Patch "x86/MCE: Serialize sysfs changes" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

x86/MCE: Serialize sysfs changes

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-mce-serialize-sysfs-changes.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From b3b7c4795ccab5be71f080774c45bbbcc75c2aaf Mon Sep 17 00:00:00 2001
From: Seunghun Han <[email protected]>
Date: Tue, 6 Mar 2018 15:21:43 +0100
Subject: x86/MCE: Serialize sysfs changes

From: Seunghun Han <[email protected]>

commit b3b7c4795ccab5be71f080774c45bbbcc75c2aaf upstream.

The check_interval file in

/sys/devices/system/machinecheck/machinecheck<cpu number>

directory is a global timer value for MCE polling. If it is changed by one CPU, mce_restart() broadcasts the event to other CPUs to delete and restart the MCE polling timer and __mcheck_cpu_init_timer() reinitializes the mce_timer variable.

If more than one CPU writes a specific value to the check_interval file concurrently, mce_timer is not protected from such concurrent accesses and all kinds of explosions happen. Since only root can write to those sysfs variables, the issue is not a big deal security-wise.

However, concurrent writes to these configuration variables is void of reason so the proper thing to do is to serialize the access with a mutex.

Boris:

- Make store_int_with_restart() use device_store_ulong() to filter out negative intervals
- Limit min interval to 1 second
- Correct locking
- Massage commit message

---
 arch/x86/kernel/cpu/mcheck/mce.c |   22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/x86-mce-serialize-sysfs-changes.patch
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@@ -57,6 +57,9 @@
 
 static DEFINE_MUTEX(mce_log_mutex);
 
+/* sysfs synchronization */
+static DEFINE_MUTEX(mce_sysfs_mutex);
+
 #define CREATE_TRACE_POINTS
 #include <trace/events/mce.h>
 

@@ -2080,6 +2083,7 @@ static ssize_t set_ignore_ce(struct devi
 	if (kstrtou64(buf, 0, &new) < 0)
 		return -EINVAL;
 
+	mutex_lock(&mce_sysfs_mutex);
 	if (mca_cfg.ignore_ce ^ !!new) {
 		if (new) {
 			/* disable ce features */

@@ -2092,6 +2096,8 @@ static ssize_t set_ignore_ce(struct devi
 			on_each_cpu(mce_enable_ce, (void *)1, 1);
 		}
 	}
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return size;
 }
 

@@ -2104,6 +2110,7 @@ static ssize_t set_cmci_disabled(struct
 	if (kstrtou64(buf, 0, &new) < 0)
 		return -EINVAL;
 
+	mutex_lock(&mce_sysfs_mutex);
 	if (mca_cfg.cmci_disabled ^ !!new) {
 		if (new) {
 			/* disable cmci */

@@ -2115,6 +2122,8 @@ static ssize_t set_cmci_disabled(struct
 			on_each_cpu(mce_enable_ce, NULL, 1);
 		}
 	}
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return size;
 }
 

@@ -2122,8 +2131,19 @@ static ssize_t store_int_with_restart(st
 				      struct device_attribute *attr,
 				      const char *buf, size_t size)
 {
-	ssize_t ret = device_store_int(s, attr, buf, size);
+	unsigned long old_check_interval = check_interval;
+	ssize_t ret = device_store_ulong(s, attr, buf, size);
+
+	if (check_interval == old_check_interval)
+		return ret;
+
+	if (check_interval < 1)
+		check_interval = 1;
+
+	mutex_lock(&mce_sysfs_mutex);
 	mce_restart();
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return ret;
 }