[tip:x86/urgent] x86/intel_rdt: Ensure a CPU remains online for the region's pseudo-locking sequence

[tip-bot for Reinette Chatre]

Commit-ID:  80b71c340f17705ec145911b9a193ea781811b16
Gitweb:     https://git.kernel.org/tip/80b71c340f17705ec145911b9a193ea781811b16
Author:     Reinette Chatre <reinette.chatre@xxxxxxxxx>
AuthorDate: Mon, 10 Dec 2018 13:21:54 -0800
Committer:  Borislav Petkov <bp@xxxxxxx>
CommitDate: Tue, 11 Dec 2018 21:59:01 +0100

x86/intel_rdt: Ensure a CPU remains online for the region's pseudo-locking sequence

The user triggers the creation of a pseudo-locked region when writing
the requested schemata to the schemata resctrl file. The pseudo-locking
of a region is required to be done on a CPU that is associated with the
cache on which the pseudo-locked region will reside. In order to run the
locking code on a specific CPU, the needed CPU has to be selected and
ensured to remain online during the entire locking sequence.

At this time, the cpu_hotplug_lock is not taken during the pseudo-lock
region creation and it is thus possible for a CPU to be selected to run
the pseudo-locking code and then that CPU to go offline before the
thread is able to run on it.

Fix this by ensuring that the cpu_hotplug_lock is taken while the CPU on
which code has to run needs to be controlled. Since the cpu_hotplug_lock
is always taken before rdtgroup_mutex the lock order is maintained.

Fixes: e0bdfe8e36f3 ("x86/intel_rdt: Support creation/removal of pseudo-locked region")
Signed-off-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
Cc: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: gavin.hindman@xxxxxxxxx
Cc: jithu.joseph@xxxxxxxxx
Cc: stable <stable@xxxxxxxxxxxxxxx>
Cc: x86-ml <x86@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/b7b17432a80f95a1fa21a1698ba643014f58ad31.1544476425.git.reinette.chatre@xxxxxxxxx
---
 arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c b/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
index 27937458c231..efa4a519f5e5 100644
--- a/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
+++ b/arch/x86/kernel/cpu/intel_rdt_ctrlmondata.c
@@ -23,6 +23,7 @@
 
 #define pr_fmt(fmt)	KBUILD_MODNAME ": " fmt
 
+#include <linux/cpu.h>
 #include <linux/kernfs.h>
 #include <linux/seq_file.h>
 #include <linux/slab.h>
@@ -310,9 +311,11 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
 		return -EINVAL;
 	buf[nbytes - 1] = '\0';
 
+	cpus_read_lock();
 	rdtgrp = rdtgroup_kn_lock_live(of->kn);
 	if (!rdtgrp) {
 		rdtgroup_kn_unlock(of->kn);
+		cpus_read_unlock();
 		return -ENOENT;
 	}
 	rdt_last_cmd_clear();
@@ -367,6 +370,7 @@ ssize_t rdtgroup_schemata_write(struct kernfs_open_file *of,
 
 out:
 	rdtgroup_kn_unlock(of->kn);
+	cpus_read_unlock();
 	return ret ?: nbytes;
 }