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

Mickaël Salaün
2024-07-05T18:02:06Z

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?

>
> [...]
> > diff --git a/fs/exec.c b/fs/exec.c
> > index 40073142288f..ea2a1867afdc 100644
> > --- a/fs/exec.c
> > +++ b/fs/exec.c
> > @@ -931,7 +931,7 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
> > .lookup_flags = LOOKUP_FOLLOW,
> > };
> >
> > - if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
> > + if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | AT_CHECK)) != 0)
> > return ERR_PTR(-EINVAL);
> > if (flags & AT_SYMLINK_NOFOLLOW)
> > open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
> [...]
> > + * To avoid race conditions leading to time-of-check to time-of-use issues,
> > + * AT_CHECK should be used with AT_EMPTY_PATH to check against a file
> > + * descriptor instead of a path.
>
> I want this enforced by the kernel. Let's not leave trivial ToCToU
> foot-guns around. i.e.:
>
> if ((flags & AT_CHECK) == AT_CHECK && (flags & AT_EMPTY_PATH) == 0)
> return ERR_PTR(-EBADF);

There are valid use cases relying on pathnames. See Linus's comment: