook
