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 <kkama...@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