[1/2] kernel: Add noaudit variant of ns_capable()

When checking the current cred for a capability in a specific user
namespace, it isn't always desirable to have the LSMs audit the check.
This patch adds a noaudit variant of ns_capable() for when those
situations arise.

The common logic between ns_capable() and the new ns_capable_noaudit()
is moved into a single, shared function to keep duplicated code to a
minimum and ease maintainability.

Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
---
 include/linux/capability.h |  5 +++++
 kernel/capability.c        | 46 ++++++++++++++++++++++++++++++++++++----------
 2 files changed, 41 insertions(+), 10 deletions(-)

Quoting Tyler Hicks (tyhicks@canonical.com):
> When checking the current cred for a capability in a specific user
> namespace, it isn't always desirable to have the LSMs audit the check.
> This patch adds a noaudit variant of ns_capable() for when those
> situations arise.
> 
> The common logic between ns_capable() and the new ns_capable_noaudit()
> is moved into a single, shared function to keep duplicated code to a
> minimum and ease maintainability.
> 
> Signed-off-by: Tyler Hicks <tyhicks@canonical.com>

Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>

> ---
>  include/linux/capability.h |  5 +++++
>  kernel/capability.c        | 46 ++++++++++++++++++++++++++++++++++++----------
>  2 files changed, 41 insertions(+), 10 deletions(-)
> 
> diff --git a/include/linux/capability.h b/include/linux/capability.h
> index 00690ff..5f3c63d 100644
> --- a/include/linux/capability.h
> +++ b/include/linux/capability.h
> @@ -206,6 +206,7 @@ extern bool has_ns_capability_noaudit(struct task_struct *t,
>  				      struct user_namespace *ns, int cap);
>  extern bool capable(int cap);
>  extern bool ns_capable(struct user_namespace *ns, int cap);
> +extern bool ns_capable_noaudit(struct user_namespace *ns, int cap);
>  #else
>  static inline bool has_capability(struct task_struct *t, int cap)
>  {
> @@ -233,6 +234,10 @@ static inline bool ns_capable(struct user_namespace *ns, int cap)
>  {
>  	return true;
>  }
> +static inline bool ns_capable_noaudit(struct user_namespace *ns, int cap)
> +{
> +	return true;
> +}
>  #endif /* CONFIG_MULTIUSER */
>  extern bool capable_wrt_inode_uidgid(const struct inode *inode, int cap);
>  extern bool file_ns_capable(const struct file *file, struct user_namespace *ns, int cap);
> diff --git a/kernel/capability.c b/kernel/capability.c
> index 45432b5..00411c8 100644
> --- a/kernel/capability.c
> +++ b/kernel/capability.c
> @@ -361,6 +361,24 @@ bool has_capability_noaudit(struct task_struct *t, int cap)
>  	return has_ns_capability_noaudit(t, &init_user_ns, cap);
>  }
>  
> +static bool ns_capable_common(struct user_namespace *ns, int cap, bool audit)
> +{
> +	int capable;
> +
> +	if (unlikely(!cap_valid(cap))) {
> +		pr_crit("capable() called with invalid cap=%u\n", cap);
> +		BUG();
> +	}
> +
> +	capable = audit ? security_capable(current_cred(), ns, cap) :
> +			  security_capable_noaudit(current_cred(), ns, cap);
> +	if (capable == 0) {
> +		current->flags |= PF_SUPERPRIV;
> +		return true;
> +	}
> +	return false;
> +}
> +
>  /**
>   * ns_capable - Determine if the current task has a superior capability in effect
>   * @ns:  The usernamespace we want the capability in
> @@ -374,19 +392,27 @@ bool has_capability_noaudit(struct task_struct *t, int cap)
>   */
>  bool ns_capable(struct user_namespace *ns, int cap)
>  {
> -	if (unlikely(!cap_valid(cap))) {
> -		pr_crit("capable() called with invalid cap=%u\n", cap);
> -		BUG();
> -	}
> -
> -	if (security_capable(current_cred(), ns, cap) == 0) {
> -		current->flags |= PF_SUPERPRIV;
> -		return true;
> -	}
> -	return false;
> +	return ns_capable_common(ns, cap, true);
>  }
>  EXPORT_SYMBOL(ns_capable);
>  
> +/**
> + * ns_capable_noaudit - Determine if the current task has a superior capability
> + * (unaudited) in effect
> + * @ns:  The usernamespace we want the capability in
> + * @cap: The capability to be tested for
> + *
> + * Return true if the current task has the given superior capability currently
> + * available for use, false if not.
> + *
> + * This sets PF_SUPERPRIV on the task if the capability is available on the
> + * assumption that it's about to be used.
> + */
> +bool ns_capable_noaudit(struct user_namespace *ns, int cap)
> +{
> +	return ns_capable_common(ns, cap, false);
> +}
> +EXPORT_SYMBOL(ns_capable_noaudit);
>  
>  /**
>   * capable - Determine if the current task has a superior capability in effect
> -- 
> 2.7.4
>