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Subject[PATCH 3.2 020/153] x86/MCE: Serialize sysfs changes
3.2.102-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Seunghun Han <kkamagui@gmail.com>

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

Signed-off-by: Seunghun Han <kkamagui@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: http://lkml.kernel.org/r/20180302202706.9434-1-kkamagui@gmail.com
[bwh: Backported to 3.2:
- MCE device is a sysdev here
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
arch/x86/kernel/cpu/mcheck/mce.c | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)

--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -52,6 +52,9 @@ static DEFINE_MUTEX(mce_chrdev_read_mute
rcu_read_lock_sched_held() || \
lockdep_is_held(&mce_chrdev_read_mutex))

+/* sysfs synchronization */
+static DEFINE_MUTEX(mce_sysfs_mutex);
+
#define CREATE_TRACE_POINTS
#include <trace/events/mce.h>

@@ -1883,6 +1886,7 @@ static ssize_t set_ignore_ce(struct sys_
if (strict_strtoull(buf, 0, &new) < 0)
return -EINVAL;

+ mutex_lock(&mce_sysfs_mutex);
if (mce_ignore_ce ^ !!new) {
if (new) {
/* disable ce features */
@@ -1895,6 +1899,8 @@ static ssize_t set_ignore_ce(struct sys_
on_each_cpu(mce_enable_ce, (void *)1, 1);
}
}
+ mutex_unlock(&mce_sysfs_mutex);
+
return size;
}

@@ -1907,6 +1913,7 @@ static ssize_t set_cmci_disabled(struct
if (strict_strtoull(buf, 0, &new) < 0)
return -EINVAL;

+ mutex_lock(&mce_sysfs_mutex);
if (mce_cmci_disabled ^ !!new) {
if (new) {
/* disable cmci */
@@ -1918,6 +1925,8 @@ static ssize_t set_cmci_disabled(struct
on_each_cpu(mce_enable_ce, NULL, 1);
}
}
+ mutex_unlock(&mce_sysfs_mutex);
+
return size;
}

@@ -1925,8 +1934,19 @@ static ssize_t store_int_with_restart(st
struct sysdev_attribute *attr,
const char *buf, size_t size)
{
- ssize_t ret = sysdev_store_int(s, attr, buf, size);
+ unsigned long old_check_interval = check_interval;
+ ssize_t ret = sysdev_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;
}

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