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

Kees Cook
2024-07-05T00:04:04Z

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.

[...]
> 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);

--
Kees Cook