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 <kkama...@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 <kkama...@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 <kkama...@gmail.com> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Cc: Tony Luck <tony.l...@intel.com> Cc: linux-edac <linux-e...@vger.kernel.org> Cc: x86-ml <x...@kernel.org> Link: http://lkml.kernel.org/r/20180302202706.9434-1-kkama...@gmail.com Signed-off-by: Borislav Petkov <b...@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; } -- 2.13.0 -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
