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

Tony Jones
2011-03-17T18:12:08Z

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>
---
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;