Re: [PATCH] SMACK: Add missing rcu_read_lock/unlock for process capability walk.

[Casey Schaufler <casey@schaufler-ca.com>]

On 4/20/2011 3:00 PM, Andi Kleen wrote:
> From: Andi Kleen <ak@linux.intel.com>
>
> smk_access_entry does a RCU list walk for a list shared with other
> threads. It relies on the caller doing rcu_read_lock().
> One caller forgot to do to this, which could lead to races
> on preemptible kernels.
>
> Move the rcu_read_lock() into smk_access_entry instead.

Nacked-by: Casey Schaufler <casey@schaufler-ca.com>

The lock was moved out of smk_access_entry in support of the
processing done in the smack_mmap_file() hook. Where do you see
a potential race, and which caller "forgot" to do the lock?

Thank you.

> Signed-off-by: Andi Kleen <ak@linux.intel.com>
> ---
>  security/smack/smack_access.c |    4 ++--
>  1 files changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
> index 7b0d3b3..43b20f3 100644
> --- a/security/smack/smack_access.c
> +++ b/security/smack/smack_access.c
> @@ -92,6 +92,7 @@ int smk_access_entry(char *subject_label, char *object_label,
>  	int may = -ENOENT;
>  	struct smack_rule *srp;
>  
> +	rcu_read_lock();
>  	list_for_each_entry_rcu(srp, rule_list, list) {
>  		if (srp->smk_subject == subject_label ||
>  		    strcmp(srp->smk_subject, subject_label) == 0) {
> @@ -102,6 +103,7 @@ int smk_access_entry(char *subject_label, char *object_label,
>  			}
>  		}
>  	}
> +	rcu_read_unlock();
>  
>  	return may;
>  }
> @@ -184,9 +186,7 @@ int smk_access_flags(char *subject_label, char *object_label, int request,
>  	 * good. A negative response from smk_access_entry()
>  	 * indicates there is no entry for this pair.
>  	 */
> -	rcu_read_lock();
>  	may = smk_access_entry(subject_label, object_label, &smack_rule_list);
> -	rcu_read_unlock();
>  
>  	if (may > 0 && (request & may) == request)
>  		goto out_audit;