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Re: PATCH [1/1]: audit: acquire creds selectively to reduce atomic op overhead



On Thu, 2011-03-17 at 11:11 -0700, Tony Jones wrote:
> On Tue, Mar 15, 2011 at 08:11:17PM +0000, David Howells wrote:
> > Eric Paris <eparis redhat com> wrote:
> > 
> > > WARN_ON(cred != current->cred && cred->refcnt != 1)
> > 
> > 'tsk->parent == current' perhaps?  Or audit_alloc() could pass a flag
> > indicating the state, or just look to see if tsk->audit_context is still NULL.
> > 
> > David
> 
> Round 3.   tsk->parent == current isn't an option as it's not set by
> copy_process until after audit_alloc.  I used a flag to provide an explicit
> indication.  I didn't have audit_alloc pass the flag into audit_filter_task
> as there is already a explicit "process creation time" comment for this static
> function. If you want it pushed all the way upto audit_alloc, let me know and
> I'll revise.
> 
> Oddly sparse didn't throw any warnings about the direct use of tsk->cred.
> 
> tony
> ---
> 
> Commit c69e8d9c01db added calls to get_task_cred and put_cred in
> audit_filter_rules.  Profiling with a large number of audit rules active on the
> exit chain shows that we are spending upto 48% in this routine for syscall
> intensive tests, most of which is in the atomic ops.
> 
> 1. The code should be accessing tsk->cred rather than tsk->real_cred.  
> 2. Since tsk is current (or tsk is being created by copy_process) access to 
> tsk->cred without rcu read lock is possible.  At the request of the audit
> maintainer, a new flag has been added to audit_filter_rules in order to make 
> this explicit and guide future code.
> 
> Signed-off-by: Tony Jones <tonyj suse de>

Acked-by: Eric Paris <eparis redhat com>

> ---
>  kernel/auditsc.c |   27 +++++++++++++++++----------
>  1 files changed, 17 insertions(+), 10 deletions(-)
> 
> diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> index f49a031..281dcf1 100644
> --- a/kernel/auditsc.c
> +++ b/kernel/auditsc.c
> @@ -443,17 +443,25 @@ static int match_tree_refs(struct audit_context *ctx, struct audit_tree *tree)
>  
>  /* Determine if any context name data matches a rule's watch data */
>  /* Compare a task_struct with an audit_rule.  Return 1 on match, 0
> - * otherwise. */
> + * otherwise.
> + *
> + * If task_creation is true, this is an explicit indication that we are
> + * filtering a task rule at task creation time.  This and tsk == current are
> + * the only situations where tsk->cred may be accessed without an rcu read lock.
> + */
>  static int audit_filter_rules(struct task_struct *tsk,
>  			      struct audit_krule *rule,
>  			      struct audit_context *ctx,
>  			      struct audit_names *name,
> -			      enum audit_state *state)
> +			      enum audit_state *state,
> +			      bool task_creation)
>  {
> -	const struct cred *cred = get_task_cred(tsk);
> +	const struct cred *cred;
>  	int i, j, need_sid = 1;
>  	u32 sid;
>  
> +	cred = rcu_dereference_check(tsk->cred, tsk == current || task_creation);
> +
>  	for (i = 0; i < rule->field_count; i++) {
>  		struct audit_field *f = &rule->fields[i];
>  		int result = 0;
> @@ -637,10 +645,8 @@ static int audit_filter_rules(struct task_struct *tsk,
>  			break;
>  		}
>  
> -		if (!result) {
> -			put_cred(cred);
> +		if (!result)
>  			return 0;
> -		}
>  	}
>  
>  	if (ctx) {
> @@ -656,7 +662,6 @@ static int audit_filter_rules(struct task_struct *tsk,
>  	case AUDIT_NEVER:    *state = AUDIT_DISABLED;	    break;
>  	case AUDIT_ALWAYS:   *state = AUDIT_RECORD_CONTEXT; break;
>  	}
> -	put_cred(cred);
>  	return 1;
>  }
>  
> @@ -671,7 +676,8 @@ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
>  
>  	rcu_read_lock();
>  	list_for_each_entry_rcu(e, &audit_filter_list[AUDIT_FILTER_TASK], list) {
> -		if (audit_filter_rules(tsk, &e->rule, NULL, NULL, &state)) {
> +		if (audit_filter_rules(tsk, &e->rule, NULL, NULL,
> +				       &state, true)) {
>  			if (state == AUDIT_RECORD_CONTEXT)
>  				*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
>  			rcu_read_unlock();
> @@ -705,7 +711,7 @@ static enum audit_state audit_filter_syscall(struct task_struct *tsk,
>  		list_for_each_entry_rcu(e, list, list) {
>  			if ((e->rule.mask[word] & bit) == bit &&
>  			    audit_filter_rules(tsk, &e->rule, ctx, NULL,
> -					       &state)) {
> +					       &state, false)) {
>  				rcu_read_unlock();
>  				ctx->current_state = state;
>  				return state;
> @@ -743,7 +749,8 @@ void audit_filter_inodes(struct task_struct *tsk, struct audit_context *ctx)
>  
>  		list_for_each_entry_rcu(e, list, list) {
>  			if ((e->rule.mask[word] & bit) == bit &&
> -			    audit_filter_rules(tsk, &e->rule, ctx, n, &state)) {
> +			    audit_filter_rules(tsk, &e->rule, ctx, n,
> +				    	       &state, false)) {
>  				rcu_read_unlock();
>  				ctx->current_state = state;
>  				return;



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