linux-kernel - [PATCH 3/3] auditsc: Do not use RCU primitive to read from cred pointer

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Date:   Fri,  7 Feb 2020 23:35:05 +0530
From:   Amol Grover <frextrite@...il.com>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        James Morris <jamorris@...ux.microsoft.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Jann Horn <jannh@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...hat.com>
Cc:     linux-kernel-mentees@...ts.linuxfoundation.org,
        linux-kernel@...r.kernel.org, linux-audit@...hat.com,
        Joel Fernandes <joel@...lfernandes.org>,
        Madhuparna Bhowmik <madhuparnabhowmik10@...il.com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Amol Grover <frextrite@...il.com>
Subject: [PATCH 3/3] auditsc: Do not use RCU primitive to read from cred pointer

task_struct::cred is only used task-synchronously and does
not require any RCU locks, hence, rcu_dereference_check is
not required to read from it.

Suggested-by: Jann Horn <jannh@...gle.com>
Co-developed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
Signed-off-by: Amol Grover <frextrite@...il.com>
---
 kernel/auditsc.c | 15 +++++----------
 1 file changed, 5 insertions(+), 10 deletions(-)

diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index 4effe01ebbe2..d3510513cdd1 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -430,24 +430,19 @@ static int audit_field_compare(struct task_struct *tsk,
 /* 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.
- *
- * 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,
-			      bool task_creation)
+			      enum audit_state *state)
 {
 	const struct cred *cred;
 	int i, need_sid = 1;
 	u32 sid;
 	unsigned int sessionid;
 
-	cred = rcu_dereference_check(tsk->cred, tsk == current || task_creation);
+	cred = tsk->cred;
 
 	for (i = 0; i < rule->field_count; i++) {
 		struct audit_field *f = &rule->fields[i];
@@ -745,7 +740,7 @@ 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, true)) {
+				       &state)) {
 			if (state == AUDIT_RECORD_CONTEXT)
 				*key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
 			rcu_read_unlock();
@@ -791,7 +786,7 @@ static enum audit_state audit_filter_syscall(struct task_struct *tsk,
 	list_for_each_entry_rcu(e, list, list) {
 		if (audit_in_mask(&e->rule, ctx->major) &&
 		    audit_filter_rules(tsk, &e->rule, ctx, NULL,
-				       &state, false)) {
+				       &state)) {
 			rcu_read_unlock();
 			ctx->current_state = state;
 			return state;
@@ -815,7 +810,7 @@ static int audit_filter_inode_name(struct task_struct *tsk,
 
 	list_for_each_entry_rcu(e, list, list) {
 		if (audit_in_mask(&e->rule, ctx->major) &&
-		    audit_filter_rules(tsk, &e->rule, ctx, n, &state, false)) {
+		    audit_filter_rules(tsk, &e->rule, ctx, n, &state)) {
 			ctx->current_state = state;
 			return 1;
 		}
-- 
2.24.1