NFS: Fix up nfs_ctx_key_to_expire()

If the cached credential exists but doesn't have any expiration callback
then exit early.
Fix up atomicity issues when replacing the credential with a new one
since the existing code could lead to refcount leaks.
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/inode.c

@@ -1024,7 +1024,7 @@ struct nfs_open_context *alloc_nfs_open_context(struct dentry *dentry,
 		ctx->cred = get_cred(filp->f_cred);
 	else
 		ctx->cred = get_current_cred();
-	ctx->ll_cred = NULL;
+	rcu_assign_pointer(ctx->ll_cred, NULL);
 	ctx->state = NULL;
 	ctx->mode = f_mode;
 	ctx->flags = 0;
@@ -1063,7 +1063,7 @@ static void __put_nfs_open_context(struct nfs_open_context *ctx, int is_sync)
 	put_cred(ctx->cred);
 	dput(ctx->dentry);
 	nfs_sb_deactive(sb);
-	put_rpccred(ctx->ll_cred);
+	put_rpccred(rcu_dereference_protected(ctx->ll_cred, 1));
 	kfree(ctx->mdsthreshold);
 	kfree_rcu(ctx, rcu_head);
 }

fs/nfs/write.c

@@ -1246,7 +1246,7 @@ nfs_key_timeout_notify(struct file *filp, struct inode *inode)
 	struct nfs_open_context *ctx = nfs_file_open_context(filp);
 
 	if (nfs_ctx_key_to_expire(ctx, inode) &&
-	    !ctx->ll_cred)
+	    !rcu_access_pointer(ctx->ll_cred))
 		/* Already expired! */
 		return -EACCES;
 	return 0;
@@ -1258,23 +1258,38 @@ nfs_key_timeout_notify(struct file *filp, struct inode *inode)
 bool nfs_ctx_key_to_expire(struct nfs_open_context *ctx, struct inode *inode)
 {
 	struct rpc_auth *auth = NFS_SERVER(inode)->client->cl_auth;
-	struct rpc_cred *cred = ctx->ll_cred;
+	struct rpc_cred *cred, *new, *old = NULL;
 	struct auth_cred acred = {
 		.cred = ctx->cred,
 	};
+	bool ret = false;
 
-	if (cred && !cred->cr_ops->crmatch(&acred, cred, 0)) {
-		put_rpccred(cred);
-		ctx->ll_cred = NULL;
-		cred = NULL;
-	}
-	if (!cred)
-		cred = auth->au_ops->lookup_cred(auth, &acred, 0);
-	if (!cred || IS_ERR(cred))
+	rcu_read_lock();
+	cred = rcu_dereference(ctx->ll_cred);
+	if (cred && !(cred->cr_ops->crkey_timeout &&
+		      cred->cr_ops->crkey_timeout(cred)))
+		goto out;
+	rcu_read_unlock();
+
+	new = auth->au_ops->lookup_cred(auth, &acred, 0);
+	if (new == cred) {
+		put_rpccred(new);
 		return true;
-	ctx->ll_cred = cred;
-	return !!(cred->cr_ops->crkey_timeout &&
-		  cred->cr_ops->crkey_timeout(cred));
+	}
+	if (IS_ERR_OR_NULL(new)) {
+		new = NULL;
+		ret = true;
+	} else if (new->cr_ops->crkey_timeout &&
+		   new->cr_ops->crkey_timeout(new))
+		ret = true;
+
+	rcu_read_lock();
+	old = rcu_dereference_protected(xchg(&ctx->ll_cred,
+					     RCU_INITIALIZER(new)), 1);
+out:
+	rcu_read_unlock();
+	put_rpccred(old);
+	return ret;
 }
 
 /*

include/linux/nfs_fs.h

@@ -81,7 +81,7 @@ struct nfs_open_context {
 	fl_owner_t flock_owner;
 	struct dentry *dentry;
 	const struct cred *cred;
-	struct rpc_cred *ll_cred;	/* low-level cred - use to check for expiry */
+	struct rpc_cred __rcu *ll_cred;	/* low-level cred - use to check for expiry */
 	struct nfs4_state *state;
 	fmode_t mode;