[PATCH 04/10] LSM: SafeSetID: refactor safesetid_security_capable()

[Kees Cook]

On Wed, Apr 10, 2019 at 9:55 AM Micah Morton <mortonm at chromium.org> wrote:
>
> From: Jann Horn <jannh at google.com>
>
> At the moment, safesetid_security_capable() has two nested conditional
> blocks, and one big comment for all the logic. Chop it up and reduce the
> amount of indentation.
>
> Signed-off-by: Jann Horn <jannh at google.com>
> Signed-off-by: Micah Morton <mortonm at chromium.org>

Reviewed-by: Kees Cook <keescook at chromium.org>

-Kees

> ---
>  security/safesetid/lsm.c | 41 +++++++++++++++++++++++++---------------
>  1 file changed, 26 insertions(+), 15 deletions(-)
>
> diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
> index 15cd13b5a211..ab429e1816c5 100644
> --- a/security/safesetid/lsm.c
> +++ b/security/safesetid/lsm.c
> @@ -55,21 +55,32 @@ static int safesetid_security_capable(const struct cred *cred,
>                                       int cap,
>                                       unsigned int opts)
>  {
> -       if (cap == CAP_SETUID &&
> -           setuid_policy_lookup(cred->uid, INVALID_UID) != SIDPOL_DEFAULT) {
> -               if (!(opts & CAP_OPT_INSETID)) {
> -                       /*
> -                        * Deny if we're not in a set*uid() syscall to avoid
> -                        * giving powers gated by CAP_SETUID that are related
> -                        * to functionality other than calling set*uid() (e.g.
> -                        * allowing user to set up userns uid mappings).
> -                        */
> -                       pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
> -                               __kuid_val(cred->uid));
> -                       return -1;
> -               }
> -       }
> -       return 0;
> +       /* We're only interested in CAP_SETUID. */
> +       if (cap != CAP_SETUID)
> +               return 0;
> +
> +       /*
> +        * If CAP_SETUID is currently used for a set*uid() syscall, we want to
> +        * let it go through here; the real security check happens later, in the
> +        * task_fix_setuid hook.
> +        */
> +       if ((opts & CAP_OPT_INSETID) != 0)
> +               return 0;
> +
> +       /*
> +        * If no policy applies to this task, allow the use of CAP_SETUID for
> +        * other purposes.
> +        */
> +       if (setuid_policy_lookup(cred->uid, INVALID_UID) == SIDPOL_DEFAULT)
> +               return 0;
> +
> +       /*
> +        * Reject use of CAP_SETUID for functionality other than calling
> +        * set*uid() (e.g. setting up userns uid mappings).
> +        */
> +       pr_warn("Operation requires CAP_SETUID, which is not available to UID %u for operations besides approved set*uid transitions\n",
> +               __kuid_val(cred->uid));
> +       return -1;
>  }
>
>  /*
> --
> 2.21.0.392.gf8f6787159e-goog
>


-- 
Kees Cook