Re: [RFC PATCH v19 1/5] exec: Add a new AT_CHECK flag to execveat(2)

Kees Cook
2024-07-08T19:38:20Z

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