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

[Seunghun Han <kkamagui@gmail.com>]

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

I found a critical 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 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.

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

To fix this problem, I changed the __mcheck_cpu_init_timer() function to
reuse mce_timer instead of initializing it. The purpose of the function is
to restart the timer and it can be archived by calling

Signed-off-by: Seunghun Han <kkamagui@gmail.com>
---
 arch/x86/kernel/cpu/mcheck/mce.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
index 3d8c573a9a27..d72f2f74f4d7 100644
--- a/arch/x86/kernel/cpu/mcheck/mce.c
+++ b/arch/x86/kernel/cpu/mcheck/mce.c
@@ -1771,7 +1771,6 @@ static void __mcheck_cpu_init_timer(void)
 {
 	struct timer_list *t = this_cpu_ptr(&mce_timer);
 
-	timer_setup(t, mce_timer_fn, TIMER_PINNED);
 	mce_start_timer(t);
 }
 
@@ -2029,8 +2028,10 @@ static void mce_enable_ce(void *all)
 		return;
 	cmci_reenable();
 	cmci_recheck();
-	if (all)
+	if (all) {
+		del_timer_sync(this_cpu_ptr(&mce_timer));
 		__mcheck_cpu_init_timer();
+	}
 }
 
 static struct bus_type mce_subsys = {
-- 
2.11.0