linux-kernel - Re: [PATCH v2 2/8] exec: Factor security_bprm_creds_for_exec out of security_bprm_set

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Message-ID: <202005191108.7A6E97831@keescook>
Date:   Tue, 19 May 2020 11:10:12 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Oleg Nesterov <oleg@...hat.com>, Jann Horn <jannh@...gle.com>,
        Greg Ungerer <gerg@...ux-m68k.org>,
        Rob Landley <rob@...dley.net>,
        Bernd Edlinger <bernd.edlinger@...mail.de>,
        linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-security-module@...r.kernel.org,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v2 2/8] exec: Factor security_bprm_creds_for_exec out of
 security_bprm_set_creds

On Mon, May 18, 2020 at 07:30:10PM -0500, Eric W. Biederman wrote:
> 
> Today security_bprm_set_creds has several implementations:
> apparmor_bprm_set_creds, cap_bprm_set_creds, selinux_bprm_set_creds,
> smack_bprm_set_creds, and tomoyo_bprm_set_creds.
> 
> Except for cap_bprm_set_creds they all test bprm->called_set_creds and
> return immediately if it is true.  The function cap_bprm_set_creds
> ignores bprm->calld_sed_creds entirely.
> 
> Create a new LSM hook security_bprm_creds_for_exec that is called just
> before prepare_binprm in __do_execve_file, resulting in a LSM hook
> that is called exactly once for the entire of exec.  Modify the bits
> of security_bprm_set_creds that only want to be called once per exec
> into security_bprm_creds_for_exec, leaving only cap_bprm_set_creds
> behind.
> 
> Remove bprm->called_set_creds all of it's former users have been moved
> to security_bprm_creds_for_exec.
> 
> Add or upate comments a appropriate to bring them up to date and
> to reflect this change.

Yup, awesome. One nit below.

Reviewed-by: Kees Cook <keescook@...omium.org>

> [...]
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 0b4e32161b77..718345dd76bb 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> [...]
> @@ -2297,8 +2297,6 @@ static int selinux_bprm_set_creds(struct linux_binprm *bprm)
>  
>  	/* SELinux context only depends on initial program or script and not
>  	 * the script interpreter */
> -	if (bprm->called_set_creds)
> -		return 0;
>  
>  	old_tsec = selinux_cred(current_cred());
>  	new_tsec = selinux_cred(bprm->cred);

As you've done in the other LSMs, I think this comment can be removed
(or moved to the top of the function) too.

-- 
Kees Cook