[tip:x86/urgent] x86/resctrl: Prevent NULL pointer dereference when local MBM is disabled

[tip-bot for Prarit Bhargava <tipbot@zytor.com>]

Commit-ID:  c7563e62a6d720aa3b068e26ddffab5f0df29263
Gitweb:     https://git.kernel.org/tip/c7563e62a6d720aa3b068e26ddffab5f0df29263
Author:     Prarit Bhargava <prarit@redhat.com>
AuthorDate: Mon, 10 Jun 2019 13:15:44 -0400
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 12 Jun 2019 10:31:50 +0200

x86/resctrl: Prevent NULL pointer dereference when local MBM is disabled

Booting with kernel parameter "rdt=cmt,mbmtotal,memlocal,l3cat,mba" and
executing "mount -t resctrl resctrl -o mba_MBps /sys/fs/resctrl" results in
a NULL pointer dereference on systems which do not have local MBM support
enabled..

BUG: kernel NULL pointer dereference, address: 0000000000000020
PGD 0 P4D 0
Oops: 0000 [#1] SMP PTI
CPU: 0 PID: 722 Comm: kworker/0:3 Not tainted 5.2.0-0.rc3.git0.1.el7_UNSUPPORTED.x86_64 #2
Workqueue: events mbm_handle_overflow
RIP: 0010:mbm_handle_overflow+0x150/0x2b0

Only enter the bandwith update loop if the system has local MBM enabled.

Fixes: de73f38f7680 ("x86/intel_rdt/mba_sc: Feedback loop to dynamically update mem bandwidth")
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: Reinette Chatre <reinette.chatre@intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20190610171544.13474-1-prarit@redhat.com

---
 arch/x86/kernel/cpu/resctrl/monitor.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 1573a0a6b525..ff6e8e561405 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -368,6 +368,9 @@ static void update_mba_bw(struct rdtgroup *rgrp, struct rdt_domain *dom_mbm)
 	struct list_head *head;
 	struct rdtgroup *entry;
 
+	if (!is_mbm_local_enabled())
+		return;
+
 	r_mba = &rdt_resources_all[RDT_RESOURCE_MBA];
 	closid = rgrp->closid;
 	rmid = rgrp->mon.rmid;