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

Eric Paris
2011-03-21T13:57:48Z

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;