[V3] x86: mce: fix kernel panic when check_interval is changed

[V3] x86: mce: fix kernel panic when check_interval is changed

[Seunghun Han]

I am Seunghun Han and a senior security researcher at National Security
Research Institute of South Korea.

I found a security issue which can make kernel panic in userspace. After
analyzing the issue carefully, I found that MCE driver in the kernel has a
problem which can be occurred in SMP environment.

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 driver
in kernel calls mce_restart() function in store_int_with_restart() function
and broadcasts the event to other CPUs to delete and restart MCE polling
timer.

The __mcheck_cpu_init_timer() function which is called by mce_restart()
function initializes the mce_timer variable, and the "lock" in mce_timer is
also reinitialized. If more than one CPU write a specific value to
check_interval file concurrently, one can initialize the "lock" in mce_timer
while the others are handling "lock" in mce_timer. This problem causes some
synchronization errors such as kernel panic and kernel hang. Other functions
such as set_ignore_ce(), set_cmci_disabled(), and mce_enable_ce() also
have synchronization problems.

It could be a security problem because the attacker could make kernel panic
by writing a value to the check_interval file in userspace, and it could be
used for Denial-of-Service (DoS) attack.

To fix this problem, I added a mce_sysfs_mutex to serialize requests for
timer and sysfs functions.

Signed-off-by: Seunghun Han <kkamagui@gmail.com>
---
Changes since v2: add a mutex to sysfs functions according to review
result.
Changes since v1: add mce_sysfs_mutex according to review result.

 arch/x86/kernel/cpu/mcheck/mce.c | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 706584681a4c..243f46a40efb 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -55,6 +55,7 @@
 #include "mce-internal.h"
 
 static DEFINE_MUTEX(mce_log_mutex);
+static DEFINE_MUTEX(mce_sysfs_mutex);
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/mce.h>
@@ -2045,8 +2046,11 @@ static void mce_enable_ce(void *all)
 		return;
 	cmci_reenable();
 	cmci_recheck();
-	if (all)
+	if (all) {
+		mutex_lock(&mce_sysfs_mutex);
 		__mcheck_cpu_init_timer();
+		mutex_unlock(&mce_sysfs_mutex);
+	}
 }
 
 static struct bus_type mce_subsys = {
@@ -2090,6 +2094,7 @@ static ssize_t set_ignore_ce(struct device *s,
 	if (kstrtou64(buf, 0, &new) < 0)
 		return -EINVAL;
 
+	mutex_lock(&mce_sysfs_mutex);
 	if (mca_cfg.ignore_ce ^ !!new) {
 		if (new) {
 			/* disable ce features */
@@ -2102,6 +2107,8 @@ static ssize_t set_ignore_ce(struct device *s,
 			on_each_cpu(mce_enable_ce, (void *)1, 1);
 		}
 	}
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return size;
 }
 
@@ -2114,6 +2121,7 @@ static ssize_t set_cmci_disabled(struct device *s,
 	if (kstrtou64(buf, 0, &new) < 0)
 		return -EINVAL;
 
+	mutex_lock(&mce_sysfs_mutex);
 	if (mca_cfg.cmci_disabled ^ !!new) {
 		if (new) {
 			/* disable cmci */
@@ -2125,6 +2133,8 @@ static ssize_t set_cmci_disabled(struct device *s,
 			on_each_cpu(mce_enable_ce, NULL, 1);
 		}
 	}
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return size;
 }
 
@@ -2132,8 +2142,16 @@ static ssize_t store_int_with_restart(struct device *s,
 				      struct device_attribute *attr,
 				      const char *buf, size_t size)
 {
+	unsigned long old_check_interval = check_interval;
 	ssize_t ret = device_store_int(s, attr, buf, size);
+
+	if (check_interval == old_check_interval)
+		return ret;
+
+	mutex_lock(&mce_sysfs_mutex);
 	mce_restart();
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return ret;
 }
 

[V3] x86: mce: fix kernel panic when check_interval is changed

[Borislav Petkov]

On Sat, Mar 03, 2018 at 05:27:06AM +0900, Seunghun Han wrote:
> I am Seunghun Han and a senior security researcher at National Security
> Research Institute of South Korea.
> 
> I found a security issue which can make kernel panic in userspace. After
> analyzing the issue carefully, I found that MCE driver in the kernel has a
> problem which can be occurred in SMP environment.
> 
> 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 driver
> in kernel calls mce_restart() function in store_int_with_restart() function
> and broadcasts the event to other CPUs to delete and restart MCE polling
> timer.
> 
> The __mcheck_cpu_init_timer() function which is called by mce_restart()
> function initializes the mce_timer variable, and the "lock" in mce_timer is
> also reinitialized. If more than one CPU write a specific value to
> check_interval file concurrently, one can initialize the "lock" in mce_timer
> while the others are handling "lock" in mce_timer. This problem causes some
> synchronization errors such as kernel panic and kernel hang. Other functions
> such as set_ignore_ce(), set_cmci_disabled(), and mce_enable_ce() also
> have synchronization problems.
> 
> It could be a security problem because the attacker could make kernel panic
> by writing a value to the check_interval file in userspace, and it could be
> used for Denial-of-Service (DoS) attack.
> 
> To fix this problem, I added a mce_sysfs_mutex to serialize requests for
> timer and sysfs functions.
> 
> Signed-off-by: Seunghun Han <kkamagui@gmail.com>
> ---
> Changes since v2: add a mutex to sysfs functions according to review
> result.
> Changes since v1: add mce_sysfs_mutex according to review result.

Thanks, I've committed the patch below. Scream if there's still
something not in order:
---
From: Seunghun Han <kkamagui@gmail.com>
Date: Sat, 3 Mar 2018 05:27:06 +0900
Subject: [PATCH] x86/MCE: Synchronize sysfs changes

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 "slow" accesses
down by synchronizing them with a mutex and thus take care of the
synchronization issue too.

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>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Tony Luck <tony.luck@intel.com>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: x86-ml <x86@kernel.org>
Link: http://lkml.kernel.org/r/20180302202706.9434-1-kkamagui@gmail.com
Signed-off-by: Borislav Petkov <bp@suse.de>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 181f6cf25895..21962c48dad7 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -56,6 +56,9 @@
 
 static DEFINE_MUTEX(mce_log_mutex);
 
+/* sysfs synchronization */
+static DEFINE_MUTEX(mce_sysfs_mutex);
+
 #define CREATE_TRACE_POINTS
 #include <trace/events/mce.h>
 
@@ -2104,6 +2107,7 @@ static ssize_t set_ignore_ce(struct device *s,
 	if (kstrtou64(buf, 0, &new) < 0)
 		return -EINVAL;
 
+	mutex_lock(&mce_sysfs_mutex);
 	if (mca_cfg.ignore_ce ^ !!new) {
 		if (new) {
 			/* disable ce features */
@@ -2116,6 +2120,8 @@ static ssize_t set_ignore_ce(struct device *s,
 			on_each_cpu(mce_enable_ce, (void *)1, 1);
 		}
 	}
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return size;
 }
 
@@ -2128,6 +2134,7 @@ static ssize_t set_cmci_disabled(struct device *s,
 	if (kstrtou64(buf, 0, &new) < 0)
 		return -EINVAL;
 
+	mutex_lock(&mce_sysfs_mutex);
 	if (mca_cfg.cmci_disabled ^ !!new) {
 		if (new) {
 			/* disable cmci */
@@ -2139,6 +2146,8 @@ static ssize_t set_cmci_disabled(struct device *s,
 			on_each_cpu(mce_enable_ce, NULL, 1);
 		}
 	}
+	mutex_unlock(&mce_sysfs_mutex);
+
 	return size;
 }
 
@@ -2146,8 +2155,19 @@ static ssize_t store_int_with_restart(struct device *s,
 				      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;
 }
 

[V3] x86: mce: fix kernel panic when check_interval is changed

[Greg Kroah-Hartman]

On Tue, Mar 06, 2018 at 11:43:20AM +0100, Borislav Petkov wrote:
> On Sat, Mar 03, 2018 at 05:27:06AM +0900, Seunghun Han wrote:
> > I am Seunghun Han and a senior security researcher at National Security
> > Research Institute of South Korea.
> > 
> > I found a security issue which can make kernel panic in userspace. After
> > analyzing the issue carefully, I found that MCE driver in the kernel has a
> > problem which can be occurred in SMP environment.
> > 
> > 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 driver
> > in kernel calls mce_restart() function in store_int_with_restart() function
> > and broadcasts the event to other CPUs to delete and restart MCE polling
> > timer.
> > 
> > The __mcheck_cpu_init_timer() function which is called by mce_restart()
> > function initializes the mce_timer variable, and the "lock" in mce_timer is
> > also reinitialized. If more than one CPU write a specific value to
> > check_interval file concurrently, one can initialize the "lock" in mce_timer
> > while the others are handling "lock" in mce_timer. This problem causes some
> > synchronization errors such as kernel panic and kernel hang. Other functions
> > such as set_ignore_ce(), set_cmci_disabled(), and mce_enable_ce() also
> > have synchronization problems.
> > 
> > It could be a security problem because the attacker could make kernel panic
> > by writing a value to the check_interval file in userspace, and it could be
> > used for Denial-of-Service (DoS) attack.
> > 
> > To fix this problem, I added a mce_sysfs_mutex to serialize requests for
> > timer and sysfs functions.
> > 
> > Signed-off-by: Seunghun Han <kkamagui@gmail.com>
> > ---
> > Changes since v2: add a mutex to sysfs functions according to review
> > result.
> > Changes since v1: add mce_sysfs_mutex according to review result.
> 
> Thanks, I've committed the patch below. Scream if there's still
> something not in order:

It would have been nice to add a cc:stable for this, but I'll try to
watch it and when it hits Linus's tree I'll queue it up there.

thanks,

greg k-h
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[V3] x86: mce: fix kernel panic when check_interval is changed

[Borislav Petkov]

On Tue, Mar 06, 2018 at 02:57:48AM -0800, Greg Kroah-Hartman wrote:
> It would have been nice to add a cc:stable for this, but I'll try to
> watch it and when it hits Linus's tree I'll queue it up there.

I can still add it - it is the top patch and I haven't sent it yet.

/me goes and adds it.

Voila!

:-)

[V3] x86: mce: fix kernel panic when check_interval is changed

[Greg Kroah-Hartman]

On Tue, Mar 06, 2018 at 12:02:08PM +0100, Borislav Petkov wrote:
> On Tue, Mar 06, 2018 at 02:57:48AM -0800, Greg Kroah-Hartman wrote:
> > It would have been nice to add a cc:stable for this, but I'll try to
> > watch it and when it hits Linus's tree I'll queue it up there.
> 
> I can still add it - it is the top patch and I haven't sent it yet.
> 
> /me goes and adds it.
> 
> Voila!
> 
> :-)

Wonderful, thanks for doing that.
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