[PATCH v2 09/10] LSM: SafeSetID: verify transitive constrainedness

[Micah Morton]

Ready for merge.

On Thu, Apr 11, 2019 at 1:38 PM Kees Cook <keescook at chromium.org> wrote:
>
> On Thu, Apr 11, 2019 at 1:12 PM Micah Morton <mortonm at chromium.org> wrote:
> >
> > From: Jann Horn <jannh at google.com>
> >
> > Someone might write a ruleset like the following, expecting that it
> > securely constrains UID 1 to UIDs 1, 2 and 3:
> >
> >     1:2
> >     1:3
> >
> > However, because no constraints are applied to UIDs 2 and 3, an attacker
> > with UID 1 can simply first switch to UID 2, then switch to any UID from
> > there. The secure way to write this ruleset would be:
> >
> >     1:2
> >     1:3
> >     2:2
> >     3:3
> >
> > , which uses "transition to self" as a way to inhibit the default-allow
> > policy without allowing anything specific.
> >
> > This is somewhat unintuitive. To make sure that policy authors don't
> > accidentally write insecure policies because of this, let the kernel verify
> > that a new ruleset does not contain any entries that are constrained, but
> > transitively unconstrained.
> >
> > 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
>
> > ---
> > Changes since the last patch: Instead of failing open when userspace
> > configures an unconstrained (and vulnerable) policy, fix up the policy
> > to make sure it is safe by restricting the un-constrained UIDs. Return
> > EINVAL from the policy write in the case that userspace writes an
> > unconstrained policy. Also move hash_add() into a small helper function.
> >  security/safesetid/securityfs.c               | 38 ++++++++++++++++++-
> >  .../selftests/safesetid/safesetid-test.c      |  4 +-
> >  2 files changed, 40 insertions(+), 2 deletions(-)
> >
> > diff --git a/security/safesetid/securityfs.c b/security/safesetid/securityfs.c
> > index 997b403c6255..d568e17dd773 100644
> > --- a/security/safesetid/securityfs.c
> > +++ b/security/safesetid/securityfs.c
> > @@ -76,6 +76,37 @@ static void release_ruleset(struct setuid_ruleset *pol)
> >         call_rcu(&pol->rcu, __release_ruleset);
> >  }
> >
> > +static void insert_rule(struct setuid_ruleset *pol, struct setuid_rule *rule)
> > +{
> > +       hash_add(pol->rules, &rule->next, __kuid_val(rule->src_uid));
> > +}
> > +
> > +static int verify_ruleset(struct setuid_ruleset *pol)
> > +{
> > +       int bucket;
> > +       struct setuid_rule *rule, *nrule;
> > +       int res = 0;
> > +
> > +       hash_for_each(pol->rules, bucket, rule, next) {
> > +               if (_setuid_policy_lookup(pol, rule->dst_uid, INVALID_UID) ==
> > +                   SIDPOL_DEFAULT) {
> > +                       pr_warn("insecure policy detected: uid %d is constrained but transitively unconstrained through uid %d\n",
> > +                               __kuid_val(rule->src_uid),
> > +                               __kuid_val(rule->dst_uid));
> > +                       res = -EINVAL;
> > +
> > +                       /* fix it up */
> > +                       nrule = kmalloc(sizeof(struct setuid_rule), GFP_KERNEL);
> > +                       if (!nrule)
> > +                               return -ENOMEM;
> > +                       nrule->src_uid = rule->dst_uid;
> > +                       nrule->dst_uid = rule->dst_uid;
> > +                       insert_rule(pol, nrule);
> > +               }
> > +       }
> > +       return res;
> > +}
> > +
> >  static ssize_t handle_policy_update(struct file *file,
> >                                     const char __user *ubuf, size_t len)
> >  {
> > @@ -128,7 +159,7 @@ static ssize_t handle_policy_update(struct file *file,
> >                         goto out_free_rule;
> >                 }
> >
> > -               hash_add(pol->rules, &rule->next, __kuid_val(rule->src_uid));
> > +               insert_rule(pol, rule);
> >                 p = end + 1;
> >                 continue;
> >
> > @@ -137,6 +168,11 @@ static ssize_t handle_policy_update(struct file *file,
> >                 goto out_free_buf;
> >         }
> >
> > +       err = verify_ruleset(pol);
> > +       /* bogus policy falls through after fixing it up */
> > +       if (err && err != -EINVAL)
> > +               goto out_free_buf;
> > +
> >         /*
> >          * Everything looks good, apply the policy and release the old one.
> >          * What we really want here is an xchg() wrapper for RCU, but since that
> > diff --git a/tools/testing/selftests/safesetid/safesetid-test.c b/tools/testing/selftests/safesetid/safesetid-test.c
> > index 4f03813d1911..8f40c6ecdad1 100644
> > --- a/tools/testing/selftests/safesetid/safesetid-test.c
> > +++ b/tools/testing/selftests/safesetid/safesetid-test.c
> > @@ -144,7 +144,9 @@ static void write_policies(void)
> >  {
> >         static char *policy_str =
> >                 "1:2\n"
> > -               "1:3\n";
> > +               "1:3\n"
> > +               "2:2\n"
> > +               "3:3\n";
> >         ssize_t written;
> >         int fd;
> >
> > --
> > 2.21.0.392.gf8f6787159e-goog
>
>
>
> --
> Kees Cook