[PATCH 02/10] LSM: SafeSetID: fix check for setresuid(new1, new2, new3)

[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>
>
> With the old code, when a process with the (real,effective,saved) UID set
> (1,1,1) calls setresuid(2,3,4), safesetid_task_fix_setuid() only checks
> whether the transition 1->2 is permitted; the transitions 1->3 and 1->4 are
> not checked. Fix this.
>
> This is also a good opportunity to refactor safesetid_task_fix_setuid() to
> be less verbose - having one branch per set*uid() syscall is unnecessary.
>
> Note that this slightly changes semantics: The UID transition check for
> UIDs that were not in the old cred struct is now always performed against
> the policy of the RUID. I think that's more consistent anyway, since the
> RUID is also the one that decides whether any policy is enforced at all.
>
> 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 | 125 +++++++++++----------------------------
>  1 file changed, 35 insertions(+), 90 deletions(-)
>
> diff --git a/security/safesetid/lsm.c b/security/safesetid/lsm.c
> index 2daecab3a4c0..5310fcf3052a 100644
> --- a/security/safesetid/lsm.c
> +++ b/security/safesetid/lsm.c
> @@ -99,20 +99,30 @@ static int safesetid_security_capable(const struct cred *cred,
>         return 0;
>  }
>
> -static int check_uid_transition(kuid_t parent, kuid_t child)
> +/*
> + * Check whether a caller with old credentials @old is allowed to switch to
> + * credentials that contain @new_uid.
> + */
> +static bool uid_permitted_for_cred(const struct cred *old, kuid_t new_uid)
>  {
> -       if (check_setuid_policy_hashtable_key_value(parent, child))
> -               return 0;
> -       pr_warn("UID transition (%d -> %d) blocked\n",
> -               __kuid_val(parent),
> -               __kuid_val(child));
> +       bool permitted;
> +
> +       /* If our old creds already had this UID in it, it's fine. */
> +       if (uid_eq(new_uid, old->uid) || uid_eq(new_uid, old->euid) ||
> +           uid_eq(new_uid, old->suid))
> +               return true;
> +
>         /*
> -        * Kill this process to avoid potential security vulnerabilities
> -        * that could arise from a missing whitelist entry preventing a
> -        * privileged process from dropping to a lesser-privileged one.
> +        * Transitions to new UIDs require a check against the policy of the old
> +        * RUID.
>          */
> -       force_sig(SIGKILL, current);
> -       return -EACCES;
> +       permitted = check_setuid_policy_hashtable_key_value(old->uid, new_uid);
> +       if (!permitted) {
> +               pr_warn("UID transition ((%d,%d,%d) -> %d) blocked\n",
> +                       __kuid_val(old->uid), __kuid_val(old->euid),
> +                       __kuid_val(old->suid), __kuid_val(new_uid));
> +       }
> +       return permitted;
>  }
>
>  /*
> @@ -125,88 +135,23 @@ static int safesetid_task_fix_setuid(struct cred *new,
>                                      int flags)
>  {
>
> -       /* Do nothing if there are no setuid restrictions for this UID. */
> +       /* Do nothing if there are no setuid restrictions for our old RUID. */
>         if (!check_setuid_policy_hashtable_key(old->uid))
>                 return 0;
>
> -       switch (flags) {
> -       case LSM_SETID_RE:
> -               /*
> -                * Users for which setuid restrictions exist can only set the
> -                * real UID to the real UID or the effective UID, unless an
> -                * explicit whitelist policy allows the transition.
> -                */
> -               if (!uid_eq(old->uid, new->uid) &&
> -                       !uid_eq(old->euid, new->uid)) {
> -                       return check_uid_transition(old->uid, new->uid);
> -               }
> -               /*
> -                * Users for which setuid restrictions exist can only set the
> -                * effective UID to the real UID, the effective UID, or the
> -                * saved set-UID, unless an explicit whitelist policy allows
> -                * the transition.
> -                */
> -               if (!uid_eq(old->uid, new->euid) &&
> -                       !uid_eq(old->euid, new->euid) &&
> -                       !uid_eq(old->suid, new->euid)) {
> -                       return check_uid_transition(old->euid, new->euid);
> -               }
> -               break;
> -       case LSM_SETID_ID:
> -               /*
> -                * Users for which setuid restrictions exist cannot change the
> -                * real UID or saved set-UID unless an explicit whitelist
> -                * policy allows the transition.
> -                */
> -               if (!uid_eq(old->uid, new->uid))
> -                       return check_uid_transition(old->uid, new->uid);
> -               if (!uid_eq(old->suid, new->suid))
> -                       return check_uid_transition(old->suid, new->suid);
> -               break;
> -       case LSM_SETID_RES:
> -               /*
> -                * Users for which setuid restrictions exist cannot change the
> -                * real UID, effective UID, or saved set-UID to anything but
> -                * one of: the current real UID, the current effective UID or
> -                * the current saved set-user-ID unless an explicit whitelist
> -                * policy allows the transition.
> -                */
> -               if (!uid_eq(new->uid, old->uid) &&
> -                       !uid_eq(new->uid, old->euid) &&
> -                       !uid_eq(new->uid, old->suid)) {
> -                       return check_uid_transition(old->uid, new->uid);
> -               }
> -               if (!uid_eq(new->euid, old->uid) &&
> -                       !uid_eq(new->euid, old->euid) &&
> -                       !uid_eq(new->euid, old->suid)) {
> -                       return check_uid_transition(old->euid, new->euid);
> -               }
> -               if (!uid_eq(new->suid, old->uid) &&
> -                       !uid_eq(new->suid, old->euid) &&
> -                       !uid_eq(new->suid, old->suid)) {
> -                       return check_uid_transition(old->suid, new->suid);
> -               }
> -               break;
> -       case LSM_SETID_FS:
> -               /*
> -                * Users for which setuid restrictions exist cannot change the
> -                * filesystem UID to anything but one of: the current real UID,
> -                * the current effective UID or the current saved set-UID
> -                * unless an explicit whitelist policy allows the transition.
> -                */
> -               if (!uid_eq(new->fsuid, old->uid)  &&
> -                       !uid_eq(new->fsuid, old->euid)  &&
> -                       !uid_eq(new->fsuid, old->suid) &&
> -                       !uid_eq(new->fsuid, old->fsuid)) {
> -                       return check_uid_transition(old->fsuid, new->fsuid);
> -               }
> -               break;
> -       default:
> -               pr_warn("Unknown setid state %d\n", flags);
> -               force_sig(SIGKILL, current);
> -               return -EINVAL;
> -       }
> -       return 0;
> +       if (uid_permitted_for_cred(old, new->uid) &&
> +           uid_permitted_for_cred(old, new->euid) &&
> +           uid_permitted_for_cred(old, new->suid) &&
> +           uid_permitted_for_cred(old, new->fsuid))
> +               return 0;
> +
> +       /*
> +        * Kill this process to avoid potential security vulnerabilities
> +        * that could arise from a missing whitelist entry preventing a
> +        * privileged process from dropping to a lesser-privileged one.
> +        */
> +       force_sig(SIGKILL, current);
> +       return -EACCES;
>  }
>
>  int add_safesetid_whitelist_entry(kuid_t parent, kuid_t child)
> --
> 2.21.0.392.gf8f6787159e-goog
>


-- 
Kees Cook