[PATCH v2 08/10] LSM: SafeSetID: add read handler

[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>
> >
> > For debugging a running system, it is very helpful to be able to see what
> > policy the system is using. Add a read handler that can dump out a copy of
> > the loaded policy.
> >
> > 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 set: Instead of doing refcounting, change
> > policy_update_lock to a mutex and hold the mutex across the policy read.
> >  security/safesetid/lsm.h        |  1 +
> >  security/safesetid/securityfs.c | 35 +++++++++++++++++++++++++++++----
> >  2 files changed, 32 insertions(+), 4 deletions(-)
> >
> > diff --git a/security/safesetid/lsm.h b/security/safesetid/lsm.h
> > index 4a34f558d964..db6d16e6bbc3 100644
> > --- a/security/safesetid/lsm.h
> > +++ b/security/safesetid/lsm.h
> > @@ -41,6 +41,7 @@ struct setuid_rule {
> >
> >  struct setuid_ruleset {
> >         DECLARE_HASHTABLE(rules, SETID_HASH_BITS);
> > +       char *policy_str;
> >         struct rcu_head rcu;
> >  };
> >
> > diff --git a/security/safesetid/securityfs.c b/security/safesetid/securityfs.c
> > index 250d59e046c1..997b403c6255 100644
> > --- a/security/safesetid/securityfs.c
> > +++ b/security/safesetid/securityfs.c
> > @@ -19,7 +19,7 @@
> >
> >  #include "lsm.h"
> >
> > -static DEFINE_SPINLOCK(policy_update_lock);
> > +static DEFINE_MUTEX(policy_update_lock);
> >
> >  /*
> >   * In the case the input buffer contains one or more invalid UIDs, the kuid_t
> > @@ -67,6 +67,7 @@ static void __release_ruleset(struct rcu_head *rcu)
> >
> >         hash_for_each_safe(pol->rules, bucket, tmp, rule, next)
> >                 kfree(rule);
> > +       kfree(pol->policy_str);
> >         kfree(pol);
> >  }
> >
> > @@ -85,6 +86,7 @@ static ssize_t handle_policy_update(struct file *file,
> >         pol = kmalloc(sizeof(struct setuid_ruleset), GFP_KERNEL);
> >         if (!pol)
> >                 return -ENOMEM;
> > +       pol->policy_str = NULL;
> >         hash_init(pol->rules);
> >
> >         p = buf = memdup_user_nul(ubuf, len);
> > @@ -92,6 +94,11 @@ static ssize_t handle_policy_update(struct file *file,
> >                 err = PTR_ERR(buf);
> >                 goto out_free_pol;
> >         }
> > +       pol->policy_str = kstrdup(buf, GFP_KERNEL);
> > +       if (pol->policy_str == NULL) {
> > +               err = -ENOMEM;
> > +               goto out_free_buf;
> > +       }
> >
> >         /* policy lines, including the last one, end with \n */
> >         while (*p != '\0') {
> > @@ -135,10 +142,10 @@ static ssize_t handle_policy_update(struct file *file,
> >          * What we really want here is an xchg() wrapper for RCU, but since that
> >          * doesn't currently exist, just use a spinlock for now.
> >          */
> > -       spin_lock(&policy_update_lock);
> > +       mutex_lock(&policy_update_lock);
> >         rcu_swap_protected(safesetid_setuid_rules, pol,
> >                            lockdep_is_held(&policy_update_lock));
> > -       spin_unlock(&policy_update_lock);
> > +       mutex_unlock(&policy_update_lock);
> >         err = len;
> >
> >  out_free_buf:
> > @@ -162,7 +169,27 @@ static ssize_t safesetid_file_write(struct file *file,
> >         return handle_policy_update(file, buf, len);
> >  }
> >
> > +static ssize_t safesetid_file_read(struct file *file, char __user *buf,
> > +                                  size_t len, loff_t *ppos)
> > +{
> > +       ssize_t res = 0;
> > +       struct setuid_ruleset *pol;
> > +       const char *kbuf;
> > +
> > +       mutex_lock(&policy_update_lock);
> > +       pol = rcu_dereference_protected(safesetid_setuid_rules,
> > +                                       lockdep_is_held(&policy_update_lock));
> > +       if (pol) {
> > +               kbuf = pol->policy_str;
> > +               res = simple_read_from_buffer(buf, len, ppos,
> > +                                             kbuf, strlen(kbuf));
> > +       }
> > +       mutex_unlock(&policy_update_lock);
> > +       return res;
> > +}
> > +
> >  static const struct file_operations safesetid_file_fops = {
> > +       .read = safesetid_file_read,
> >         .write = safesetid_file_write,
> >  };
> >
> > @@ -181,7 +208,7 @@ static int __init safesetid_init_securityfs(void)
> >                 goto error;
> >         }
> >
> > -       policy_file = securityfs_create_file("whitelist_policy", 0200,
> > +       policy_file = securityfs_create_file("whitelist_policy", 0600,
> >                         policy_dir, NULL, &safesetid_file_fops);
> >         if (IS_ERR(policy_file)) {
> >                 ret = PTR_ERR(policy_file);
> > --
> > 2.21.0.392.gf8f6787159e-goog
>
>
>
> --
> Kees Cook