Merge branch 'access-creds'

The access() (and faccessat()) credentials change can cause an
unnecessary load on the RCU machinery because every access() call ends
up freeing the temporary access credential using RCU.

This isn't really noticeable on small machines, but if you have hundreds
of cores you can cause huge slowdowns due to RCU storms.

It's easy to avoid: the temporary access crededntials aren't actually
normally accessed using RCU at all, so we can avoid the whole issue by
just marking them as such.

* access-creds:
  access: avoid the RCU grace period for the temporary subjective credentials

1 files changed, 19 insertions, 2 deletions

diff --git a/kernel/cred.c b/kernel/cred.c
index f9a0ce66c9c3..c0a4c12d38b2 100644
--- a/kernel/cred.c
+++ b/kernel/cred.c
@@ -144,7 +144,10 @@ void __put_cred(struct cred *cred)
 	BUG_ON(cred == current->cred);
 	BUG_ON(cred == current->real_cred);
 
-	call_rcu(&cred->rcu, put_cred_rcu);
+	if (cred->non_rcu)
+		put_cred_rcu(&cred->rcu);
+	else
+		call_rcu(&cred->rcu, put_cred_rcu);
 }
 EXPORT_SYMBOL(__put_cred);
 
@@ -261,6 +264,7 @@ struct cred *prepare_creds(void)
 	old = task->cred;
 	memcpy(new, old, sizeof(struct cred));
 
+	new->non_rcu = 0;
 	atomic_set(&new->usage, 1);
 	set_cred_subscribers(new, 0);
 	get_group_info(new->group_info);
@@ -544,7 +548,19 @@ const struct cred *override_creds(const struct cred *new)
 
 	validate_creds(old);
 	validate_creds(new);
-	get_cred(new);
+
+	/*
+	 * NOTE! This uses 'get_new_cred()' rather than 'get_cred()'.
+	 *
+	 * That means that we do not clear the 'non_rcu' flag, since
+	 * we are only installing the cred into the thread-synchronous
+	 * '->cred' pointer, not the '->real_cred' pointer that is
+	 * visible to other threads under RCU.
+	 *
+	 * Also note that we did validate_creds() manually, not depending
+	 * on the validation in 'get_cred()'.
+	 */
+	get_new_cred((struct cred *)new);
 	alter_cred_subscribers(new, 1);
 	rcu_assign_pointer(current->cred, new);
 	alter_cred_subscribers(old, -1);
@@ -681,6 +697,7 @@ struct cred *prepare_kernel_cred(struct task_struct *daemon)
 	validate_creds(old);
 
 	*new = *old;
+	new->non_rcu = 0;
 	atomic_set(&new->usage, 1);
 	set_cred_subscribers(new, 0);
 	get_uid(new->user);