linux-kernel - [PATCH 2/3] cred: Do not use RCU primitives to access cred pointer

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Message-Id: <20200207180504.4200-2-frextrite@gmail.com>
Date:   Fri,  7 Feb 2020 23:35:04 +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 2/3] cred: Do not use RCU primitives to access cred pointer

Since task_struct::cred can only be used task-synchronously,
and is not visible to other threads under RCU context,
we do not require RCU primitives to read/write to it and incur
heavy barriers.

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>
---
 include/linux/cred.h | 5 ++---
 kernel/cred.c        | 6 +++---
 2 files changed, 5 insertions(+), 6 deletions(-)

diff --git a/include/linux/cred.h b/include/linux/cred.h
index 18639c069263..5973791e5fe4 100644
--- a/include/linux/cred.h
+++ b/include/linux/cred.h
@@ -291,11 +291,10 @@ static inline void put_cred(const struct cred *_cred)
 /**
  * current_cred - Access the current task's subjective credentials
  *
- * Access the subjective credentials of the current task.  RCU-safe,
- * since nobody else can modify it.
+ * Access the subjective credentials of the current task.
  */
 #define current_cred() \
-	rcu_dereference_protected(current->cred, 1)
+	(current->cred)
 
 /**
  * current_real_cred - Access the current task's objective credentials
diff --git a/kernel/cred.c b/kernel/cred.c
index 809a985b1793..3956c31d068d 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -485,7 +485,7 @@ int commit_creds(struct cred *new)
 	if (new->user != old->user)
 		atomic_inc(&new->user->processes);
 	rcu_assign_pointer(task->real_cred, new);
-	rcu_assign_pointer(task->cred, new);
+	task->cred = new;
 	if (new->user != old->user)
 		atomic_dec(&old->user->processes);
 	alter_cred_subscribers(old, -2);
@@ -562,7 +562,7 @@ const struct cred *override_creds(const struct cred *new)
 	 */
 	get_new_cred((struct cred *)new);
 	alter_cred_subscribers(new, 1);
-	rcu_assign_pointer(current->cred, new);
+	current->cred = new;
 	alter_cred_subscribers(old, -1);
 
 	kdebug("override_creds() = %p{%d,%d}", old,
@@ -590,7 +590,7 @@ void revert_creds(const struct cred *old)
 	validate_creds(old);
 	validate_creds(override);
 	alter_cred_subscribers(old, 1);
-	rcu_assign_pointer(current->cred, old);
+	current->cred = old;
 	alter_cred_subscribers(override, -1);
 	put_cred(override);
 }
-- 
2.24.1