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

[Kees Cook]

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